by petergtz

petergtz / pegomock

Pegomock is a powerful, yet simple mocking framework for the Go programming language

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PegoMock is a mocking framework for the Go programming language. It integrates well with Go's built-in

package, but can be used in other contexts too. It is based on golang/mock, but uses a DSL closely related to Mockito.

Getting Pegomock


go get
go get

This will download the package and install an executable

in your

See also sectionTracking the pegomock tool in your project for a per-project control of the tool version.

Getting Started

Using Pegomock with Golang’s XUnit-style Tests

The preferred way is:

import (

func TestUsingMocks(t *testing.T) { mock := NewMockPhoneBook(pegomock.WithT(t))

// use your mock here


Alternatively, you can set a global fail handler within your test:

func TestUsingMocks(t *testing.T) {

mock := NewMockPhoneBook()

// use your mock here


Note: In this case, Pegomock uses a global (singleton) fail handler. This has the benefit that you don’t need to pass the fail handler down to each test, but does mean that you cannot run your XUnit style tests in parallel with Pegomock.

If you configure both a global fail handler and a specific one for your mock, the specific one overrides the global fail handler.

Using Pegomock with Ginkgo

When a Pegomock verification fails, it calls a

. This is a function that you must provide using

If you’re using Ginkgo, all you need to do is:


before you start your test suite.

Avoiding Ginkgo Naming Collision with

Ginkgo introduced a new keyword in its DSL:

. This causes name collisions when dot-importing both Ginkgo and Pegomock. To avoid this, you can use a different dot-import for Pegomock which uses
instead of
. Example:
package some_test

import ( . "" . "" )

var _ = Describe("Some function", func() { When("certain condition", func() { It("succeeds", func() { mock := NewMockPhoneBook() Whenever(mock.GetPhoneNumber(EqString("Tom"))).ThenReturn("123-456-789") }) }) })

Generating Your First Mock and Using It

Let's assume you have:

type Display interface {
    Show(text string)

The simplest way is to call

from within your go package specifying the interface by its name:
cd path/to/package
pegomock generate Display

This will generate a

file which you can now use in your tests:
// creating mock
display := NewMockDisplay()

// using the mock display.Show("Hello World!")

// verifying display.VerifyWasCalledOnce().Show("Hello World!")

Why yet Another Mocking Framework for Go?

I've looked at some of the other frameworks, but found none of them satisfying: - GoMock seemed overly complicated when setting up mocks and verifying them. The command line interface is also not quite intuitive. That said, Pegomock is based on the GoMock, reusing mostly the mockgen code. - Counterfeiter uses a DSL that I didn't find expressive enough. It often seems to need more lines of code too. In one of its samples, it uses e.g.:

fake.DoThings("stuff", 5)

str, num := fake.DoThingsArgsForCall(0) Expect(str).To(Equal("stuff")) Expect(num).To(Equal(uint64(5)))

In Pegomock, this can be written as simple as:

fake.DoThings("stuff", 5)
fake.VerifyWasCalledOnce().DoThings("stuff", 5)

  • Hel uses a new and interesting approach to setting up and verifying mocks. However, I wonder how flexible it actually is. E.g. how about providing a callback function when stubbing? Can this be modeled with its current approach using channels?

In addition, Pegomock provides a "watch" command similar to Ginkgo, which constantly watches over changes in an interface and updates its mocks. It gives the framework a much more dynamic feel, similar to mocking frameworks in Ruby or Java.

Using Mocks In Your Tests

Verifying Behavior


type Display interface {
    Show(text string)


// creating mock:
display := NewMockDisplay()

// using the mock: display.Show("Hello World!")

// verifying: display.VerifyWasCalledOnce().Show("Hello World!")



type PhoneBook interface {
    GetPhoneNumber(name string) string


// creating the mock
phoneBook := NewMockPhoneBook()

// stubbing: When(phoneBook.GetPhoneNumber("Tom")).ThenReturn("345-123-789") When(phoneBook.GetPhoneNumber("Invalid")).ThenPanic("Invalid Name")

// prints "345-123-789": fmt.Println(phoneBook.GetPhoneNumber("Tom"))

// panics: fmt.Println(phoneBook.GetPhoneNumber("Invalid"))

// prints "", because GetPhoneNumber("Dan") was not stubbed fmt.Println(phoneBook.GetPhoneNumber("Dan"))

// Although it is possible to verify a stubbed invocation, usually it's redundant // If your code cares what GetPhoneNumber("Tom") returns, then something else breaks (often even before a verification gets executed). // If your code doesn't care what GetPhoneNumber("Tom") returns, then it should not be stubbed.

