ruby-style-guide

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rubocop-hq /ruby-style-guide

A community-driven Ruby coding style guide

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= The Ruby Style Guide :idprefix: :idseparator: - :sectanchors: :sectlinks: :toc: preamble :toclevels: 1 ifndef::backend-pdf[] :toc-title: pass:[

Table of Contents

] endif::[] :source-highlighter: rouge

== Introduction

[quote, Officer Alex J. Murphy / RoboCop]


Role models are important.


ifdef::env-github[] TIP: You can find a beautiful version of this guide with much improved navigation at https://rubystyle.guide. endif::[]

This Ruby style guide recommends best practices so that real-world Ruby programmers can write code that can be maintained by other real-world Ruby programmers. A style guide that reflects real-world usage gets used, while a style guide that holds to an ideal that has been rejected by the people it is supposed to help risks not getting used at all - no matter how good it is.

The guide is separated into several sections of related guidelines. We've tried to add the rationale behind the guidelines (if it's omitted we've assumed it's pretty obvious).

We didn't come up with all the guidelines out of nowhere - they are mostly based on the professional experience of the editors, feedback and suggestions from members of the Ruby community and various highly regarded Ruby programming resources, such as https://pragprog.com/book/ruby4/programming-ruby-1-9-2-0["Programming Ruby"] and http://www.amazon.com/Ruby-Programming-Language-David-Flanagan/dp/0596516177["The Ruby Programming Language"].

This style guide evolves over time as additional conventions are identified and past conventions are rendered obsolete by changes in Ruby itself.

ifdef::env-github[] You can generate a PDF copy of this guide using https://asciidoctor.org/docs/asciidoctor-pdf/[AsciiDoctor PDF], and an HTML copy https://asciidoctor.org/docs/convert-documents/#converting-a-document-to-html[with] https://asciidoctor.org/#installation[AsciiDoctor] using the following commands:

[source,shell]

Generates README.pdf

asciidoctor-pdf -a allow-uri-read README.adoc

Generates README.html

asciidoctor

[TIP]

Install the

rouge
gem to get nice syntax highlighting in the generated document.

[source,shell]

gem install rouge

==== endif::[]

[TIP]

If you're into Rails or RSpec you might want to check out the complementary https://github.com/rubocop-hq/rails-style-guide[Ruby on Rails Style Guide] and https://github.com/rubocop-hq/rspec-style-guide[RSpec Style Guide].

TIP: https://github.com/rubocop-hq/rubocop[RuboCop] is a static code analyzer (linter) and formatter, based on this style guide.

=== Guiding Principles

[quote, Harold Abelson, Structure and Interpretation of Computer Programs]


Programs must be written for people to read, and only incidentally for machines to execute.


It's common knowledge that code is read much more often than it is written. The guidelines provided here are intended to improve the readability of code and make it consistent across the wide spectrum of Ruby code. They are also meant to reflect real-world usage of Ruby instead of a random ideal. When we had to choose between a very established practice and a subjectively better alternative we've opted to recommend the established practice.footnote:[Occasionally we might suggest to the reader to consider some alternatives, though.]

There are some areas in which there is no clear consensus in the Ruby community regarding a particular style (like string literal quoting, spacing inside hash literals, dot position in multi-line method chaining, etc.). In such scenarios all popular styles are acknowledged and it's up to you to pick one and apply it consistently.

Ruby had existed for over 15 years by the time the guide was created, and the language's flexibility and lack of common standards have contributed to the creations of numerous styles for just about everything. Rallying people around the cause of community standards took a lot of time and energy, and we still have a lot of ground to cover.

Ruby is famously optimized for programmer happiness. We'd like to believe that this guide is going to help you optimize for maximum programmer happiness.

=== A Note about Consistency

[quote, Ralph Waldo Emerson]


A foolish consistency is the hobgoblin of little minds, adored by little statesmen and philosophers and divines.


A style guide is about consistency. Consistency with this style guide is important. Consistency within a project is more important. Consistency within one class or method is the most important.

However, know when to be inconsistent -- sometimes style guide recommendations just aren't applicable. When in doubt, use your best judgment. Look at other examples and decide what looks best. And don't hesitate to ask!

In particular: do not break backwards compatibility just to comply with this guide!

Some other good reasons to ignore a particular guideline:

  • When applying the guideline would make the code less readable, even for someone who is used to reading code that follows this style guide.
  • To be consistent with surrounding code that also breaks it (maybe for historic reasons) -- although this is also an opportunity to clean up someone else's mess (in true XP style).
  • Because the code in question predates the introduction of the guideline and there is no other reason to be modifying that code.
  • When the code needs to remain compatible with older versions of Ruby that don't support the feature recommended by the style guide.

=== Translations

Translations of the guide are available in the following languages:

  • https://github.com/JuanitoFatas/ruby-style-guide/blob/master/README-zhCN.md[Chinese Simplified]
  • https://github.com/JuanitoFatas/ruby-style-guide/blob/master/README-zhTW.md[Chinese Traditional]
  • https://github.com/HassanTC/ruby-style-guide/blob/master/README-EgAr.md[Egyptian Arabic]
  • https://github.com/gauthier-delacroix/ruby-style-guide/blob/master/README-frFR.md[French]
  • https://github.com/fortissimo1997/ruby-style-guide/blob/japanese/README.ja.md[Japanese]
  • https://github.com/dalzony/ruby-style-guide/blob/master/README-koKR.md[Korean]
  • https://github.com/rubensmabueno/ruby-style-guide/blob/master/README-PT-BR.md[Portuguese (pt-BR)]
  • https://github.com/arbox/ruby-style-guide/blob/master/README-ruRU.md[Russian]
  • https://github.com/alemohamad/ruby-style-guide/blob/master/README-esLA.md[Spanish]
  • https://github.com/CQBinh/ruby-style-guide/blob/master/README-viVN.md[Vietnamese]

NOTE: These translations are not maintained by our editor team, so their quality and level of completeness may vary. The translated versions of the guide often lag behind the upstream English version.

== Source Code Layout

[quote, Jerry Coffin (on indentation)]


Nearly everybody is convinced that every style but their own is ugly and unreadable. Leave out the "but their own" and they're probably right...


=== Source Encoding [[utf-8]]

Use

UTF-8
as the source file encoding.

TIP: UTF-8 has been the default source file encoding since Ruby 2.0.

=== Tabs or Spaces? [[tabs-or-spaces]]

Use only spaces for indentation. No hard tabs.

=== Indentation [[spaces-indentation]]

Use two spaces per indentation level (aka soft tabs).

[source,ruby]

bad - four spaces

def somemethod dosomething end

good

def somemethod dosomething

end

=== Maximum Line Length [[max-line-length]]

Limit lines to 80 characters.

TIP: Most editors and IDEs have configurations options to help you with that. They would typically highlight lines that exceed the length limit.

.Why Bother with 80 characters in a World of Modern Widescreen Displays?


A lot of people these days feel that a maximum line length of 80 characters is just a remnant of the past and makes little sense today. After all - modern displays can easily fit 200+ characters on a single line. Still, there are some important benefits to be gained from sticking to shorter lines of code.

First, and foremost - numerous studies have shown that humans read much faster vertically and very long lines of text impede the reading process. As noted earlier, one of the guiding principles of this style guide is to optimize the code we write for human consumption.

Additionally, limiting the required editor window width makes it possible to have several files open side-by-side, and works well when using code review tools that present the two versions in adjacent columns.

The default wrapping in most tools disrupts the visual structure of the code, making it more difficult to understand. The limits are chosen to avoid wrapping in editors with the window width set to 80, even if the tool places a marker glyph in the final column when wrapping lines. Some web based tools may not offer dynamic line wrapping at all.

Some teams strongly prefer a longer line length. For code maintained exclusively or primarily by a team that can reach agreement on this issue, it is okay to increase the line length limit up to 100 characters, or all the way up to 120 characters. Please, restrain the urge to go beyond 120 characters.


=== No Trailing Whitespace [[no-trailing-whitespace]]

Avoid trailing whitespace.

TIP: Most editors and IDEs have configuration options to visualize trailing whitespace and to remove it automatically on save.

=== Line Endings [[crlf]]

Use Unix-style line endings.footnote:[*BSD/Solaris/Linux/macOS users are covered by default, Windows users have to be extra careful.]

[TIP]

If you're using Git you might want to add the following configuration setting to protect your project from Windows line endings creeping in:

[source,bash]

$ git config --global core.autocrlf true

====

=== Should I Terminate Files with a Newline? [[newline-eof]]

End each file with a newline.

TIP: This should be done by through editor configuration, not manually.

=== Should I Terminate Expressions with

;
? [[no-semicolon]]

Don't use

;
to terminate statements and expressions.

[source,ruby]

bad

puts 'foobar'; # superfluous semicolon

good

puts 'foobar'

=== One Expression Per Line [[one-expression-per-line]]

Use one expression per line.

[source,ruby]

bad

puts 'foo'; puts 'bar' # two expressions on the same line

good

puts 'foo' puts 'bar'

puts 'foo', 'bar' # this applies to puts in particular

=== Spaces and Operators [[spaces-operators]]

Use spaces around operators, after commas, colons and semicolons. Whitespace might be (mostly) irrelevant to the Ruby interpreter, but its proper use is the key to writing easily readable code.

[source,ruby]

bad

sum=1+2 a,b=1,2 class FooError<StandardError;end

good

sum = 1 + 2 a, b = 1, 2

class FooError < StandardError; end

There are a few exceptions:

  • Exponent operator:

[source,ruby]

bad

e = M * c ** 2

good

e = M * c**2

  • Slash in rational literals:

[source,ruby]

bad

o_scale = 1 / 48r

good

o_scale = 1/48r

  • Safe navigation operator:

[source,ruby]

bad

foo &. bar foo &.bar foo&. bar

good

foo&.bar

=== Spaces and Braces [[spaces-braces]]

No spaces after

(
,
[
or before
]
,
)
. Use spaces around
{
and before
}
.

[source,ruby]

bad

some( arg ).other [ 1, 2, 3 ].each{|e| puts e}

good

some(arg).other

[1, 2, 3].each { |e| puts e }

{
and
}
deserve a bit of clarification, since they are used for block and hash literals, as well as string interpolation.

For hash literals two styles are considered acceptable. The first variant is slightly more readable (and arguably more popular in the Ruby community in general). The second variant has the advantage of adding visual difference between block and hash literals. Whichever one you pick - apply it consistently.

[source,ruby]

good - space after { and before }

{ one: 1, two: 2 }

good - no space after { and before }

{one: 1, two: 2}

With interpolated expressions, there should be no padded-spacing inside the braces.

[source,ruby]

bad

"From: #{ user.firstname }, #{ user.lastname }"

good

"From: #{user.firstname}, #{user.lastname}"

=== No Space after Bang [[no-space-bang]]

No space after

!
.

[source,ruby]

bad

! something

good

!something

=== No Space inside Range Literals [[no-space-inside-range-literals]]

No space inside range literals.

[source,ruby]

bad

1 .. 3 'a' ... 'z'

good

1..3

'a'...'z'

=== Indent

when
to
case
[[indent-when-to-case]]

Indent

when
as deep as
case
.

[source,ruby]

bad

case when song.name == 'Misty' puts 'Not again!' when song.duration > 120 puts 'Too long!' when Time.now.hour > 21 puts "It's too late" else song.play end

good

case when song.name == 'Misty' puts 'Not again!' when song.duration > 120 puts 'Too long!' when Time.now.hour > 21 puts "It's too late" else song.play

end

.A Bit of History


This is the style established in both "The Ruby Programming Language" and "Programming Ruby". Historically it is derived from the fact that

case
and
switch
statements are not blocks, hence should not be indented, and the
when
and
else
keywords are labels (compiled in the C language, they are literally labels for
JMP
calls).

=== Indent Conditional Assignment [[indent-conditional-assignment]]

When assigning the result of a conditional expression to a variable, preserve the usual alignment of its branches.

[source,ruby]

bad - pretty convoluted

kind = case year when 1850..1889 then 'Blues' when 1890..1909 then 'Ragtime' when 1910..1929 then 'New Orleans Jazz' when 1930..1939 then 'Swing' when 1940..1950 then 'Bebop' else 'Jazz' end

result = if somecond calcsomething else calcsomethingelse end

good - it's apparent what's going on

kind = case year when 1850..1889 then 'Blues' when 1890..1909 then 'Ragtime' when 1910..1929 then 'New Orleans Jazz' when 1930..1939 then 'Swing' when 1940..1950 then 'Bebop' else 'Jazz' end

result = if somecond calcsomething else calcsomethingelse end

good (and a bit more width efficient)

kind = case year when 1850..1889 then 'Blues' when 1890..1909 then 'Ragtime' when 1910..1929 then 'New Orleans Jazz' when 1930..1939 then 'Swing' when 1940..1950 then 'Bebop' else 'Jazz' end

result = if somecond calcsomething else calcsomethingelse

end

=== Empty Lines between Methods [[empty-lines-between-methods]]

Use empty lines between method definitions and also to break up methods into logical paragraphs internally.

[source,ruby]

def some_method data = initialize(options)

data.manipulate!

data.result end

def some_method result

end

=== Two or More Empty Lines [[two-or-more-empty-lines]]

Don't use several empty lines in a row.

[source,ruby]

bad - It has two empty lines.

some_method

some_method

good

some_method

some_method

=== Empty Lines around Attribute Accessor [[empty-lines-around-attribute-accessor]]

Use empty lines around attribute accessor.

[source,ruby]

bad

class Foo attr_reader :foo def foo # do something... end end

good

class Foo attr_reader :foo

def foo # do something... end

end

=== Empty Lines around Access Modifier [[empty-lines-around-access-modifier]]

Use empty lines around attribute modifier.

[source,ruby]

bad

class Foo def bar; end private def baz; end end

good

class Foo def bar; end

private

def baz; end

end

=== Empty Lines around Bodies [[empty-lines-around-bodies]]

Don't use empty lines around method, class, module, block bodies.

