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4.6K Stars 297 Forks ISC License 145 Commits 55 Opened issues


Implements a deep pretty printer for Go data structures to aid in debugging

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Build Status ISC License Coverage Status

Go-spew implements a deep pretty printer for Go data structures to aid in debugging. A comprehensive suite of tests with 100% test coverage is provided to ensure proper functionality. See

for the gocov coverage report. Go-spew is licensed under the liberal ISC license, so it may be used in open source or commercial projects.

If you're interested in reading about how this package came to life and some of the challenges involved in providing a deep pretty printer, there is a blog post about it here.




go doc
style documentation for the project can be viewed online without installing this package by using the excellent GoDoc site here:

You can also view the documentation locally once the package is installed with the

tool by running
godoc -http=":6060"
and pointing your browser to http://localhost:6060/pkg/


$ go get -u

Quick Start

Add this import line to the file you're working in:

import ""

To dump a variable with full newlines, indentation, type, and pointer information use Dump, Fdump, or Sdump:

spew.Dump(myVar1, myVar2, ...)
spew.Fdump(someWriter, myVar1, myVar2, ...)
str := spew.Sdump(myVar1, myVar2, ...)

Alternatively, if you would prefer to use format strings with a compacted inline printing style, use the convenience wrappers Printf, Fprintf, etc with %v (most compact), %+v (adds pointer addresses), %#v (adds types), or %#+v (adds types and pointer addresses):

spew.Printf("myVar1: %v -- myVar2: %+v", myVar1, myVar2)
spew.Printf("myVar3: %#v -- myVar4: %#+v", myVar3, myVar4)
spew.Fprintf(someWriter, "myVar1: %v -- myVar2: %+v", myVar1, myVar2)
spew.Fprintf(someWriter, "myVar3: %#v -- myVar4: %#+v", myVar3, myVar4)

Debugging a Web Application Example

Here is an example of how you can use

to help debug a web application. Please be sure to wrap your output using the
function for safety reasons. You should also only use this debugging technique in a development environment, never in production.
package main

import ( "fmt" "html" "net/http"



func handler(w http.ResponseWriter, r *http.Request) { w.Header().Set("Content-Type", "text/html") fmt.Fprintf(w, "Hi there, %s!", r.URL.Path[1:]) fmt.Fprintf(w, "") }

func main() { http.HandleFunc("/", handler) http.ListenAndServe(":8080", nil) }

Sample Dump Output

(main.Foo) {
 unexportedField: (*main.Bar)(0xf84002e210)({
  flag: (main.Flag) flagTwo,
  data: (uintptr) 
 ExportedField: (map[interface {}]interface {}) {
  (string) "one": (bool) true
([]uint8) {
 00000000  11 12 13 14 15 16 17 18  19 1a 1b 1c 1d 1e 1f 20  |............... |
 00000010  21 22 23 24 25 26 27 28  29 2a 2b 2c 2d 2e 2f 30  |!"#$%&'()*+,-./0|
 00000020  31 32                                             |12|

Sample Formatter Output

Double pointer to a uint8:

      %v: 5
     %+v: (0xf8400420d0->0xf8400420c8)5
     %#v: (**uint8)5
    %#+v: (**uint8)(0xf8400420d0->0xf8400420c8)5

Pointer to circular struct with a uint8 field and a pointer to itself:

      %v: {1 }
     %+v: (0xf84003e260){ui8:1 c:(0xf84003e260)}
     %#v: (*main.circular){ui8:(uint8)1 c:(*main.circular)}
    %#+v: (*main.circular)(0xf84003e260){ui8:(uint8)1 c:(*main.circular)(0xf84003e260)}

Configuration Options

Configuration of spew is handled by fields in the ConfigState type. For convenience, all of the top-level functions use a global state available via the spew.Config global.

It is also possible to create a ConfigState instance that provides methods equivalent to the top-level functions. This allows concurrent configuration options. See the ConfigState documentation for more details.

* Indent
    String to use for each indentation level for Dump functions.
    It is a single space by default.  A popular alternative is "\t".

  • MaxDepth Maximum number of levels to descend into nested data structures. There is no limit by default.

  • DisableMethods Disables invocation of error and Stringer interface methods. Method invocation is enabled by default.

  • DisablePointerMethods Disables invocation of error and Stringer interface methods on types which only accept pointer receivers from non-pointer variables. This option relies on access to the unsafe package, so it will not have any effect when running in environments without access to the unsafe package such as Google App Engine or with the "safe" build tag specified. Pointer method invocation is enabled by default.

  • DisablePointerAddresses DisablePointerAddresses specifies whether to disable the printing of pointer addresses. This is useful when diffing data structures in tests.

  • DisableCapacities DisableCapacities specifies whether to disable the printing of capacities for arrays, slices, maps and channels. This is useful when diffing data structures in tests.

  • ContinueOnMethod Enables recursion into types after invoking error and Stringer interface methods. Recursion after method invocation is disabled by default.

  • SortKeys Specifies map keys should be sorted before being printed. Use this to have a more deterministic, diffable output. Note that only native types (bool, int, uint, floats, uintptr and string) and types which implement error or Stringer interfaces are supported, with other types sorted according to the reflect.Value.String() output which guarantees display stability. Natural map order is used by default.

  • SpewKeys SpewKeys specifies that, as a last resort attempt, map keys should be spewed to strings and sorted by those strings. This is only considered if SortKeys is true.

Unsafe Package Dependency

This package relies on the unsafe package to perform some of the more advanced features, however it also supports a "limited" mode which allows it to work in environments where the unsafe package is not available. By default, it will operate in this mode on Google App Engine and when compiled with GopherJS. The "safe" build tag may also be specified to force the package to build without using the unsafe package.


Go-spew is licensed under the copyfree ISC License.

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