// Not convinced? See phoneBook.VerifyWasCalledOnce().GetPhoneNumber("Tom")

  • By default, for all methods that return a value, a mock will return zero values.
  • Once stubbed, the method will always return a stubbed value, regardless of how many times it is called.
  • ThenReturn
    supports chaining, i.e.
    etc. The mock will return the values in the same order the chaining was done. The values from the last
    will be returned indefinitely when the number of call exceeds the

Stubbing Functions That Have no Return Value

Stubbing functions that have no return value requires a slightly different approach, because such functions cannot be passed directly to another function. However, we can wrap them in an anonymous function:

// creating mock:
display := NewMockDisplay()

// stubbing When(func() { display.Show("Hello World!") }).ThenPanic("Panicking")

// panics: display.Show("Hello World!")

Argument Matchers

Pegomock provides matchers for stubbing and verification.


display := NewMockDisplay()

// Calling mock display.Show("Hello again!")

// Verification: display.VerifyWasCalledOnce().Show(AnyString())


phoneBook := NewMockPhoneBook()

// Stubbing: When(phoneBook.GetPhoneNumber(AnyString())).ThenReturn("123-456-789")

// Prints "123-456-789": fmt.Println(phoneBook.GetPhoneNumber("Dan")) // Also prints "123-456-789": fmt.Println(phoneBook.GetPhoneNumber("Tom"))

Important: When you use argument matchers, you must always use them for all arguments:

// Incorrect, panics:
When(contactList.getContactByFullName("Dan", AnyString())).thenReturn(Contact{...})
// Correct:
When(contactList.getContactByFullName(EqString("Dan"), AnyString())).thenReturn(Contact{...})

Writing Your Own Argument Matchers


matchers for types used in mock methods, can now be auto-generated while generating the mock. So writing your own argument matchers is not necessary for most use cases. See section The Pegomock CLI for more information.

You can also write your own matchers for non-basic types. E.g. if you have a

struct MyType
, you can write an Equals and Any matcher like this: ```go func EqMyType(value MyType) MyType { RegisterMatcher(&EqMatcher{Value: value}) return MyType{} }

func AnyMyType() MyType { RegisterMatcher(NewAnyMatcher(reflect.TypeOf(MyType{}))) return MyType{} } ```

Verifying the Number of Invocations

display := NewMockDisplay()

// Calling mock display.Show("Hello") display.Show("Hello, again") display.Show("And again")

// Verification: display.VerifyWasCalled(Times(3)).Show(AnyString()) // or: display.VerifyWasCalled(AtLeast(3)).Show(AnyString()) // or: display.VerifyWasCalled(Never()).Show("This one was never called")

Verifying in Order

display1 := NewMockDisplay()
display2 := NewMockDisplay()

// Calling mocks display1.Show("One") display1.Show("Two") display2.Show("Another two") display1.Show("Three")

// Verification: inOrderContext := new(InOrderContext) display1.VerifyWasCalledInOrder(Once(), inOrderContext).Show("One") display2.VerifyWasCalledInOrder(Once(), inOrderContext).Show("Another two") display1.VerifyWasCalledInOrder(Once(), inOrderContext).Show("Three")

Note that it's not necessary to verify the call for

if that one is not of any interested. An
only verifies that the verifications that are done, are in order.

Stubbing with Callbacks

phoneBook := NewMockPhoneBook()

// Stubbing: When(phoneBook.GetPhoneNumber(AnyString())).Then(func(params []Param) ReturnValues { return []ReturnValue{fmt.Sprintf("1-800-CALL-%v", strings.ToUpper(params[0]))} },

// Prints "1-800-CALL-DAN": fmt.Println(phoneBook.GetPhoneNumber("Dan")) // Prints "1-800-CALL-TOM": fmt.Println(phoneBook.GetPhoneNumber("Tom"))

Verifying with Argument Capture

In some cases it can be useful to capture the arguments from mock invocations and assert on them separately. This method is only recommended if the techniques using matchers are not sufficient.

display := NewMockDisplay()

// Calling mock display.Show("Hello") display.Show("Hello, again") display.Show("And again")

// Verification and getting captured arguments text := display.VerifyWasCalled(AtLeast(1)).Show(AnyString()).GetCapturedArguments()

// Captured arguments are from last invocation Expect(text).To(Equal("And again"))

You can also get all captured arguments:

// Verification and getting all captured arguments
texts := display.VerifyWasCalled(AtLeast(1)).Show(AnyString()).GetAllCapturedArguments()

// Captured arguments are a slice Expect(texts).To(ConsistOf("Hello", "Hello, again", "And again"))