[source,ruby]

bad

class Foo

def foo

begin

do_something do

something

end

rescue

something

end

end

end

good

class Foo def foo begin do_something do something end rescue something end end

end

=== Trailing Comma in Method Arguments [[no-trailing-params-comma]]

Avoid comma after the last parameter in a method call, especially when the parameters are not on separate lines.

[source,ruby]

bad - easier to move/add/remove parameters, but still not preferred

some_method( size, count, color, )

bad

some_method(size, count, color, )

good

some_method(size, count, color)

=== Spaces around Equals [[spaces-around-equals]]

Use spaces around the

=
operator when assigning default values to method parameters:

[source,ruby]

bad

def some_method(arg1=:default, arg2=nil, arg3=[]) # do something... end

good

def some_method(arg1 = :default, arg2 = nil, arg3 = []) # do something...

end

While several Ruby books suggest the first style, the second is much more prominent in practice (and arguably a bit more readable).

=== Line Continuation in Expressions [[no-trailing-backslash]]

Avoid line continuation with

\
where not required. In practice, avoid using line continuations for anything but string concatenation.

[source,ruby]

bad (\ is not needed here)

result = 1 - \ 2

bad (\ is required, but still ugly as hell)

result = 1 \ - 2

good

result = 1 - 2

long_string = 'First part of the long string' \

' and second part of the long string'

=== Multi-line Method Chains [[consistent-multi-line-chains]]

Adopt a consistent multi-line method chaining style. There are two popular styles in the Ruby community, both of which are considered good - leading

.
and trailing
.
.

==== Leading

.
[[leading-dot-in-multi-line-chains]]

When continuing a chained method invocation on another line, keep the

.
on the second line.

[source,ruby]

bad - need to consult first line to understand second line

one.two.three. four

good - it's immediately clear what's going on the second line

one.two.three

.four

==== Trailing

.
[[trailing-dot-in-multi-line-chains]]

When continuing a chained method invocation on another line, include the

.
on the first line to indicate that the expression continues.

[source,ruby]

bad - need to read ahead to the second line to know that the chain continues

one.two.three .four

good - it's immediately clear that the expression continues beyond the first line

one.two.three.

four

A discussion on the merits of both alternative styles can be found https://github.com/rubocop-hq/ruby-style-guide/pull/176[here].

=== Method Arguments Alignment [[no-double-indent]]

Align the arguments of a method call if they span more than one line. When aligning arguments is not appropriate due to line-length constraints, single indent for the lines after the first is also acceptable.

[source,ruby]

starting point (line is too long)

def send_mail(source) Mailer.deliver(to: '[email protected]', from: '[email protected]', subject: 'Important message', body: source.text) end

bad (double indent)

def send_mail(source) Mailer.deliver( to: '[email protected]', from: '[email protected]', subject: 'Important message', body: source.text) end

good

def send_mail(source) Mailer.deliver(to: '[email protected]', from: '[email protected]', subject: 'Important message', body: source.text) end

good (normal indent)

def send_mail(source) Mailer.deliver( to: '[email protected]', from: '[email protected]', subject: 'Important message', body: source.text )

end

=== Implicit Options Hash [[no-braces-opts-hash]]

IMPORTANT: As of Ruby 2.7 braces around an options hash are no longer optional.

Omit the outer braces around an implicit options hash.

[source,ruby]

bad

user.set({ name: 'John', age: 45, permissions: { read: true } })

good

user.set(name: 'John', age: 45, permissions: { read: true })

=== DSL Method Calls [[no-dsl-decorating]]

Omit both the outer braces and parentheses for methods that are part of an internal DSL.

[source,ruby]

class Person < ActiveRecord::Base # bad validates(:name, { presence: true, length: { within: 1..10 } })

# good validates :name, presence: true, length: { within: 1..10 }

end

=== Space in Method Calls [[parens-no-spaces]]

Do not put a space between a method name and the opening parenthesis.

[source,ruby]

bad

puts (x + y)

good

puts(x + y)

=== Multi-line Arrays Alignment [[align-multiline-arrays]]

Align the elements of array literals spanning multiple lines.

[source,ruby]

bad - single indent

menu_item = %w[Spam Spam Spam Spam Spam Spam Spam Spam Baked beans Spam Spam Spam Spam Spam]

good

menu_item = %w[ Spam Spam Spam Spam Spam Spam Spam Spam Baked beans Spam Spam Spam Spam Spam ]

good

menu_item = %w[Spam Spam Spam Spam Spam Spam Spam Spam

Baked beans Spam Spam Spam Spam Spam]

== Naming Conventions

[quote, Phil Karlton]


The only real difficulties in programming are cache invalidation and naming things.


=== English for Identifiers [[english-identifiers]]

Name identifiers in English.

[source,ruby]

bad - identifier using non-ascii characters

заплата = 1_000

bad - identifier is a Bulgarian word, written with Latin letters (instead of Cyrillic)

zaplata = 1_000

good

salary = 1_000

=== Snake Case for Symbols, Methods and Variables [[snake-case-symbols-methods-vars]]

Use

snake_case
for symbols, methods and variables.

[source,ruby]

bad

:'some symbol' :SomeSymbol :someSymbol

someVar = 5

def someMethod # some code end

def SomeMethod # some code end

good

:some_symbol

some_var = 5

def some_method # some code

end

=== Identifiers with a Numeric Suffix [[snake-case-symbols-methods-vars-with-numbers]]

Do not separate numbers from letters on symbols, methods and variables.

[source,ruby]

bad

:somesym1

somevar1 = 1

var_10 = 10

def somemethod1 # some code end

good

:some_sym1

some_var1 = 1

var10 = 10

def some_method1 # some code

end

=== CamelCase for Classes [[camelcase-classes]]

Use

CamelCase
for classes and modules. (Keep acronyms like HTTP, RFC, XML uppercase).

[source,ruby]

bad

class Someclass # some code end

class Some_Class # some code end

class SomeXml # some code end

class XmlSomething # some code end

good

class SomeClass # some code end

class SomeXML # some code end

class XMLSomething # some code

end

=== Snake Case for Files [[snake-case-files]]

Use

snake_case
for naming files, e.g.
hello_world.rb
.

=== Snake Case for Directories [[snake-case-dirs]]

Use

snake_case
for naming directories, e.g.
lib/hello_world/hello_world.rb
.

=== One Class per File [[one-class-per-file]]

Aim to have just a single class/module per source file. Name the file name as the class/module, but replacing

CamelCase
with
snake_case
.

=== Screaming Snake Case for Constants [[screaming-snake-case]]

Use

SCREAMING_SNAKE_CASE
for other constants (those that don't refer to classes and modules).

[source,ruby]

bad

SomeConst = 5

good

SOME_CONST = 5

=== Predicate Methods Suffix [[bool-methods-qmark]]

The names of predicate methods (methods that return a boolean value) should end in a question mark (i.e.

Array#empty?
). Methods that don't return a boolean, shouldn't end in a question mark.

[source,ruby]

bad

def even(value) end

good

def even?(value)

end

=== Predicate Methods Prefix [[bool-methods-prefix]]

Avoid prefixing predicate methods with the auxiliary verbs such as

is
,
does
, or
can
. These words are redundant and inconsistent with the style of boolean methods in the Ruby core library, such as
empty?
and
include?
.

[source,ruby]

bad

class Person def is_tall? true end

def canplaybasketball? false end

def doeslikecandy? true end end

good

class Person def tall? true end

def basketball_player? false end

def likes_candy? true end

end

=== Dangerous Method Suffix [[dangerous-method-bang]]

The names of potentially dangerous methods (i.e. methods that modify

self
or the arguments,
exit!
(doesn't run the finalizers like
exit
does), etc) should end with an exclamation mark if there exists a safe version of that dangerous method.

[source,ruby]

bad - there is no matching 'safe' method

class Person def update! end end

good

class Person def update end end

good

class Person def update! end

def update end

end

=== Relationship between Safe and Dangerous Methods [[safe-because-unsafe]]

Define the non-bang (safe) method in terms of the bang (dangerous) one if possible.

[source,ruby]

class Array def flatten_once! res = []

each do |e|
  [*e].each { |f| res << f }
end

replace(res)

end

def flattenonce dup.flattenonce! end

end

=== Unused Variables Prefix [[underscore-unused-vars]]

Prefix with

+_+
unused block parameters and local variables. It's also acceptable to use just
+_+
(although it's a bit less descriptive). This convention is recognized by the Ruby interpreter and tools like RuboCop and will suppress their unused variable warnings.

[source,ruby]

bad

result = hash.map { |k, v| v + 1 }

def something(x) unusedvar, usedvar = something_else(x) # some code end

good

result = hash.map { |_k, v| v + 1 }

def something(x) unusedvar, usedvar = somethingelse(x) # some code end

good

result = hash.map { |_, v| v + 1 }

def something(x) , usedvar = something_else(x) # some code

end

===

other
Parameter [[other-arg]]

When defining binary operators and operator-alike methods, name the parameter

other
for operators with "symmetrical" semantics of operands. Symmetrical semantics means both sides of the operator are typically of same or coercible types.

Operators and operator-alike methods with symmetrical semantics (the parameter should be named

other
):
+
,
-
,
+*+
,
/
,
%
,
**
,
==
,
>
,
<
,
|
,
&
,
^
,
eql?
,
equal?
.

Operators with non-symmetrical semantics (the parameter should not be named

other
):
<<
,
[]
(collection/item relations between operands),
===
(pattern/matchable relations).

Note that the rule should be followed only if both sides of the operator have the same semantics. Prominent exception in Ruby core is, for example,

Array#*(int)
.

[source,ruby]

good

def +(other) # body omitted end

bad

def <<(other) @internal << other end

good

def <<(item) @internal << item end

bad

Returns some string multiplied
other
times

def *(other) # body omitted end

good

Returns some string multiplied
num
times

def *(num) # body omitted

end

== Flow of Control

===

for
Loops [[no-for-loops]]

Do not use

for
, unless you know exactly why. Most of the time iterators should be used instead.
for
is implemented in terms of
each
(so you're adding a level of indirection), but with a twist -
for
doesn't introduce a new scope (unlike
each
) and variables defined in its block will be visible outside it.

[source,ruby]

arr = [1, 2, 3]

bad

for elem in arr do puts elem end

note that elem is accessible outside of the for loop

elem # => 3

good

arr.each { |elem| puts elem }

elem is not accessible outside each's block

elem # => NameError: undefined local variable or method `elem'

===

then
in Multi-line Expression [[no-then]]

Do not use

then
for multi-line
if
/
unless
/
when
.

[source,ruby]

bad

if some_condition then # body omitted end

bad

case foo when bar then # body omitted end

good

if some_condition # body omitted end

good

case foo when bar # body omitted

end

=== Condition Placement [[same-line-condition]]

Always put the condition on the same line as the

if
/
unless
in a multi-line conditional.

[source,ruby]

bad

if somecondition dosomething dosomethingelse end

good

if somecondition dosomething dosomethingelse

end

=== Ternary Operator vs

if
[[ternary-operator]]

Prefer the ternary operator(

?:
) over
if/then/else/end
constructs. It's more common and obviously more concise.

[source,ruby]

bad

result = if somecondition then something else somethingelse end

good

result = somecondition ? something : somethingelse

=== Nested Ternary Operators [[no-nested-ternary]]

Use one expression per branch in a ternary operator. This also means that ternary operators must not be nested. Prefer

if/else
constructs in these cases.

[source,ruby]

bad

somecondition ? (nestedcondition ? nestedsomething : nestedsomethingelse) : somethingelse

good

if somecondition nestedcondition ? nestedsomething : nestedsomethingelse else somethingelse

end

=== Semicolon in

if
[[no-semicolon-ifs]]

Do not use

if x; ...
. Use the ternary operator instead.

[source,ruby]

bad

result = if somecondition; something else somethingelse end

good

result = somecondition ? something : somethingelse

===

case
vs
if-else
[[case-vs-if-else]]

Prefer

case
over
if-elsif
when compared value is same in each clause.

[source,ruby]

bad

if status == :active performaction elsif status == :inactive || status == :hibernating checktimeout else final_action end

good

case status when :active performaction when :inactive, :hibernating checktimeout else final_action

end

=== Returning Result from

if
/
case
[[use-if-case-returns]]

Leverage the fact that

if
and
case
are expressions which return a result.

[source,ruby]

bad

if condition result = x else result = y end

good

result = if condition x else y

end

=== One-line Cases [[one-line-cases]]

Use

when x then ...
for one-line cases.

NOTE: The alternative syntax

when x: ...
has been removed as of Ruby 1.9.

=== Semicolon in

when
[[no-when-semicolons]]

Do not use

when x; ...
. See the previous rule.

===

!
vs
not
[[bang-not-not]]

Use

!
instead of
not
.

[source,ruby]

bad - parentheses are required because of op precedence

x = (not something)

good

x = !something

=== Double Negation [[no-bang-bang]]

Avoid unnecessary uses of

!!

!!
converts a value to boolean, but you don't need this explicit conversion in the condition of a control expression; using it only obscures your intention.

Consider using it only when there is a valid reason to restrict the result

true
or
false
. Examples include outputing to a particular format or API like JSON, or as the return value of a
predicate?
method. In these cases, also consider doing a nil check instead:
!something.nil?
.

[source,ruby]

bad

x = 'test'

obscure nil check

if !!x # body omitted end

good

x = 'test' if x # body omitted end

good

def named? !name.nil? end

good

def banned? !!banned_until&.future?

end

===

and
/
or
[[no-and-or-or]]

The

and
and
or
keywords are banned. The minimal added readability is just not worth the high probability of introducing subtle bugs. For boolean expressions, always use
&&
and
||
instead. For flow control, use
if
and
unless
;
&&
and
||
are also acceptable but less clear.

[source,ruby]

bad

boolean expression

ok = gotneededarguments and argumentsarevalid

control flow

document.save or raise("Failed to save document!")

good

boolean expression

ok = gotneededarguments && argumentsarevalid

control flow

raise("Failed to save document!") unless document.save

ok

control flow

document.save || raise("Failed to save document!")

=== Multi-line Ternary Operator [[no-multiline-ternary]]

Avoid multi-line

?:
(the ternary operator); use
if
/
unless
instead.

===

if
as a Modifier [[if-as-a-modifier]]

Prefer modifier

if
/
unless
usage when you have a single-line body. Another good alternative is the usage of control flow
&&
/
||
.

[source,ruby]

bad

if somecondition dosomething end

good

dosomething if somecondition

another good option

somecondition && dosomething

=== Multi-line

if
Modifiers [[no-multiline-if-modifiers]]

Avoid modifier

if
/
unless
usage at the end of a non-trivial multi-line block.

[source,ruby]

bad

10.times do # multi-line body omitted end if some_condition

good

if some_condition 10.times do # multi-line body omitted end

end

=== Nested Modifiers [[no-nested-modifiers]]

Avoid nested modifier

if
/
unless
/
while
/
until
usage. Prefer
&&
/
||
if appropriate.

[source,ruby]

bad

dosomething if othercondition if some_condition

good

dosomething if somecondition && other_condition

===

if
vs
unless
[[unless-for-negatives]]

Prefer

unless
over
if
for negative conditions (or control flow
||
).

[source,ruby]

bad

dosomething if !somecondition

bad

dosomething if not somecondition

good

dosomething unless somecondition

another good option

somecondition || dosomething

=== Using

else
with
unless
[[no-else-with-unless]]

Do not use

unless
with
else
. Rewrite these with the positive case first.

[source,ruby]

bad

unless success? puts 'failure' else puts 'success' end

good

if success? puts 'success' else puts 'failure'

end

=== Parentheses around Condition [[no-parens-around-condition]]

Don't use parentheses around the condition of a control expression.

[source,ruby]

bad

if (x > 10) # body omitted end

good

if x > 10 # body omitted

end

NOTE: There is an exception to this rule, namely <>.

=== Multi-line

while do
[[no-multiline-while-do]]

Do not use

while/until condition do
for multi-line
while/until
.

[source,ruby]

bad

while x > 5 do # body omitted end

until x > 5 do # body omitted end

good

while x > 5 # body omitted end

until x > 5 # body omitted

end

===

while
as a Modifier [[while-as-a-modifier]]

Prefer modifier

while/until
usage when you have a single-line body.

[source,ruby]

bad

while somecondition dosomething end

good

dosomething while somecondition

===

while
vs
until
[[until-for-negatives]]

Prefer

until
over
while
for negative conditions.

[source,ruby]

bad

dosomething while !somecondition

good

dosomething until somecondition

=== Infinite Loop [[infinite-loop]]

Use

Kernel#loop
instead of
while
/
until
when you need an infinite loop.

[source,ruby]

bad

while true do_something end

until false do_something end

good

loop do do_something

end

===

loop
with
break
[[loop-with-break]]

Use

Kernel#loop
with
break
rather than
begin/end/until
or
begin/end/while
for post-loop tests.

[source,ruby]

bad

begin puts val val += 1 end while val < 0

good

loop do puts val val += 1 break unless val < 0

end

=== Explicit

return
[[no-explicit-return]]

Avoid

return
where not required for flow of control.

[source,ruby]

bad

def somemethod(somearr) return some_arr.size end

good

def somemethod(somearr) some_arr.size

end

=== Explicit

self
[[no-self-unless-required]]

Avoid

self
where not required. (It is only required when calling a
self
write accessor, methods named after reserved words, or overloadable operators.)

[source,ruby]

bad

def ready? if self.lastreviewedat > self.lastupdatedat self.worker.update(self.content, self.options) self.status = :in_progress end self.status == :verified end

good

def ready? if lastreviewedat > lastupdatedat worker.update(content, options) self.status = :in_progress end status == :verified

end

=== Shadowing Methods [[no-shadowing]]

As a corollary, avoid shadowing methods with local variables unless they are both equivalent.

[source,ruby]

class Foo attr_accessor :options

# ok def initialize(options) self.options = options # both options and self.options are equivalent here end

# bad def do_something(options = {}) unless options[:when] == :later output(self.options[:message]) end end

# good def do_something(params = {}) unless params[:when] == :later output(options[:message]) end end

end

=== Safe Assignment in Condition [[safe-assignment-in-condition]]

Don't use the return value of

=
(an assignment) in conditional expressions unless the assignment is wrapped in parentheses. This is a fairly popular idiom among Rubyists that's sometimes referred to as safe assignment in condition.

[source,ruby]

bad (+ a warning)

if v = array.grep(/foo/) do_something(v) # some code end

good (MRI would still complain, but RuboCop won't)

if (v = array.grep(/foo/)) do_something(v) # some code end

good

v = array.grep(/foo/) if v do_something(v) # some code

end

===

BEGIN
Blocks [[no-BEGIN-blocks]]

Avoid the use of

BEGIN
blocks.

===

END
Blocks [[no-END-blocks]]

Do not use

END
blocks. Use
Kernel#at_exit
instead.

[source,ruby]

bad

END { puts 'Goodbye!' }

good

at_exit { puts 'Goodbye!' }

=== Nested Conditionals [[no-nested-conditionals]]

Avoid use of nested conditionals for flow of control.

Prefer a guard clause when you can assert invalid data. A guard clause is a conditional statement at the top of a function that bails out as soon as it can.

[source,ruby]

bad

def computething(thing) if thing[:foo] updatewithbar(thing[:foo]) if thing[:foo][:bar] partialcompute(thing) else re_compute(thing) end end end

good

def computething(thing) return unless thing[:foo] updatewithbar(thing[:foo]) return recompute(thing) unless thing[:foo][:bar] partial_compute(thing)

end

Prefer

next
in loops instead of conditional blocks.

[source,ruby]

bad

[0, 1, 2, 3].each do |item| if item > 1 puts item end end

good

[0, 1, 2, 3].each do |item| next unless item > 1 puts item

end

== Exceptions

===

raise
vs
fail
[[prefer-raise-over-fail]]

Prefer

raise
over
fail
for exceptions.

[source,ruby]

bad

fail SomeException, 'message'

good

raise SomeException, 'message'

=== Raising Explicit

RuntimeError
[[no-explicit-runtimeerror]]

Don't specify

RuntimeError
explicitly in the two argument version of
raise
.

[source,ruby]

bad

raise RuntimeError, 'message'

good - signals a RuntimeError by default

raise 'message'

=== Exception Class Messages [[exception-class-messages]]

Prefer supplying an exception class and a message as two separate arguments to

raise
, instead of an exception instance.

[source,ruby]

bad

raise SomeException.new('message')

Note that there is no way to do
raise SomeException.new('message'), backtrace
.

good

raise SomeException, 'message'

Consistent with
raise SomeException, 'message', backtrace
.


===

return
from
ensure
[[no-return-ensure]]

Do not return from an

ensure
block. If you explicitly return from a method inside an
ensure
block, the return will take precedence over any exception being raised, and the method will return as if no exception had been raised at all. In effect, the exception will be silently thrown away.

[source,ruby]

bad

def foo raise ensure return 'very bad idea'

end

=== Implicit

begin
[[begin-implicit]]

Use implicit begin blocks where possible.

[source,ruby]

bad

def foo begin # main logic goes here rescue # failure handling goes here end end

good

def foo # main logic goes here rescue # failure handling goes here

end

=== Contingency Methods [[contingency-methods]]

Mitigate the proliferation of

begin
blocks by using contingency methods (a term coined by Avdi Grimm).

[source,ruby]

bad

begin somethingthatmight_fail rescue IOError # handle IOError end

begin somethingelsethatmightfail rescue IOError # handle IOError end

good

def withioerror_handling yield rescue IOError # handle IOError end

withioerrorhandling { somethingthatmightfail }

withioerrorhandling { somethingelsethatmight_fail }

=== Suppressing Exceptions [[dont-hide-exceptions]]

Don't suppress exceptions.

[source,ruby]

bad

begin do_something # an exception occurs here rescue SomeError end

good

begin dosomething # an exception occurs here rescue SomeError handleexception end

good

begin do_something # an exception occurs here rescue SomeError # Notes on why exception handling is not performed end

good

do_something rescue nil

=== Using

rescue
as a Modifier [[no-rescue-modifiers]]

Avoid using

rescue
in its modifier form.

[source,ruby]

bad - this catches exceptions of StandardError class and its descendant classes

readfile rescue handleerror($!)

good - this catches only the exceptions of Errno::ENOENT class and its descendant classes

def foo readfile rescue Errno::ENOENT => e handleerror(e)

end

=== Using Exceptions for Flow of Control [[no-exceptional-flows]]

Don't use exceptions for flow of control.

[source,ruby]

bad

begin n / d rescue ZeroDivisionError puts 'Cannot divide by 0!' end

good

if d.zero? puts 'Cannot divide by 0!' else n / d

end

=== Blind Rescues [[no-blind-rescues]]

Avoid rescuing the

Exception
class. This will trap signals and calls to
exit
, requiring you to
kill -9
the process.

[source,ruby]

bad

begin # calls to exit and kill signals will be caught (except kill -9) exit rescue Exception puts "you didn't really want to exit, right?" # exception handling end

good

begin # a blind rescue rescues from StandardError, not Exception as many # programmers assume. rescue => e # exception handling end

also good

begin # an exception occurs here rescue StandardError => e # exception handling

end

=== Exception Rescuing Ordering [[exception-ordering]]

Put more specific exceptions higher up the rescue chain, otherwise they'll never be rescued from.

[source,ruby]

bad

begin # some code rescue StandardError => e # some handling rescue IOError => e # some handling that will never be executed end

good

begin # some code rescue IOError => e # some handling rescue StandardError => e # some handling

end

=== Release External Resources [[release-resources]]

Release external resources obtained by your program in an

ensure
block.

[source,ruby]

f = File.open('testfile') begin # .. process rescue # .. handle error ensure f.close if f

end

=== Auto-release External Resources [[auto-release-resources]]

Use versions of resource obtaining methods that do automatic resource cleanup when possible.

[source,ruby]

bad - you need to close the file descriptor explicitly

f = File.open('testfile')

some action on the file

f.close

good - the file descriptor is closed automatically

File.open('testfile') do |f| # some action on the file

end

=== Standard Exceptions [[standard-exceptions]]

Prefer the use of exceptions from the standard library over introducing new exception classes.

== Assignment & Comparison

=== Parallel Assignment [[parallel-assignment]]

Avoid the use of parallel assignment for defining variables. Parallel assignment is allowed when it is the return of a method call, used with the splat operator, or when used to swap variable assignment. Parallel assignment is less readable than separate assignment.

[source,ruby]

bad

a, b, c, d = 'foo', 'bar', 'baz', 'foobar'

good

a = 'foo' b = 'bar' c = 'baz' d = 'foobar'

good - swapping variable assignment

Swapping variable assignment is a special case because it will allow you to

swap the values that are assigned to each variable.

a = 'foo' b = 'bar'

a, b = b, a puts a # => 'bar' puts b # => 'foo'

good - method return

def multi_return [1, 2] end

first, second = multi_return

good - use with splat

first, *list = [1, 2, 3, 4] # first => 1, list => [2, 3, 4]

hello_array = *'Hello' # => ["Hello"]

a = *(1..3) # => [1, 2, 3]

=== Dealing with Trailing Underscore Variables in Destructuring Assignment [[trailing-underscore-variables]]

Avoid the use of unnecessary trailing underscore variables during parallel assignment. Named underscore variables are to be preferred over underscore variables because of the context that they provide. Trailing underscore variables are necessary when there is a splat variable defined on the left side of the assignment, and the splat variable is not an underscore.

[source,ruby]

bad

foo = 'one,two,three,four,five'

Unnecessary assignment that does not provide useful information

first, second, _ = foo.split(',') first, , _ = foo.split(',') first, * = foo.split(',')

good

foo = 'one,two,three,four,five'

The underscores are needed to show that you want all elements

except for the last number of underscore elements

*beginning, _ = foo.split(',') *beginning, something, _ = foo.split(',')

a, = foo.split(',') a, b, = foo.split(',')

Unnecessary assignment to an unused variable, but the assignment

provides us with useful information.

first, _second = foo.split(',') first, _second, = foo.split(',')

first, *_ending = foo.split(',')

=== Self-assignment [[self-assignment]]

Use shorthand self assignment operators whenever applicable.

[source,ruby]

bad

x = x + y x = x * y x = x**y x = x / y x = x || y x = x && y

good

x += y x = y x *= y x /= y x ||= y

x &&= y

=== Conditional Variable Initialization Shorthand [[double-pipe-for-uninit]]

Use

||=
to initialize variables only if they're not already initialized.

[source,ruby]

bad

name = name ? name : 'Bozhidar'

bad

name = 'Bozhidar' unless name

good - set name to 'Bozhidar', only if it's nil or false

name ||= 'Bozhidar'

[WARNING]

Don't use

||=
to initialize boolean variables. (Consider what would happen if the current value happened to be
false
.)

[source,ruby]

bad - would set enabled to true even if it was false

enabled ||= true

good

enabled = true if enabled.nil?

====

=== Existence Check Shorthand [[double-amper-preprocess]]

Use

&&=
to preprocess variables that may or may not exist. Using
&&=
will change the value only if it exists, removing the need to check its existence with
if
.

[source,ruby]

bad

if something something = something.downcase end

bad

something = something ? something.downcase : nil

ok

something = something.downcase if something

good

something = something && something.downcase

better

something &&= something.downcase

=== Identity Comparison [[identity-comparison]]

Prefer

equal?
over
==
when comparing
object_id
.
Object#equal?
is provided to compare objects for identity, and in contrast
Object#==
is provided for the purpose of doing value comparison.

[source,ruby]

bad

foo.objectid == bar.objectid

good

foo.equal?(bar)

Similarly, prefer using

Hash#compare_by_identity
than using
object_id
for keys:

[source,ruby]

bad

hash = {} hash[foo.objectid] = :bar if hash.key?(baz.objectid) # ...

good

hash = {}.comparebyidentity hash[foo] = :bar

if hash.key?(baz) # ...