Verifying with Asynchronous Mock Invocations

When the code exercising the mock is run as part of a Goroutine, it's necessary to verify in a polling fashion until a timeout kicks in.

can help here: ```go display := NewMockDisplay()

go func() { doSomething() display.Show("Hello") }()

display.VerifyWasCalledEventually(Once(), 2*time.Second).Show("Hello") ```

The Pegomock CLI


Install it via:

go install

Tracking the pegomock tool in your project

Go modules allow to pin not only a package but also a tool (that is, an executable). The steps are:

  1. Use a file named
    with contents similar to this: ```go // +build tools

// This file will never be compiled (see the build constraint above); it is // used to record dependencies on build tools with the Go modules machinery. // See

package tools

import ( _ "" )

2. Set `$GOBIN` to a `bin` directory relative to your repo (this defines where tool dependencies will be installed).
2. Install the tool with `go install`:
console $ cd /path/to/myproject $ export GOBIN=$PWD/bin $ go install
3. Use that `$GOBIN` when invoking `pegomock` for that project:
console $ $GOBIN/pegomock ...
console $ export PATH=$GOBIN:$PATH $ pegomock ... ```

See Tools as dependencies for details.

Generating Mocks

Pegomock can generate mocks in two different ways:

  1. by parsing source code Go files

    pegomock generate [] 
  2. by building a Go package and using reflection

    pegomock generate [] [] 

Flags can be any of the following:

  • --output,-o
    : Output file; defaults to mocktest.go.
  • --package
    : Package of the generated code; defaults to the package from which pegomock was executed suffixed with _test
  • --generate-matchers,-m
    : This will auto-generate argument matchers and place them in a
    directory alongside the mock source code itself.

For more flags, run:

pegomock --help

Generating Mocks with

There are a number of shortcomings in the current reflection-based implementation. To overcome these, there is now an option to use a new, experimental implementation that is based on To use it when generating your mocks, invoke

like this:
pegomock generate --use-experimental-model-gen [] [] 

What are the benefits? - The current default uses the reflect package to introspect the interface for which a mock should be generated. But reflection cannot determine method parameter names, only types. This forces the generator to generate them based on a pattern. In a code editor with code assistence, those pattern-based names (such as

) are non-descriptive and provide less help while writing code. The new implementation properly parses the source (including all dependent packages) and subsequently uses the same names as used in the interface definition. - With the current default you cannot generate an interface that lives in the
package. It's due to the way this implementation works: it imports the interface's package into temporarily generated code that gets compiled on the fly. This compilation fails, because there are now two
functions. - The new implementation is simpler and will probably become the default in the future, because it will be easier to maintain.

What are the drawbacks? - There is only one drawback: maturity. The new implementation is not complete yet, and also might have some bugs that still need to be fixed.

Users of Pegomock are encouraged to use this new option and report any problems by opening an issue. Help to stabilize it is greatly appreciated.

Generating mocks with
go generate

can be used with
go generate
. Simply add the directive to your source file.

Here's an example for a Display interface used by a calculator program:

// package/path/to/display/display.go

package display

type Display interface { Show(text string) }

// package/path/to/calculator/calculator_test.go

package calculator_test

//go:generate pegomock generate package/path/to/display Display

// Use generated mock mockDisplay := NewMockDisplay() ...

Generating it:

cd package/path/to/calculator
go generate

Note: While you could add the directive adjacent to the interface definition, the author's opinion is that this violates clean dependency management and would pollute the package of the interface. It's better to generate the mock in the same package, where it is used (if this coincides with the interface package, that's fine). That way, not only stays the interface's package clean, the tests also don't need to prefix the mock with a package, or use a dot-import.

Continuously Generating Mocks


command lets Pegomock generate mocks continuously on every change to an interface:
pegomock watch

For this, Pegomock expects an

file in the package directory where the mocks should be generated. In fact,
pegomock watch
will create it for you if it doesn't exist yet. The contents of the file are similar to the ones of the
# Any line starting with a # is treated as comment.

interface name without package specifies an Interface in the current package:


generates a mock for SomeInterfacetaken from mypackage:

path/to/my/mypackage SomeInterface

you can also specify a Go file:


and use most of the flags from the "generate" command

--output my_special_output.go MyInterface

Flags can be:

  • --recursive,-r
    : Recursively watch sub-directories as well.

Removing Generated Mocks

Sometimes it can be useful to systematically remove all mocks and matcher files generated by Pegomock. For this purpose, there is the

command. By simply calling it from the current directory
pegomock remove
it will remove all Pegomock-generated files in the current directory. It supports additional flags, such as
to recursively remove all Pegomock-generated files in sub-directories as well. To see all possible options, run:
pegomock remove --help

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