Note that

Set
also has
Set#compare_by_identity
available.

=== Explicit Use of the Case Equality Operator [[no-case-equality]]

Avoid explicit use of the case equality operator

===
. As its name implies it is meant to be used implicitly by
case
expressions and outside of them it yields some pretty confusing code.

[source,ruby]

bad

Array === something (1..100) === 7 /something/ === some_string

good

something.is_a?(Array) (1..100).include?(7)

some_string.match?(/something/)

NOTE: With direct subclasses of

BasicObject
, using
is_a?
is not an option since
BasicObject
doesn't provide that method (it's defined in
Object
). In those rare cases it's OK to use
===
.

===

is_a?
vs
kind_of?
[[is-a-vs-kind-of]]

Prefer

is_a?
over
kind_of?
. The two methods are synonyms, but
is_a?
is the more commonly used name in the wild.

[source,ruby]

bad

something.kind_of?(Array)

good

something.is_a?(Array)

===

is_a?
vs
instance_of?
[[is-a-vs-instance-of]]

Prefer

is_a?
over
instance_of?
.

While the two methods are similar,

is_a?
will consider the whole inheritance chain (superclasses and included modules), which is what you normally would want to do.
instance_of?
, on the other hand, only returns
true
if an object is an instance of that exact class you're checking for, not a subclass.

[source,ruby]

bad

something.instance_of?(Array)

good

something.is_a?(Array)

===

==
vs
eql?
[[eql]]

Do not use

eql?
when using
==
will do. The stricter comparison semantics provided by
eql?
are rarely needed in practice.

[source,ruby]

bad - eql? is the same as == for strings

'ruby'.eql? some_str

good

'ruby' == some_str

1.0.eql? x # eql? makes sense here if want to differentiate between Integer and Float 1

== Blocks, Procs & Lambdas

=== Proc Application Shorthand [[single-action-blocks]]

Use the Proc invocation shorthand when the invoked method is the only operation of a block.

[source,ruby]

bad

names.map { |name| name.upcase }

good

names.map(&:upcase)

=== Single-line Blocks Delimiters [[single-line-blocks]]

Prefer

{...}
over
do...end
for single-line blocks. Avoid using
{...}
for multi-line blocks (multi-line chaining is always ugly). Always use
do...end
for "control flow" and "method definitions" (e.g. in Rakefiles and certain DSLs). Avoid
do...end
when chaining.

[source,ruby]

names = %w[Bozhidar Filipp Sarah]

bad

names.each do |name| puts name end

good

names.each { |name| puts name }

bad

names.select do |name| name.start_with?('S') end.map { |name| name.upcase }

good

names.select { |name| name.start_with?('S') }.map(&:upcase)

Some will argue that multi-line chaining would look OK with the use of {...}, but they should ask themselves - is this code really readable and can the blocks' contents be extracted into nifty methods?

=== Explicit Block Argument [[block-argument]]

Consider using explicit block argument to avoid writing block literal that just passes its arguments to another block.

[source,ruby]

require 'tempfile'

bad

def withtmpdir Dir.mktmpdir do |tmpdir| Dir.chdir(tmpdir) { |dir| yield dir } # block just passes arguments end end

good

def withtmpdir(&block) Dir.mktmpdir do |tmpdir| Dir.chdir(tmpdir, &block) end end

withtmpdir do |dir| puts "dir is accessible as a parameter and pwd is set: #{dir}"

end

=== Trailing Comma in Block Parameters [[no-trailing-parameters-comma]]

Avoid comma after the last parameter in a block, except in cases where only a single argument is present and its removal would affect functionality (for instance, array destructuring).

[source,ruby]

bad - easier to move/add/remove parameters, but still not preferred

[[1, 2, 3], [4, 5, 6]].each do |a, b, c,| a + b + c end

good

[[1, 2, 3], [4, 5, 6]].each do |a, b, c| a + b + c end

bad

[[1, 2, 3], [4, 5, 6]].each { |a, b, c,| a + b + c }

good

[[1, 2, 3], [4, 5, 6]].each { |a, b, c| a + b + c }

good - this comma is meaningful for array destructuring

[[1, 2, 3], [4, 5, 6]].map { |a,| a }

=== Nested Method Definitions [[no-nested-methods]]

Do not use nested method definitions, use lambda instead. Nested method definitions actually produce methods in the same scope (e.g. class) as the outer method. Furthermore, the "nested method" will be redefined every time the method containing its definition is invoked.

[source,ruby]

bad

def foo(x) def bar(y) # body omitted end

bar(x) end

good - the same as the previous, but no bar redefinition on every foo call

def bar(y) # body omitted end

def foo(x) bar(x) end

also good

def foo(x) bar = ->(y) { ... } bar.call(x)

end

=== Multi-line Lambda Definition [[lambda-multi-line]]

Use the new lambda literal syntax for single-line body blocks. Use the

lambda
method for multi-line blocks.

[source,ruby]

bad

l = lambda { |a, b| a + b } l.call(1, 2)

correct, but looks extremely awkward

l = ->(a, b) do tmp = a * 7 tmp * b / 50 end

good

l = ->(a, b) { a + b } l.call(1, 2)

l = lambda do |a, b| tmp = a * 7 tmp * b / 50

end

=== Stabby Lambda Definition with Parameters [[stabby-lambda-with-args]]

Don't omit the parameter parentheses when defining a stabby lambda with parameters.

[source,ruby]

bad

l = ->x, y { something(x, y) }

good

l = ->(x, y) { something(x, y) }

=== Stabby Lambda Definition without Parameters [[stabby-lambda-no-args]]

Omit the parameter parentheses when defining a stabby lambda with no parameters.

[source,ruby]

bad

l = ->() { something }

good

l = -> { something }

===

proc
vs
Proc.new
[[proc]]

Prefer

proc
over
Proc.new
.

[source,ruby]

bad

p = Proc.new { |n| puts n }

good

p = proc { |n| puts n }

=== Proc Invocation [[proc-call]]

Prefer

proc.call()
over
proc[]
or
proc.()
for both lambdas and procs.

[source,ruby]

bad - looks similar to Enumeration access

l = ->(v) { puts v } l[1]

good - most compact form, but might be confusing for newcomers to Ruby

l = ->(v) { puts v } l.(1)

good - a bit verbose, but crystal clear

l = ->(v) { puts v }

l.call(1)

== Methods

=== Short Methods [[short-methods]]

Avoid methods longer than 10 LOC (lines of code). Ideally, most methods will be shorter than 5 LOC. Empty lines do not contribute to the relevant LOC.

=== No Single-line Methods [[no-single-line-methods]]

Avoid single-line methods. Although they are somewhat popular in the wild, there are a few peculiarities about their definition syntax that make their use undesirable. At any rate - there should be no more than one expression in a single-line method.

[source,ruby]

bad

def toomuch; something; somethingelse; end

okish - notice that the first ; is required

def nobracesmethod; body end

okish - notice that the second ; is optional

def nobracesmethod; body; end

okish - valid syntax, but no ; makes it kind of hard to read

def some_method() body end

good

def some_method body

end

One exception to the rule are empty-body methods.

[source,ruby]

good

def no_op; end

=== Double Colons [[double-colons]]

Use

::
only to reference constants (this includes classes and modules) and constructors (like
Array()
or
Nokogiri::HTML()
). Do not use
::
for regular method invocation.

[source,ruby]

bad

SomeClass::somemethod someobject::some_method

good

SomeClass.somemethod someobject.somemethod SomeModule::SomeClass::SOMECONST

SomeModule::SomeClass()

=== Colon Method Definition [[colon-method-definition]]

Do not use

::
to define class methods.

[source,ruby]

bad

class Foo def self::some_method end end

good

class Foo def self.some_method end

end

=== Method Definition Parentheses [[method-parens]]

Use

def
with parentheses when there are parameters. Omit the parentheses when the method doesn't accept any parameters.

[source,ruby]

bad

def some_method() # body omitted end

good

def some_method # body omitted end

bad

def somemethodwith_parameters param1, param2 # body omitted end

good

def somemethodwith_parameters(param1, param2) # body omitted

end

=== Method Invocation Parentheses [[method-invocation-parens]]

Use parentheses around the arguments of method invocations, especially if the first argument begins with an open parenthesis

(
, as in
f((3 + 2) + 1)
.

[source,ruby]

bad

x = Math.sin y

good

x = Math.sin(y)

bad

array.delete e

good

array.delete(e)

bad

temperance = Person.new 'Temperance', 30

good

temperance = Person.new('Temperance', 30)

==== Method Calls with No Arguments [[method-invocation-parens-no-args]]

Always omit parentheses for method calls with no arguments.

[source,ruby]

bad

Kernel.exit!() 2.even?() fork() 'test'.upcase()

good

Kernel.exit! 2.even? fork

'test'.upcase

==== Methods That are Part of an Internal DSL [[method-invocation-parens-internal-dsl]]

Always omit parentheses for methods that are part of an internal DSL (e.g., Rake, Rails, RSpec):

[source,ruby]

bad

validates(:name, presence: true)

good

validates :name, presence: true

==== Methods That Have "keyword" Status in Ruby [[method-invocation-parens-keyword]]

Always omit parentheses for methods that have "keyword" status in Ruby.

NOTE: Unfortunately, it's not exactly clear which methods have "keyword" status. There is agreement that declarative methods have "keyword" status. However, there's less agreement on which non-declarative methods, if any, have "keyword" status.

===== Declarative Methods That Have "keyword" Status in Ruby [[method-invocation-parens-declarative-keyword]]

Always omit parentheses for declarative methods (a.k.a. DSL methods or macro methods) that have "keyword" status in Ruby (e.g., various

Module
instance methods):

[source,ruby]

class Person # bad attrreader(:name, :age) # good attrreader :name, :age

# body omitted

end

===== Non-Declarative Methods That Have "keyword" Status in Ruby [[method-invocation-parens-non-declarative-keyword]]

For non-declarative methods with "keyword" status (e.g., various

Kernel
instance methods), two styles are considered acceptable. By far the most popular style is to omit parentheses. Rationale: The code reads better, and method calls look more like keywords. A less-popular style, but still acceptable, is to include parentheses. Rationale: The methods have ordinary semantics, so why treat them differently, and it's easier to achieve a uniform style by not worrying about which methods have "keyword" status. Whichever one you pick, apply it consistently.

[source,ruby]

good (most popular)

puts temperance.age system 'ls' exit 1

also good (less popular)

puts(temperance.age) system('ls')

exit(1)

==== Using

super
with Arguments [[super-with-args]]

Always use parentheses when calling

super
with arguments:

[source,ruby]

bad

super name, age

good

super(name, age)

IMPORTANT: When calling

super
without arguments,
super
and
super()
mean different things. Decide what is appropriate for your usage.

=== Too Many Params [[too-many-params]]

Avoid parameter lists longer than three or four parameters.

=== Optional Arguments [[optional-arguments]]

Define optional arguments at the end of the list of arguments. Ruby has some unexpected results when calling methods that have optional arguments at the front of the list.

[source,ruby]

bad

def some_method(a = 1, b = 2, c, d) puts "#{a}, #{b}, #{c}, #{d}" end

somemethod('w', 'x') # => '1, 2, w, x' somemethod('w', 'x', 'y') # => 'w, 2, x, y' some_method('w', 'x', 'y', 'z') # => 'w, x, y, z'

good

def some_method(c, d, a = 1, b = 2) puts "#{a}, #{b}, #{c}, #{d}" end

somemethod('w', 'x') # => '1, 2, w, x' somemethod('w', 'x', 'y') # => 'y, 2, w, x'

some_method('w', 'x', 'y', 'z') # => 'y, z, w, x'

=== Boolean Keyword Arguments [[boolean-keyword-arguments]]

Use keyword arguments when passing boolean argument to a method.

[source,ruby]

bad

def some_method(bar = false) puts bar end

bad - common hack before keyword args were introduced

def some_method(options = {}) bar = options.fetch(:bar, false) puts bar end

good

def some_method(bar: false) puts bar end

some_method # => false

some_method(bar: true) # => true

=== Keyword Arguments vs Optional Arguments [[keyword-arguments-vs-optional-arguments]]

Prefer keyword arguments over optional arguments.

[source,ruby]

bad

def some_method(a, b = 5, c = 1) # body omitted end

good

def some_method(a, b: 5, c: 1) # body omitted

end

=== Keyword Arguments vs Option Hashes [[keyword-arguments-vs-option-hashes]]

Use keyword arguments instead of option hashes.

[source,ruby]

bad

def some_method(options = {}) bar = options.fetch(:bar, false) puts bar end

good

def some_method(bar: false) puts bar

end

=== Private Global Methods [[private-global-methods]]

If you really need "global" methods, add them to Kernel and make them private.

== Classes & Modules

=== Consistent Classes [[consistent-classes]]

Use a consistent structure in your class definitions.

[source,ruby]

class Person # extend and include go first extend SomeModule include AnotherModule

# inner classes CustomError = Class.new(StandardError)

# constants are next SOME_CONSTANT = 20

# afterwards we have attribute macros attr_reader :name

# followed by other macros (if any) validates :name

# public class methods are next in line def self.some_method end

# initialization goes between class methods and other instance methods def initialize end

# followed by other public instance methods def some_method end

# protected and private methods are grouped near the end protected

def someprotectedmethod end

private

def someprivatemethod end

end

=== Mixin Grouping [[mixin-grouping]]

Split multiple mixins into separate statements.

[source,ruby]

bad

class Person include Foo, Bar end

good

class Person # multiple mixins go in separate statements include Foo include Bar

end

=== Single-line Classes [[single-line-classes]]

Prefer a two-line format for class definitions with no body. It is easiest to read, understand, and modify.

[source,ruby]

bad

FooError = Class.new(StandardError)

okish

class FooError < StandardError; end

ok

class FooError < StandardError

end

NOTE: Many editors/tools will fail to understand properly the usage of

Class.new
. Someone trying to locate the class definition might try a grep "class FooError". A final difference is that the name of your class is not available to the
inherited
callback of the base class with the
Class.new
form. In general it's better to stick to the basic two-line style.

=== File Classes [[file-classes]]

Don't nest multi-line classes within classes. Try to have such nested classes each in their own file in a folder named like the containing class.

[source,ruby]

bad

foo.rb

class Foo class Bar # 30 methods inside end

class Car # 20 methods inside end

# 30 methods inside end

good

foo.rb

class Foo # 30 methods inside end

foo/bar.rb

class Foo class Bar # 30 methods inside end end

foo/car.rb

class Foo class Car # 20 methods inside end

end

=== Namespace Definition [[namespace-definition]]

Define (and reopen) namespaced classes and modules using explicit nesting. Using the scope resolution operator can lead to surprising constant lookups due to Ruby's https://cirw.in/blog/constant-lookup.html[lexical scoping], which depends on the module nesting at the point of definition.

[source,ruby]

module Utilities class Queue end end

bad

class Utilities::Store Module.nesting # => [Utilities::Store]

def initialize # Refers to the top level ::Queue class because Utilities isn't in the # current nesting chain. @queue = Queue.new end end

good

module Utilities class WaitingList Module.nesting # => [Utilities::WaitingList, Utilities]

def initialize
  @queue = Queue.new # Refers to Utilities::Queue
end

end

end

=== Modules vs Classes [[modules-vs-classes]]

Prefer modules to classes with only class methods. Classes should be used only when it makes sense to create instances out of them.

[source,ruby]

bad

class SomeClass def self.some_method # body omitted end

def self.someothermethod # body omitted end end

good

module SomeModule module_function

def some_method # body omitted end

def someothermethod # body omitted end

end

===

module_function
[[module-function]]

Prefer the use of

module_function
over
extend self
when you want to turn a module's instance methods into class methods.

[source,ruby]

bad

module Utilities extend self

def parse_something(string) # do stuff here end

def otherutilitymethod(number, string) # do some more stuff end end

good

module Utilities module_function

def parse_something(string) # do stuff here end

def otherutilitymethod(number, string) # do some more stuff end

end

=== Liskov [[liskov]]

When designing class hierarchies make sure that they conform to the https://en.wikipedia.org/wiki/Liskovsubstitutionprinciple[Liskov Substitution Principle].

=== SOLID design [[solid-design]]

Try to make your classes as https://en.wikipedia.org/wiki/SOLID[SOLID] as possible.

=== Define

to_s
[[define-to-s]]

Always supply a proper

to_s
method for classes that represent domain objects.

[source,ruby]

class Person attrreader :firstname, :last_name

def initialize(firstname, lastname) @firstname = firstname @lastname = lastname end

def tos "#{firstname} #{last_name}" end

end

===

attr
Family [[attr_family]]

Use the

attr
family of functions to define trivial accessors or mutators.

[source,ruby]

bad

class Person def initialize(firstname, lastname) @firstname = firstname @lastname = lastname end

def firstname @firstname end

def lastname @lastname end end

good

class Person attrreader :firstname, :last_name

def initialize(firstname, lastname) @firstname = firstname @lastname = lastname end

end

=== Accessor/Mutator Method Names [[accessormutatormethod_names]]

For accessors and mutators, avoid prefixing method names with

get_
and
set_
. It is a Ruby convention to use attribute names for accessors (readers) and
attr_name=
for mutators (writers).

[source,ruby]

bad

class Person def getname "#{@firstname} #{@last_name}" end

def setname(name) @firstname, @last_name = name.split(' ') end end

good

class Person def name "#{@firstname} #{@lastname}" end

def name=(name) @firstname, @lastname = name.split(' ') end

end

===

attr
[[attr]]

Avoid the use of

attr
. Use
attr_reader
and
attr_accessor
instead.

[source,ruby]

bad - creates a single attribute accessor (deprecated in Ruby 1.9)

attr :something, true attr :one, :two, :three # behaves as attr_reader

good

attr_accessor :something

attr_reader :one, :two, :three

===

Struct.new
[[struct-new]]

Consider using

Struct.new
, which defines the trivial accessors, constructor and comparison operators for you.

[source,ruby]

good

class Person attraccessor :firstname, :last_name

def initialize(firstname, lastname) @firstname = firstname @lastname = lastname end end

better

Person = Struct.new(:firstname, :lastname) do

end

=== No Extend

Struct.new
[[no-extend-struct-new]]

Don't extend an instance initialized by

Struct.new
. Extending it introduces a superfluous class level and may also introduce weird errors if the file is required multiple times.

[source,ruby]

bad

class Person < Struct.new(:firstname, :lastname) end

good

Person = Struct.new(:firstname, :lastname)

=== Duck Typing [[duck-typing]]

Prefer https://en.wikipedia.org/wiki/Duck_typing[duck-typing] over inheritance.

[source,ruby]

bad

class Animal # abstract method def speak end end

extend superclass

class Duck < Animal def speak puts 'Quack! Quack' end end

extend superclass

class Dog < Animal def speak puts 'Bau! Bau!' end end

good

class Duck def speak puts 'Quack! Quack' end end

class Dog def speak puts 'Bau! Bau!' end

end

=== No Class Vars [[no-class-vars]]

Avoid the usage of class (

@@
) variables due to their "nasty" behavior in inheritance.

[source,ruby]

class Parent @@class_var = 'parent'

def self.printclassvar puts @@class_var end end

class Child < Parent @@class_var = 'child' end

Parent.printclassvar # => will print 'child'

As you can see all the classes in a class hierarchy actually share one class variable. Class instance variables should usually be preferred over class variables.

=== Leverage Access Modifiers (e.g.

private
and
protected
) [[visibility]]

Assign proper visibility levels to methods (

private
,
protected
) in accordance with their intended usage. Don't go off leaving everything
public
(which is the default). After all we're coding in Ruby now, not in Python.

=== Access Modifiers Indentation [[indent-public-private-protected]]

Indent the

public
,
protected
, and
private
methods as much as the method definitions they apply to. Leave one blank line above the visibility modifier and one blank line below in order to emphasize that it applies to all methods below it.

[source,ruby]

good

class SomeClass def public_method # some code end

private

def private_method # some code end

def anotherprivatemethod # some code end

end

=== Defining Class Methods [[def-self-class-methods]]

Use

def self.method
to define class methods. This makes the code easier to refactor since the class name is not repeated.

[source,ruby]

class TestClass # bad def TestClass.some_method # body omitted end

# good def self.someothermethod # body omitted end

# Also possible and convenient when you # have to define many class methods. class << self def first_method # body omitted end

def second_method_etc
  # body omitted
end

end

end

=== Alias Method Lexically [[alias-method-lexically]]

Prefer

alias
when aliasing methods in lexical class scope as the resolution of
self
in this context is also lexical, and it communicates clearly to the user that the indirection of your alias will not be altered at runtime or by any subclass unless made explicit.

[source,ruby]

class Westerner def first_name @names.first end

alias givenname firstname

end

Since

alias
, like
def
, is a keyword, prefer bareword arguments over symbols or strings. In other words, do
alias foo bar
, not
alias :foo :bar
.

Also be aware of how Ruby handles aliases and inheritance: an alias references the method that was resolved at the time the alias was defined; it is not dispatched dynamically.

[source,ruby]

class Fugitive < Westerner def first_name 'Nobody' end

end

In this example,

Fugitive#given_name
would still call the original
Westerner#first_name
method, not
Fugitive#first_name
. To override the behavior of
Fugitive#given_name
as well, you'd have to redefine it in the derived class.

[source,ruby]

class Fugitive < Westerner def first_name 'Nobody' end

alias givenname firstname

end

===

alias_method
[[alias-method]]

Always use

alias_method
when aliasing methods of modules, classes, or singleton classes at runtime, as the lexical scope of
alias
leads to unpredictability in these cases.

[source,ruby]

module Mononymous def self.included(other) other.classeval { aliasmethod :fullname, :givenname } end end

class Sting < Westerner include Mononymous

end

=== Class and

self
[[class-and-self]]

When class (or module) methods call other such methods, omit the use of a leading

self
or own name followed by a
.
when calling other such methods. This is often seen in "service classes" or other similar concepts where a class is treated as though it were a function. This convention tends to reduce repetitive boilerplate in such classes.

[source,ruby]

class TestClass # bad -- more work when class renamed/method moved def self.call(param1, param2) TestClass.new(param1).call(param2) end

# bad -- more verbose than necessary def self.call(param1, param2) self.new(param1).call(param2) end

# good def self.call(param1, param2) new(param1).call(param2) end

# ...other methods...

end

=== Defining Constants within a Block [[no-constant-definition-in-block]]

Do not define constants within a block, since the block's scope does not isolate or namespace the constant in any way.

Define the constant outside of the block instead, or use a variable or method if defining the constant in the outer scope would be problematic.

[source,ruby]

bad - FILESTOLINT is now defined globally

task :lint do FILESTOLINT = Dir['lib/*.rb'] # ... end

good - filestolint is only defined inside the block

task :lint do filestolint = Dir['lib/*.rb'] # ...

end

== Classes: Constructors

=== Factory Methods [[factory-methods]]

Consider adding factory methods to provide additional sensible ways to create instances of a particular class.

[source,ruby]

class Person def self.create(options_hash) # body omitted end

end

=== Disjunctive Assignment in Constructor [[disjunctive-assignment-in-constructor]]

In constructors, avoid unnecessary disjunctive assignment (

||=
) of instance variables. Prefer plain assignment. In ruby, instance variables (beginning with an
@
) are nil until assigned a value, so in most cases the disjunction is unnecessary.

[source,ruby]

bad

def initialize @x ||= 1 end

good

def initialize @x = 1

end

== Comments

[quote, Steve McConnell]


Good code is its own best documentation. As you're about to add a comment, ask yourself, "How can I improve the code so that this comment isn't needed?". Improve the code and then document it to make it even clearer.


=== No Comments [[no-comments]]

Write self-documenting code and ignore the rest of this section. Seriously!

=== Rationale Comments [[rationale-comments]]

If the how can be made self-documenting, but not the why (e.g. the code works around non-obvious library behavior, or implements an algorithm from an academic paper), add a comment explaining the rationale behind the code.

[source,ruby]

bad

x = BuggyClass.something.dup

def computedependencygraph ...30 lines of recursive graph merging... end

good

BuggyClass returns an internal object, so we have to dup it to modify it.

x = BuggyClass.something.dup

This is algorithm 6.4(a) from Worf & Yar's Amazing Graph Algorithms (2243).

def computedependencygraph ...30 lines of recursive graph merging...

end

=== English Comments [[english-comments]]

Write comments in English.

=== Hash Space [[hash-space]]

Use one space between the leading

#
character of the comment and the text of the comment.

=== English Syntax [[english-syntax]]

Comments longer than a word are capitalized and use punctuation. Use https://en.wikipedia.org/wiki/Sentence_spacing[one space] after periods.

=== No Superfluous Comments [[no-superfluous-comments]]

Avoid superfluous comments.

[source,ruby]

bad

counter += 1 # Increments counter by one.

=== Comment Upkeep [[comment-upkeep]]

Keep existing comments up-to-date. An outdated comment is worse than no comment at all.

=== Refactor, Don't Comment [[refactor-dont-comment]]

[quote, old programmers maxim, 'http://eloquentruby.com/blog/2011/03/07/good-code-and-good-jokes/[through Russ Olsen]']


Good code is like a good joke: it needs no explanation.


Avoid writing comments to explain bad code. Refactor the code to make it self-explanatory. ("Do or do not - there is no try." Yoda)

== Comment Annotations

=== Annotations Placement [[annotate-above]]

Annotations should usually be written on the line immediately above the relevant code.

[source,ruby]

bad

def bar baz(:quux) # FIXME: This has crashed occasionally since v3.2.1. end

good

def bar # FIXME: This has crashed occasionally since v3.2.1. baz(:quux)

end

=== Annotations Keyword Format [[annotate-keywords]]

The annotation keyword is followed by a colon and a space, then a note describing the problem.

[source,ruby]

bad

def bar # FIXME This has crashed occasionally since v3.2.1. baz(:quux) end

good

def bar # FIXME: This has crashed occasionally since v3.2.1. baz(:quux)

end

=== Multi-line Annotations Indentation [[indent-annotations]]

If multiple lines are required to describe the problem, subsequent lines should be indented three spaces after the

#
(one general plus two for indentation purpose).

[source,ruby]

def bar # FIXME: This has crashed occasionally since v3.2.1. It may # be related to the BarBazUtil upgrade. baz(:quux)

end

=== Inline Annotations [[rare-eol-annotations]]

In cases where the problem is so obvious that any documentation would be redundant, annotations may be left at the end of the offending line with no note. This usage should be the exception and not the rule.

[source,ruby]

def bar sleep 100 # OPTIMIZE

end

===

TODO
[[todo]]

Use

TODO
to note missing features or functionality that should be added at a later date.

===

FIXME
[[fixme]]

Use

FIXME
to note broken code that needs to be fixed.

===

OPTIMIZE
[[optimize]]

Use

OPTIMIZE
to note slow or inefficient code that may cause performance problems.

===

HACK
[[hack]]

Use

HACK
to note code smells where questionable coding practices were used and should be refactored away.

===

REVIEW
[[review]]

Use

REVIEW
to note anything that should be looked at to confirm it is working as intended. For example:
REVIEW: Are we sure this is how the client does X currently?

=== Document Annotations [[document-annotations]]

Use other custom annotation keywords if it feels appropriate, but be sure to document them in your project's

README
or similar.

== Magic Comments

=== Magic Comments First [[magic-comments-first]]

Place magic comments above all code and documentation in a file (except shebangs, which are discussed next).

[source,ruby]

bad

Some documentation about Person

frozenstringliteral: true

class Person end

good

frozenstringliteral: true

Some documentation about Person

class Person

end

=== Below Shebang [[below-shebang]]

Place magic comments below shebangs when they are present in a file.

[source,ruby]

bad

frozenstringliteral: true

!/usr/bin/env ruby

App.parse(ARGV)

good

!/usr/bin/env ruby

frozenstringliteral: true

App.parse(ARGV)

=== One Magic Comment per Line [[one-magic-comment-per-line]]

Use one magic comment per line if you need multiple.

[source,ruby]

bad

-- frozenstringliteral: true; encoding: ascii-8bit --

good

frozenstringliteral: true

encoding: ascii-8bit


=== Separate Magic Comments from Code [[separate-magic-comments-from-code]]

Separate magic comments from code and documentation with a blank line.

[source,ruby]

bad

frozenstringliteral: true

Some documentation for Person

class Person # Some code end

good

frozenstringliteral: true

Some documentation for Person

class Person # Some code

end

== Collections

=== Literal Array and Hash [[literal-array-hash]]

Prefer literal array and hash creation notation (unless you need to pass parameters to their constructors, that is).

[source,ruby]

bad

arr = Array.new hash = Hash.new

good

arr = [] arr = Array.new(10) hash = {}

hash = Hash.new(0)

===

%w
[[percent-w]]

Prefer

%w
to the literal array syntax when you need an array of words (non-empty strings without spaces and special characters in them). Apply this rule only to arrays with two or more elements.

[source,ruby]

bad

STATES = ['draft', 'open', 'closed']

good

STATES = %w[draft open closed]

===

%i
[[percent-i]]

Prefer

%i
to the literal array syntax when you need an array of symbols (and you don't need to maintain Ruby 1.9 compatibility). Apply this rule only to arrays with two or more elements.

[source,ruby]

bad

STATES = [:draft, :open, :closed]

good

STATES = %i[draft open closed]

=== No Trailing Array Commas [[no-trailing-array-commas]]

Avoid comma after the last item of an

Array
or
Hash
literal, especially when the items are not on separate lines.

[source,ruby]

bad - easier to move/add/remove items, but still not preferred

VALUES = [ 1001, 2020, 3333, ]

bad

VALUES = [1001, 2020, 3333, ]

good

VALUES = [1001, 2020, 3333]

=== No Gappy Arrays [[no-gappy-arrays]]

Avoid the creation of huge gaps in arrays.

[source,ruby]

arr = []

arr[100] = 1 # now you have an array with lots of nils

===

first
and
last
[[first-and-last]]

When accessing the first or last element from an array, prefer

first
or
last
over
[0]
or
[-1]
.

=== Set vs Array [[set-vs-array]]

Use

Set
instead of
Array
when dealing with unique elements.
Set
implements a collection of unordered values with no duplicates. This is a hybrid of
Array
's intuitive inter-operation facilities and
Hash
's fast lookup.

=== Symbols as Keys [[symbols-as-keys]]

Prefer symbols instead of strings as hash keys.

[source,ruby]

bad

hash = { 'one' => 1, 'two' => 2, 'three' => 3 }

good

hash = { one: 1, two: 2, three: 3 }

=== No Mutable Keys [[no-mutable-keys]]

Avoid the use of mutable objects as hash keys.

=== Hash Literals [[hash-literals]]

Use the Ruby 1.9 hash literal syntax when your hash keys are symbols.

[source,ruby]

bad

hash = { :one => 1, :two => 2, :three => 3 }

good

hash = { one: 1, two: 2, three: 3 }

=== Hash Literal as Last Array Item [[hash-literal-as-last-array-item]]

Wrap hash literal in braces if it is a last array item.

[source,ruby]

bad

[1, 2, one: 1, two: 2]

good

[1, 2, { one: 1, two: 2 }]

=== No Mixed Hash Syntaxes [[no-mixed-hash-syntaxes]]

Don't mix the Ruby 1.9 hash syntax with hash rockets in the same hash literal. When you've got keys that are not symbols stick to the hash rockets syntax.

[source,ruby]

bad

{ a: 1, 'b' => 2 }

good

{ :a => 1, 'b' => 2 }

===

Hash#key?
[[hash-key]]

Use

Hash#key?
instead of
Hash#has_key?
and
Hash#value?
instead of
Hash#has_value?
.

[source,ruby]

bad

hash.haskey?(:test) hash.hasvalue?(value)

good

hash.key?(:test)

hash.value?(value)

===

Hash#each
[[hash-each]]

Use

Hash#each_key
instead of
Hash#keys.each
and
Hash#each_value
instead of
Hash#values.each
.

[source,ruby]

bad

hash.keys.each { |k| p k } hash.values.each { |v| p v } hash.each { |k, v| p k } hash.each { |k, v| p v }

good

hash.each_key { |k| p k }

hash.each_value { |v| p v }

===

Hash#fetch
[[hash-fetch]]

Use

Hash#fetch
when dealing with hash keys that should be present.

[source,ruby]

heroes = { batman: 'Bruce Wayne', superman: 'Clark Kent' }

bad - if we make a mistake we might not spot it right away

heroes[:batman] # => 'Bruce Wayne' heroes[:supermann] # => nil

good - fetch raises a KeyError making the problem obvious

heroes.fetch(:supermann)

===

Hash#fetch
defaults [[hash-fetch-defaults]]

Introduce default values for hash keys via

Hash#fetch
as opposed to using custom logic.

[source,ruby]

batman = { name: 'Bruce Wayne', is_evil: false }

bad - if we just use || operator with falsy value we won't get the expected result

batman[:is_evil] || true # => true

good - fetch works correctly with falsy values

batman.fetch(:is_evil, true) # => false

=== Use Hash Blocks [[use-hash-blocks]]

Prefer the use of the block instead of the default value in

Hash#fetch
if the code that has to be evaluated may have side effects or be expensive.

[source,ruby]

batman = { name: 'Bruce Wayne' }

bad - if we use the default value, we eager evaluate it

so it can slow the program down if done multiple times

batman.fetch(:powers, obtainbatmanpowers) # obtainbatmanpowers is an expensive call

good - blocks are lazy evaluated, so only triggered in case of KeyError exception

batman.fetch(:powers) { obtainbatmanpowers }

===

Hash#values_at
[[hash-values-at]]

Use

Hash#values_at
when you need to retrieve several values consecutively from a hash.

[source,ruby]

bad

email = data['email'] username = data['nickname']

good

email, username = data.values_at('email', 'nickname')

===

Hash#transform_keys
and
Hash#transform_values
[[hash-transform-methods]]

Prefer

transform_keys
or
transform_values
over
each_with_object
or
map
when transforming just the keys or just the values of a hash.

[source,ruby]

bad

{a: 1, b: 2}.eachwithobject({}) { |(k, v), h| h[k] = v * v } {a: 1, b: 2}.map { |k, v| [k.tos, v] }.toh

good

{a: 1, b: 2}.transform_values { |v| v * v }

{a: 1, b: 2}.transformkeys { |k| k.tos }

=== Ordered Hashes [[ordered-hashes]]

Rely on the fact that as of Ruby 1.9 hashes are ordered.

=== No Modifying Collections [[no-modifying-collections]]

Do not modify a collection while traversing it.

=== Accessing Elements Directly [[accessing-elements-directly]]

When accessing elements of a collection, avoid direct access via

[n]
by using an alternate form of the reader method if it is supplied. This guards you from calling
[]
on
nil
.

[source,ruby]

bad

Regexp.last_match[1]

good

Regexp.last_match(1)

=== Provide Alternate Accessor to Collections [[provide-alternate-accessor-to-collections]]

When providing an accessor for a collection, provide an alternate form to save users from checking for

nil
before accessing an element in the collection.

[source,ruby]

bad

def awesomethings @awesomethings end

good

def awesomethings(index = nil) if index && @awesomethings @awesomethings[index] else @awesomethings end

end

===

map
/
find
/
select
/
reduce
/
include?
/
size
[[map-find-select-reduce-include-size]]

Prefer

map
over
collect
,
find
over
detect
,
select
over
find_all
,
reduce
over
inject
,
include?
over
member?
and
size
over
length
. This is not a hard requirement; if the use of the alias enhances readability, it's ok to use it. The rhyming methods are inherited from Smalltalk and are not common in other programming languages. The reason the use of
select
is encouraged over
find_all
is that it goes together nicely with
reject
and its name is pretty self-explanatory.

===

count
vs
size
[[count-vs-size]]

Don't use

count
as a substitute for
size
. For
Enumerable
objects other than
Array
it will iterate the entire collection in order to determine its size.

[source,ruby]

bad

some_hash.count

good

some_hash.size

===

flat_map
[[flat-map]]

Use

flat_map
instead of
map
+
flatten
. This does not apply for arrays with a depth greater than 2, i.e. if
users.first.songs == ['a', ['b','c']]
, then use
map + flatten
rather than
flat_map
.
flat_map
flattens the array by 1, whereas
flatten
flattens it all the way.

[source,ruby]

bad

all_songs = users.map(&:songs).flatten.uniq

good

allsongs = users.flatmap(&:songs).uniq

===

reverse_each
[[reverse-each]]

Prefer

reverse_each
to
reverse.each
because some classes that
include Enumerable
will provide an efficient implementation. Even in the worst case where a class does not provide a specialized implementation, the general implementation inherited from
Enumerable
will be at least as efficient as using
reverse.each
.

[source,ruby]

bad

array.reverse.each { ... }

good

array.reverse_each { ... }

== Numbers

=== Underscores in Numerics [[underscores-in-numerics]]

Add underscores to large numeric literals to improve their readability.

[source,ruby]

bad - how many 0s are there?

num = 1000000

good - much easier to parse for the human brain

num = 1000000

=== Numeric Literal Prefixes [[numeric-literal-prefixes]]

Prefer lowercase letters for numeric literal prefixes.

0o
for octal,
0x
for hexadecimal and
0b
for binary. Do not use
0d
prefix for decimal literals.

[source,ruby]

bad

num = 01234 num = 0O1234 num = 0X12AB num = 0B10101 num = 0D1234 num = 0d1234

good - easier to separate digits from the prefix

num = 0o1234 num = 0x12AB num = 0b10101

num = 1234

=== Integer Type Checking [[integer-type-checking]]

Use

Integer
to check type of an integer number. Since
Fixnum
is platform-dependent, checking against it will return different results on 32-bit and 64-bit machines.

[source,ruby]

timestamp = Time.now.to_i

bad

timestamp.isa?(Fixnum) timestamp.isa?(Bignum)

good

timestamp.is_a?(Integer)

=== Random Numbers [[random-numbers]]

Prefer to use ranges when generating random numbers instead of integers with offsets, since it clearly states your intentions. Imagine simulating a roll of a dice:

[source,ruby]

bad

rand(6) + 1

good

rand(1..6)

=== Float Division [[float-division]]

When performing float-division on two integers, either use

fdiv
or convert one-side integer to float.

[source,ruby]

bad

a.tof / b.tof

good

a.tof / b a / b.tof

a.fdiv(b)

=== Float Comparison [[float-comparison]]

Avoid (in)equality comparisons of floats as they are unreliable.

Floating point values are inherently inaccurate, and comparing them for exact equality is almost never the desired semantics. Comparison via the

==/!=
operators checks floating-point value representation to be exactly the same, which is very unlikely if you perform any arithmetic operations involving precision loss.

[source,ruby]

bad

x == 0.1 x != 0.1

good - using BigDecimal

x.tod == 0.1.tod

good - not an actual float comparison

x == Float::INFINITY

good

(x - 0.1).abs < Float::EPSILON

good

tolerance = 0.0001 (x - 0.1).abs < tolerance

Or some other epsilon based type of comparison:

https://www.embeddeduse.com/2019/08/26/qt-compare-two-floats/


=== Exponential Notation [[exponential-notation]]

When using exponential notation for numbers, prefer using the normalized scientific notation, which uses a mantissa between 1 (inclusive) and 10 (exclusive). Omit the exponent altogether if it is zero.

The goal is to avoid confusion between powers of ten and exponential notation, as one quickly reading

10e7
could think it's 10 to the power of 7 (one then 7 zeroes) when it's actually 10 to the power of 8 (one then 8 zeroes). If you want 10 to the power of 7, you should do
1e7
.

| power notation | exponential notation | output | |----------------|----------------------|----------| | 10 ** 7 | 1e7 | 10000000 | | 10 ** 6 | 1e6 | 1000000 | | 10 ** 7 | 10e6 | 10000000 |

One could favor the alternative engineering notation, in which the exponent must always be a multiple of 3 for easy conversion to the thousand / million / ... system.

[source,ruby]

bad

10e6 0.3e4 11.7e5 3.14e0

good

1e7 3e3 1.17e6

3.14

Alternative : engineering notation:

[source,ruby]

bad

3.2e7 0.1e5 12e4

good

1e6 17e6

0.98e9

== Strings

=== String Interpolation [[string-interpolation]]

Prefer string interpolation and string formatting to string concatenation:

[source,ruby]

bad

emailwithname = user.name + ' <' + user.email + '>'

good

emailwithname = "#{user.name} <#{user.email}>"

good

emailwithname = format('%s <%s>', user.name, user.email)

=== Consistent String Literals [[consistent-string-literals]]

Adopt a consistent string literal quoting style. There are two popular styles in the Ruby community, both of which are considered good - single quotes by default and double quotes by default.

NOTE: The string literals in this guide are using single quotes by default.

==== Single Quote [[consistent-string-literals-single-quote]]

Prefer single-quoted strings when you don't need string interpolation or special symbols such as

\t
,
\n
,
'
, etc.

[source,ruby]

bad

name = "Bozhidar"

name = 'De\'Andre'

good

name = 'Bozhidar'

name = "De'Andre"

==== Double Quote [[consistent-string-literals-double-quote]]

Prefer double-quotes unless your string literal contains " or escape characters you want to suppress.

[source,ruby]

bad

name = 'Bozhidar'

sarcasm = "I \"like\" it."

good

name = "Bozhidar"

sarcasm = 'I "like" it.'

=== No Character Literals [[no-character-literals]]

Don't use the character literal syntax

?x
. Since Ruby 1.9 it's basically redundant -
?x
would be interpreted as
'x'
(a string with a single character in it).

[source,ruby]

bad

char = ?c

good

char = 'c'

=== Curlies Interpolate [[curlies-interpolate]]

Don't leave out

{}
around instance and global variables being interpolated into a string.

[source,ruby]

class Person attrreader :firstname, :last_name

def initialize(firstname, lastname) @firstname = firstname @lastname = lastname end

# bad - valid, but awkward def tos "#@firstname #@last_name" end

# good def tos "#{@firstname} #{@last_name}" end end

$global = 0

bad

puts "$global = #$global"

good

puts "$global = #{$global}"

=== No

to_s
[[no-to-s]]

Don't use

Object#to_s
on interpolated objects. It's invoked on them automatically.

[source,ruby]

bad

message = "This is the #{result.to_s}."

good

message = "This is the #{result}."

=== String Concatenation [[concat-strings]]

Avoid using

String#+
when you need to construct large data chunks. Instead, use
String#<<
. Concatenation mutates the string instance in-place and is always faster than
String#+
, which creates a bunch of new string objects.

[source,ruby]

bad

html = '' html += '

Page title

'

paragraphs.each do |paragraph| html += "

#{paragraph}

" end

good and also fast

html = '' html << '

Page title

'

paragraphs.each do |paragraph| html << "

#{paragraph}

"

end

=== Don't Abuse

gsub
[[dont-abuse-gsub]]

Don't use

String#gsub
in scenarios in which you can use a faster and more specialized alternative.

[source,ruby]

url = 'http://example.com' str = 'lisp-case-rules'

bad

url.gsub('http://', 'https://') str.gsub('-', '_')

good

url.sub('http://', 'https://')

str.tr('-', '_')

===

sprintf
[[sprintf]]

Prefer the use of

sprintf
and its alias
format
over the fairly cryptic
String#%
method.

[source,ruby]

bad

'%d %d' % [20, 10]

=> '20 10'

good

sprintf('%d %d', 20, 10)

=> '20 10'

good

sprintf('%d %d', first: 20, second: 10)

=> '20 10'

format('%d %d', 20, 10)

=> '20 10'

good

format('%d %d', first: 20, second: 10)

=> '20 10'


=== Named Format Tokens [[named-format-tokens]]

When using named format string tokens, favor

%s
over
%{name}
because it encodes information about the type of the value.

[source,ruby]

bad

format('Hello, %{name}', name: 'John')

good

format('Hello, %s', name: 'John')

=== Long Strings [[heredoc-long-strings]]

Break long strings into multiple lines but don't concatenate them with

+
. If you want to add newlines, use heredoc. Otherwise use
\
:

[source,ruby]

bad

"Lorem Ipsum is simply dummy text of the printing and typesetting industry. " + "Lorem Ipsum has been the industry's standard dummy text ever since the 1500s, " + "when an unknown printer took a galley of type and scrambled it to make a type specimen book."

good

<<~LOREM Lorem Ipsum is simply dummy text of the printing and typesetting industry. Lorem Ipsum has been the industry's standard dummy text ever since the 1500s, when an unknown printer took a galley of type and scrambled it to make a type specimen book. LOREM

good

"Lorem Ipsum is simply dummy text of the printing and typesetting industry. "\ "Lorem Ipsum has been the industry's standard dummy text ever since the 1500s, "\

"when an unknown printer took a galley of type and scrambled it to make a type specimen book."

== Heredocs

=== Squiggly Heredocs [[squiggly-heredocs]]

Use Ruby 2.3's squiggly heredocs for nicely indented multi-line strings.

[source,ruby]

bad - using Powerpack String#strip_margin

code = <<-RUBY.stripmargin('|') |def test | somemethod | other_method |end RUBY

also bad

code = <<-RUBY def test somemethod othermethod end RUBY

good

code = <<~RUBY def test somemethod othermethod end

RUBY

=== Heredoc Delimiters [[heredoc-delimiters]]

Use descriptive delimiters for heredocs. Delimiters add valuable information about the heredoc content, and as an added bonus some editors can highlight code within heredocs if the correct delimiter is used.

[source,ruby]

bad

code = <<~END def foo bar end END

good

code = <<~RUBY def foo bar end RUBY

good

code = <<~SUMMARY An imposing black structure provides a connection between the past and the future in this enigmatic adaptation of a short story by revered sci-fi author Arthur C. Clarke.

SUMMARY

=== Heredoc Method Calls [[heredoc-method-calls]]

Place method calls with heredoc receivers on the first line of the heredoc definition. The bad form has significant potential for error if a new line is added or removed.

[source,ruby]

bad

query = <<~SQL select foo from bar SQL .strip_indent

good

query = <<~SQL.strip_indent select foo from bar

SQL

=== Heredoc Argument Closing Parentheses [[heredoc-argument-closing-parentheses]]

Place the closing parenthesis for method calls with heredoc arguments on the first line of the heredoc definition. The bad form has potential for error if the new line before the closing parenthesis is removed.

[source,ruby]

bad

foo(<<~SQL select foo from bar SQL )

good

foo(<<~SQL) select foo from bar

SQL

== Date & Time

===

Time.now
[[time-now]]

Prefer

Time.now
over
Time.new
when retrieving the current system time.

=== No

DateTime
[[no-datetime]]

Don't use

DateTime
unless you need to account for historical calendar reform - and if you do, explicitly specify the
start
argument to clearly state your intentions.

[source,ruby]

bad - uses DateTime for current time

DateTime.now

good - uses Time for current time

Time.now

bad - uses DateTime for modern date

DateTime.iso8601('2016-06-29')

good - uses Date for modern date

Date.iso8601('2016-06-29')

good - uses DateTime with start argument for historical date

DateTime.iso8601('1751-04-23', Date::ENGLAND)

== Regular Expressions

[quote, Jamie Zawinski]


Some people, when confronted with a problem, think "I know, I'll use regular expressions." Now they have two problems.


=== Plain Text Search [[no-regexp-for-plaintext]]

Don't use regular expressions if you just need plain text search in string.

[source,ruby]

foo = 'I am an example string'

bad - using a regular expression is an overkill here

foo ~= /example/

good

foo['example']

=== Using Regular Expressions as String Indexes [[regexp-string-index]]

For simple constructions you can use regexp directly through string index.

[source,ruby]

match = string[/regexp/] # get content of matched regexp first_group = string[/text(grp)/, 1] # get content of captured group

string[/text (grp)/, 1] = 'replace' # string => 'text replace'

=== Prefer Non-capturing Groups [[non-capturing-regexp]]

Use non-capturing groups when you don't use the captured result.

[source,ruby]

bad

/(first|second)/

good

/(?:first|second)/

=== Do not mix named and numbered captures [[do-not-mix-named-and-numbered-captures]]

Do not mix named captures and numbered captures in a Regexp literal. Because numbered capture is ignored if they're mixed.

[source,ruby]

bad - There is no way to access
(BAR)
capturing.

m = /(?FOO)(BAR)/.match('FOOBAR') p m[:foo] # => "FOO" p m[1] # => "FOO" p m[2] # => nil - not "BAR"

good - Both captures are accessible with names.

m = /(?FOO)(?BAR)/.match('FOOBAR') p m[:foo] # => "FOO" p m[:bar] # => "BAR"

good -
(?:BAR)
is non-capturing grouping.

m = /(?FOO)(?:BAR)/.match('FOOBAR') p m[:foo] # => "FOO"

good - Both captures are accessible with numbers.

m = /(FOO)(BAR)/.match('FOOBAR') p m[1] # => "FOO"

p m[2] # => "BAR"

=== Refer named regexp captures by name [[refer-named-regexp-captures-by-name]]

Prefer using names to refer named regexp captures instead of numbers.

[source,ruby]

bad

m = /(?FOO)(?BAR)/.match('FOOBAR') p m[1] # => "FOO" p m[2] # => "BAR"

good

m = /(?FOO)(?BAR)/.match('FOOBAR') p m[:foo] # => "FOO"

p m[:bar] # => "BAR"

=== Avoid Perl-style Last Regular Expression Group Matchers [[no-perl-regexp-last-matchers]]

Don't use the cryptic Perl-legacy variables denoting last regexp group matches (

$1
,
$2
, etc). Use
Regexp.last_match(n)
instead.

[source,ruby]

/(regexp)/ =~ string ...

bad

process $1

good

process Regexp.last_match(1)

=== Avoid Numbered Groups [[no-numbered-regexes]]

Avoid using numbered groups as it can be hard to track what they contain. Named groups can be used instead.

[source,ruby]

bad

/(regexp)/ =~ string

some code

process Regexp.last_match(1)

good

/(?regexp)/ =~ string

some code

process meaningful_var

=== Limit Escapes [[limit-escapes]]

Character classes have only a few special characters you should care about:

^
,
-
,
\
,
]
, so don't escape
.
or brackets in
[]
.

=== Caret and Dollar Regexp [[caret-and-dollar-regexp]]

Be careful with

^
and
$
as they match start/end of line, not string endings. If you want to match the whole string use:
\A
and
\z
(not to be confused with
\Z
which is the equivalent of
/\n?\z/
).

[source,ruby]

string = "some injection\nusername" string[/^username$/] # matches

string[/\Ausername\z/] # doesn't match

=== Multi-line Regular Expressions [[multi-line-regexes]]

Use

x
(free-spacing) modifier for multi-line regexps.

NOTE: That's known as http://www.regular-expressions.info/freespacing.html[free-spacing mode]. In this mode leading and trailing whitespace is ignored.

[source,ruby]

bad

regex = /start\ \s\ (group)\ (?:alt1|alt2)\ end/

good

regexp = / start \s (group) (?:alt1|alt2) end

/x

=== Comment Complex Regular Expressions [[comment-regexes]]

Use

x
modifier for complex regexps. This makes them more readable and you can add some useful comments.

[source,ruby]

regexp = / start # some text \s # white space char (group) # first group (?:alt1|alt2) # some alternation end

/x

=== Use

gsub
with a Block or a Hash for Complex Replacements [[gsub-blocks]]

For complex replacements

sub
/
gsub
can be used with a block or a hash.

[source,ruby]

words = 'foo bar' words.sub(/f/, 'f' => 'F') # => 'Foo bar'

words.gsub(/\w+/) { |word| word.capitalize } # => 'Foo Bar'

== Percent Literals

===

%q
shorthand [[percent-q-shorthand]]

Use

%()
(it's a shorthand for
%Q
) for single-line strings which require both interpolation and embedded double-quotes. For multi-line strings, prefer heredocs.

[source,ruby]

bad (no interpolation needed)

%(

Some text
)

should be '
Some text
'

bad (no double-quotes)

%(This is #{quality} style)

should be "This is #{quality} style"

bad (multiple lines)

%(

\n#{exclamation}\n
)

should be a heredoc.

good (requires interpolation, has quotes, single line)

%( #{name})

===

%q
[[percent-q]]

Avoid

%()
or the equivalent
%q()
unless you have a string with both
'
and
"
in it. Regular string literals are more readable and should be preferred unless a lot of characters would have to be escaped in them.

[source,ruby]

bad

name = %q(Bruce Wayne) time = %q(8 o'clock) question = %q("What did you say?")

good

name = 'Bruce Wayne' time = "8 o'clock" question = '"What did you say?"'

quote = %q(

"What did you say?"

)

===

%r
[[percent-r]]

Use

%r
only for regular expressions matching at least one
/
character.

[source,ruby]

bad

%r{\s+}

good

%r{^/(.*)$}

%r{^/blog/2011/(.*)$}

===

%x
[[percent-x]]

Avoid the use of

%x
unless you're going to invoke a command with backquotes in it (which is rather unlikely).

[source,ruby]

bad

date = %x(date)

good

date =

date

echo = %x(echo
date
)

===

%s
[[percent-s]]

Avoid the use of

%s
. It seems that the community has decided
:"some string"
is the preferred way to create a symbol with spaces in it.

=== Percent Literal Braces [[percent-literal-braces]]

Use the braces that are the most appropriate for the various kinds of percent literals.

  • ()
    for string literals (
    %q
    ,
    %Q
    ).
  • []
    for array literals (
    %w
    ,
    %i
    ,
    %W
    ,
    %I
    ) as it is aligned with the standard array literals.
  • {}
    for regexp literals (
    %r
    ) since parentheses often appear inside regular expressions. That's why a less common character with
    {
    is usually the best delimiter for
    %r
    literals.
  • ()
    for all other literals (e.g.
    %s
    ,
    %x
    )

[source,ruby]

bad

%q{"Test's king!", John said.}

good

%q("Test's king!", John said.)

bad

%w(one two three) %i(one two three)

good

%w[one two three] %i[one two three]

bad

%r((\w+)-(\d+)) %r{\w{1,2}\d{2,5}}

good

%r{(\w+)-(\d+)}

%r|\w{1,2}\d{2,5}|

== Metaprogramming

=== No Needless Metaprogramming [[no-needless-metaprogramming]]

Avoid needless metaprogramming.

=== No Monkey Patching [[no-monkey-patching]]

Do not mess around in core classes when writing libraries (do not monkey-patch them).

=== Block

class_eval
[[block-class-eval]]

The block form of

class_eval
is preferable to the string-interpolated form.

==== Supply Location [[class-eval-supply-location]]

When you use the string-interpolated form, always supply

+__FILE__+
and
+__LINE__+
, so that your backtraces make sense:

[source,ruby]

classeval 'def userelativemodelnaming?; true; end', FILE, LINE

====

define_method
[[class-eval-define_method]]

define_method
is preferable to
class_eval { def ... }

===

eval
Comment Docs [[eval-comment-docs]]

When using

class_eval
(or other
eval
) with string interpolation, add a comment block showing its appearance if interpolated (a practice used in Rails code):

[source,ruby]

from activesupport/lib/activesupport/coreext/string/output_safety.rb

UNSAFESTRINGMETHODS.each do |unsafemethod| if 'String'.respondto?(unsafemethod) classeval <<-EOT, FILE, LINE + 1 def #{unsafemethod}(params, &block) # def capitalize(params, &block) tostr.#{unsafemethod}(*params, &block) # tostr.capitalize(*params, &block) end # end

  def #{unsafe_method}!(*params)              # def capitalize!(*params)
    @dirty = true                             #   @dirty = true
    super                                     #   super
  end                                         # end
EOT

end

end

=== No

method_missing
[[no-method-missing]]

Avoid using

method_missing
for metaprogramming because backtraces become messy, the behavior is not listed in
#methods
, and misspelled method calls might silently work, e.g.
nukes.luanch_state = false
. Consider using delegation, proxy, or
define_method
instead. If you must use
method_missing
:
  • Be sure to http://blog.marc-andre.ca/2010/11/methodmissing-politely.html[also define
    respond_to_missing?
    ]
  • Only catch methods with a well-defined prefix, such as
    find_by_*
    --make your code as assertive as possible.
  • Call
    super
    at the end of your statement
  • Delegate to assertive, non-magical methods:

[source,ruby]

bad

def methodmissing(meth, *params, &block) if /^findby(?.*)/ =~ meth # ... lots of code to do a findby else super end end

good

def methodmissing(meth, *params, &block) if /^findby(?.*)/ =~ meth findby(prop, *params, &block) else super end end

best of all, though, would to define_method as each findable attribute is declared


=== Prefer

public_send
[[prefer-public-send]]

Prefer

public_send
over
send
so as not to circumvent
private
/
protected
visibility.

[source,ruby]

We have an ActiveModel Organization that includes concern Activatable

module Activatable extend ActiveSupport::Concern

included do beforecreate :createtoken end

private

def reset_token # some code end

def create_token # some code end

def activate! # some code end end

class Organization < ActiveRecord::Base include Activatable end

linux_organization = Organization.find(...)

BAD - violates privacy

linuxorganization.send(:resettoken)

GOOD - should throw an exception

linuxorganization.publicsend(:reset_token)

=== Prefer

+__send__+
[[prefer-send]]

Prefer

+__send__+
over
send
, as
send
may overlap with existing methods.

[source,ruby]

require 'socket'

u1 = UDPSocket.new u1.bind('127.0.0.1', 4913) u2 = UDPSocket.new u2.connect('127.0.0.1', 4913)

Won't send a message to the receiver obj.

Instead it will send a message via UDP socket.

u2.send :sleep, 0

Will actually send a message to the receiver obj.

u2.send ...

== API Documentation [[api-documentation]]

=== YARD

Use https://yardoc.org/[YARD] and its conventions for API documentation.

=== RD (Block) Comments [[no-block-comments]]

Don't use block comments. They cannot be preceded by whitespace and are not as easy to spot as regular comments.

[source,ruby]

bad

=begin comment line another comment line =end

good

comment line

another comment line


.From Perl's POD to RD


This is not a really a block comment syntax, but more of an attempt to emulate Perl's https://perldoc.perl.org/perlpod.html[POD] documentation system.

There's an https://github.com/uwabami/rdtool[rdtool] for Ruby that's pretty similar to POD. Basically

rdtool
scans a file for
=begin
and
=end
pairs, and extracts the text between them all. This text is assumed to be documentation in https://github.com/uwabami/rdtool/blob/master/doc/rd-draft.rd[RD format]. You can read more about it https://ruby-doc.com/docs/ProgrammingRuby/html/rdtool.html[here].

RD predated the rise of RDoc and YARD and was effectively obsoleted by them.footnote:[According to this https://en.wikipedia.org/wiki/RubyDocumentformat[Wikipedia article] the format used to be popular until the early 2000s when it was superseded by RDoc.]


== Gemfile and Gemspec

=== No

RUBY_VERSION
in the gemspec [[no-ruby-version-in-the-gemspec]]

The gemspec should not contain

RUBY_VERSION
as a condition to switch dependencies.
RUBY_VERSION
is determined by
rake release
, so users may end up with wrong dependency.

[source,ruby]

bad

Gem::Specification.new do |s| if RUBYVERSION >= '2.5' s.addruntimedependency 'gema' else s.addruntimedependency 'gem_b' end

end

Fix by either:

  • Post-install messages.
  • Add both gems as dependency (if permissible).
  • If development dependencies, move to Gemfile.

== Misc

=== No Flip-flops [[no-flip-flops]]

Avoid the use of flip-flops.

NOTE: They are deprecated as of Ruby 2.6.

=== No non-

nil
Checks [[no-non-nil-checks]]

Don't do explicit non-

nil
checks unless you're dealing with boolean values.

[source,ruby]

bad

dosomething if !something.nil? dosomething if something != nil

good

do_something if something

good - dealing with a boolean

def valueset? [email protected]boolean.nil?

end

=== Global Input/Output Streams [[global-stdout]]

Use

$stdout/$stderr/$stdin
instead of
STDOUT/STDERR/STDIN
.
STDOUT/STDERR/STDIN
are constants, and while you can actually reassign (possibly to redirect some stream) constants in Ruby, you'll get an interpreter warning if you do so.

[source,ruby]

bad

STDOUT.puts('hello')

hash = { out: STDOUT, key: value }

def m(out = STDOUT) out.puts('hello') end

good

$stdout.puts('hello')

hash = { out: $stdout, key: value }

def m(out = $stdout) out.puts('hello')

end

NOTE: The only valid use-case for the stream constants is obtaining references to the original streams (assuming you've redirected some of the global vars).

=== Warn [[warn]]

Use

warn
instead of
$stderr.puts
. Apart from being more concise and clear,
warn
allows you to suppress warnings if you need to (by setting the warn level to 0 via
-W0
).

[source,ruby]

bad

$stderr.puts 'This is a warning!'

good

warn 'This is a warning!'

===

Array#join
[[array-join]]

Prefer the use of

Array#join
over the fairly cryptic
Array#*
with a string argument.

[source,ruby]

bad

%w[one two three] * ', '

=> 'one, two, three'

good

%w[one two three].join(', ')

=> 'one, two, three'


=== Array Coercion [[array-coercion]]

Use

Array()
instead of explicit
Array
check or
[*var]
, when dealing with a variable you want to treat as an Array, but you're not certain it's an array.

[source,ruby]

bad

paths = [paths] unless paths.isa?(Array) paths.each { |path| dosomething(path) }

bad (always creates a new Array instance)

[*paths].each { |path| do_something(path) }

good (and a bit more readable)

Array(paths).each { |path| do_something(path) }

=== Ranges or

between
[[ranges-or-between]]

Use ranges or

Comparable#between?
instead of complex comparison logic when possible.

[source,ruby]

bad

do_something if x >= 1000 && x <= 2000

good

do_something if (1000..2000).include?(x)

good

do_something if x.between?(1000, 2000)

=== Predicate Methods [[predicate-methods]]

Prefer the use of predicate methods to explicit comparisons with

==
. Numeric comparisons are OK.

[source,ruby]

bad

if x % 2 == 0 end

if x % 2 == 1 end

if x == nil end

good

if x.even? end

if x.odd? end

if x.nil? end

if x.zero? end

if x == 0

end

=== No Cryptic Perlisms [[no-cryptic-perlisms]]

Avoid using Perl-style special variables (like

$:
,
$;
, etc). They are quite cryptic and their use in anything but one-liner scripts is discouraged.

[source,ruby]

bad

$:.unshift File.dirname(FILE)

good

$LOADPATH.unshift File.dirname(FILE_)

Use the human-friendly aliases provided by the

English
library if required.

[source,ruby]

bad

print $', $$

good

require 'English'

print $POSTMATCH, $PID

=== Use

require_relative
whenever possible

For all your internal dependencies, you should use

require_relative
. Use of
require
should be reserved for external dependencies

[source,ruby]

bad

require 'set' require 'mygem/spec/helper' require 'mygem/lib/something'

good

require 'set' require_relative 'helper'

require_relative '../lib/something'

This way is more expressive (making clear which dependency is internal or not) and more efficient (as

require_relative
doesn't have to try all of
$LOAD_PATH
contrary to
require
).

=== Always Warn [[always-warn]]

Write

ruby -w
safe code.

=== No Optional Hash Params [[no-optional-hash-params]]

Avoid hashes as optional parameters. Does the method do too much? (Object initializers are exceptions for this rule).

=== Instance Vars [[instance-vars]]

Use module instance variables instead of global variables.

[source,ruby]

bad

$foo_bar = 1

good

module Foo class << self attr_accessor :bar end end

Foo.bar = 1

===

OptionParser
[[optionparser]]

Use

OptionParser
for parsing complex command line options and
ruby -s
for trivial command line options.

=== No Param Mutations [[no-param-mutations]]

Do not mutate parameters unless that is the purpose of the method.

=== Three is the Number Thou Shalt Count [[three-is-the-number-thou-shalt-count]]

Avoid more than three levels of block nesting.

=== Functional Code [[functional-code]]

Code in a functional way, avoiding mutation when that makes sense.

[source,ruby]

a = []; [1, 2, 3].each { |i| a << i * 2 } # bad a = [1, 2, 3].map { |i| i * 2 } # good

a = {}; [1, 2, 3].each { |i| a[i] = i * 17 } # bad a = [1, 2, 3].reduce({}) { |h, i| h[i] = i * 17; h } # good

a = [1, 2, 3].eachwithobject({}) { |i, h| h[i] = i * 17 } # good

=== No explicit

.rb
to
require
[[no-explicit-rb-to-require]]

Omit the

.rb
extension for filename passed to
require
and
require_relative
.

NOTE: If the extension is omitted, Ruby tries adding '.rb', '.so', and so on to the name until found. If the file named cannot be found, a

LoadError
will be raised. There is an edge case where
foo.so
file is loaded instead of a
LoadError
if
foo.so
file exists when
require 'foo.rb'
will be changed to
require 'foo'
, but that seems harmless.

[source,ruby]

bad

require 'foo.rb' require_relative '../foo.rb'

good

require 'foo' require 'foo.so' require_relative '../foo'

require_relative '../foo.so'

== Tools

Here are some tools to help you automatically check Ruby code against this guide.

=== RuboCop

https://github.com/rubocop-hq/rubocop[RuboCop] is a Ruby static code analyzer and formatter, based on this style guide. RuboCop already covers a significant portion of the guide and has https://docs.rubocop.org/en/stable/integrationwithother_tools/[plugins] for most popular Ruby editors and IDEs.

TIP: RuboCop's cops (code checks) have links to the guidelines that they are based on, as part of their metadata.

=== RubyMine

http://www.jetbrains.com/ruby/[RubyMine]'s code inspections are http://confluence.jetbrains.com/display/RUBYDEV/RubyMine+Inspections[partially based] on this guide.

== History

This guide started its life as an internal company Ruby coding guidelines (written by https://github.com/bbatsov[Bozhidar Batsov]). Bozhidar had always been bothered as a Ruby developer about one thing - Python developers had a great programming style reference (https://www.python.org/dev/peps/pep-0008/[PEP-8]) and Rubyists never got an official guide, documenting Ruby coding style and best practices. Bozhidar firmly believed that style matters. He also believed that a great hacker community, such as Ruby has, should be quite capable of producing this coveted document. The rest is history...

At some point Bozhidar decided that the work he was doing might be interesting to members of the Ruby community in general and that the world had little need for another internal company guideline. But the world could certainly benefit from a community-driven and community-sanctioned set of practices, idioms and style prescriptions for Ruby programming.

Bozhidar served as the guide's only editor for a few years, before a team of editors was formed once the project transitioned to RuboCop HQ.

Since the inception of the guide we've received a lot of feedback from members of the exceptional Ruby community around the world. Thanks for all the suggestions and the support! Together we can make a resource beneficial to each and every Ruby developer out there.

== Sources of Inspiration

Many people, books, presentations, articles and other style guides influenced the community Ruby style guide. Here are some of them:

  • https://en.wikipedia.org/wiki/TheElementsof_Style[The Elements of Style]
  • https://en.wikipedia.org/wiki/TheElementsofProgrammingStyle[The Elements of Programming Style]
  • https://www.python.org/dev/peps/pep-0008/[PEP-8]
  • https://pragprog.com/book/ruby4/programming-ruby-1-9-2-0["Programming Ruby"]
  • http://www.amazon.com/Ruby-Programming-Language-David-Flanagan/dp/0596516177["The Ruby Programming Language"]

== Contributing

The guide is still a work in progress - some guidelines are lacking examples, some guidelines don't have examples that illustrate them clearly enough. Improving such guidelines is a great (and simple way) to help the Ruby community!

In due time these issues will (hopefully) be addressed - just keep them in mind for now.

Nothing written in this guide is set in stone. It's our desire to work together with everyone interested in Ruby coding style, so that we could ultimately create a resource that will be beneficial to the entire Ruby community.

Feel free to open tickets or send pull requests with improvements. Thanks in advance for your help!

You can also support the project (and RuboCop) with financial contributions via https://www.patreon.com/bbatsov[Patreon].

=== How to Contribute?

It's easy, just follow the contribution guidelines below:

  • https://help.github.com/articles/fork-a-repo[Fork] https://github.com/rubocop-hq/ruby-style-guide[rubocop-hq/ruby-style-guide] on GitHub
  • Make your feature addition or bug fix in a feature branch.
  • Include a http://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html[good description] of your changes
  • Push your feature branch to GitHub
  • Send a https://help.github.com/articles/using-pull-requests[Pull Request]

== Colophon

This guide is written in http://asciidoc.org/[AsciiDoc] and is published as HTML using https://asciidoctor.org/[AsciiDoctor]. The HTML version of the guide is hosted on GitHub Pages.

Originally the guide was written in Markdown, but was converted to AsciiDoc in 2019.

== License

image:https://i.creativecommons.org/l/by/3.0/88x31.png[Creative Commons License] This work is licensed under a http://creativecommons.org/licenses/by/3.0/deed.en_US[Creative Commons Attribution 3.0 Unported License]

== Spread the Word

A community-driven style guide is of little use to a community that doesn't know about its existence. Tweet about the guide, share it with your friends and colleagues. Every comment, suggestion or opinion we get makes the guide just a little bit better. And we want to have the best possible guide, don't we?

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