1go-spew
2=======
3
4[](https://travis-ci.org/davecgh/go-spew)
5[](http://copyfree.org)
6[](https://coveralls.io/r/davecgh/go-spew?branch=master)
7
8Go-spew implements a deep pretty printer for Go data structures to aid in
9debugging. A comprehensive suite of tests with 100% test coverage is provided
10to ensure proper functionality. See `test_coverage.txt` for the gocov coverage
11report. Go-spew is licensed under the liberal ISC license, so it may be used in
12open source or commercial projects.
13
14If you're interested in reading about how this package came to life and some
15of the challenges involved in providing a deep pretty printer, there is a blog
16post about it
17[here](https://web.archive.org/web/20160304013555/https://blog.cyphertite.com/go-spew-a-journey-into-dumping-go-data-structures/).
18
19## Documentation
20
21[](http://godoc.org/github.com/davecgh/go-spew/spew)
22
23Full `go doc` style documentation for the project can be viewed online without
24installing this package by using the excellent GoDoc site here:
25http://godoc.org/github.com/davecgh/go-spew/spew
26
27You can also view the documentation locally once the package is installed with
28the `godoc` tool by running `godoc -http=":6060"` and pointing your browser to
29http://localhost:6060/pkg/github.com/davecgh/go-spew/spew
30
31## Installation
32
33```bash
34$ go get -u github.com/davecgh/go-spew/spew
35```
36
37## Quick Start
38
39Add this import line to the file you're working in:
40
41```Go
42import "github.com/davecgh/go-spew/spew"
43```
44
45To dump a variable with full newlines, indentation, type, and pointer
46information use Dump, Fdump, or Sdump:
47
48```Go
49spew.Dump(myVar1, myVar2, ...)
50spew.Fdump(someWriter, myVar1, myVar2, ...)
51str := spew.Sdump(myVar1, myVar2, ...)
52```
53
54Alternatively, if you would prefer to use format strings with a compacted inline
55printing style, use the convenience wrappers Printf, Fprintf, etc with %v (most
56compact), %+v (adds pointer addresses), %#v (adds types), or %#+v (adds types
57and pointer addresses):
58
59```Go
60spew.Printf("myVar1: %v -- myVar2: %+v", myVar1, myVar2)
61spew.Printf("myVar3: %#v -- myVar4: %#+v", myVar3, myVar4)
62spew.Fprintf(someWriter, "myVar1: %v -- myVar2: %+v", myVar1, myVar2)
63spew.Fprintf(someWriter, "myVar3: %#v -- myVar4: %#+v", myVar3, myVar4)
64```
65
66## Debugging a Web Application Example
67
68Here is an example of how you can use `spew.Sdump()` to help debug a web application. Please be sure to wrap your output using the `html.EscapeString()` function for safety reasons. You should also only use this debugging technique in a development environment, never in production.
69
70```Go
71package main
72
73import (
74 "fmt"
75 "html"
76 "net/http"
77
78 "github.com/davecgh/go-spew/spew"
79)
80
81func handler(w http.ResponseWriter, r *http.Request) {
82 w.Header().Set("Content-Type", "text/html")
83 fmt.Fprintf(w, "Hi there, %s!", r.URL.Path[1:])
84 fmt.Fprintf(w, "<!--\n" + html.EscapeString(spew.Sdump(w)) + "\n-->")
85}
86
87func main() {
88 http.HandleFunc("/", handler)
89 http.ListenAndServe(":8080", nil)
90}
91```
92
93## Sample Dump Output
94
95```
96(main.Foo) {
97 unexportedField: (*main.Bar)(0xf84002e210)({
98 flag: (main.Flag) flagTwo,
99 data: (uintptr) <nil>
100 }),
101 ExportedField: (map[interface {}]interface {}) {
102 (string) "one": (bool) true
103 }
104}
105([]uint8) {
106 00000000 11 12 13 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f 20 |............... |
107 00000010 21 22 23 24 25 26 27 28 29 2a 2b 2c 2d 2e 2f 30 |!"#$%&'()*+,-./0|
108 00000020 31 32 |12|
109}
110```
111
112## Sample Formatter Output
113
114Double pointer to a uint8:
115```
116 %v: <**>5
117 %+v: <**>(0xf8400420d0->0xf8400420c8)5
118 %#v: (**uint8)5
119 %#+v: (**uint8)(0xf8400420d0->0xf8400420c8)5
120```
121
122Pointer to circular struct with a uint8 field and a pointer to itself:
123```
124 %v: <*>{1 <*><shown>}
125 %+v: <*>(0xf84003e260){ui8:1 c:<*>(0xf84003e260)<shown>}
126 %#v: (*main.circular){ui8:(uint8)1 c:(*main.circular)<shown>}
127 %#+v: (*main.circular)(0xf84003e260){ui8:(uint8)1 c:(*main.circular)(0xf84003e260)<shown>}
128```
129
130## Configuration Options
131
132Configuration of spew is handled by fields in the ConfigState type. For
133convenience, all of the top-level functions use a global state available via the
134spew.Config global.
135
136It is also possible to create a ConfigState instance that provides methods
137equivalent to the top-level functions. This allows concurrent configuration
138options. See the ConfigState documentation for more details.
139
140```
141* Indent
142 String to use for each indentation level for Dump functions.
143 It is a single space by default. A popular alternative is "\t".
144
145* MaxDepth
146 Maximum number of levels to descend into nested data structures.
147 There is no limit by default.
148
149* DisableMethods
150 Disables invocation of error and Stringer interface methods.
151 Method invocation is enabled by default.
152
153* DisablePointerMethods
154 Disables invocation of error and Stringer interface methods on types
155 which only accept pointer receivers from non-pointer variables. This option
156 relies on access to the unsafe package, so it will not have any effect when
157 running in environments without access to the unsafe package such as Google
158 App Engine or with the "safe" build tag specified.
159 Pointer method invocation is enabled by default.
160
161* DisablePointerAddresses
162 DisablePointerAddresses specifies whether to disable the printing of
163 pointer addresses. This is useful when diffing data structures in tests.
164
165* DisableCapacities
166 DisableCapacities specifies whether to disable the printing of capacities
167 for arrays, slices, maps and channels. This is useful when diffing data
168 structures in tests.
169
170* ContinueOnMethod
171 Enables recursion into types after invoking error and Stringer interface
172 methods. Recursion after method invocation is disabled by default.
173
174* SortKeys
175 Specifies map keys should be sorted before being printed. Use
176 this to have a more deterministic, diffable output. Note that
177 only native types (bool, int, uint, floats, uintptr and string)
178 and types which implement error or Stringer interfaces are supported,
179 with other types sorted according to the reflect.Value.String() output
180 which guarantees display stability. Natural map order is used by
181 default.
182
183* SpewKeys
184 SpewKeys specifies that, as a last resort attempt, map keys should be
185 spewed to strings and sorted by those strings. This is only considered
186 if SortKeys is true.
187
188```
189
190## Unsafe Package Dependency
191
192This package relies on the unsafe package to perform some of the more advanced
193features, however it also supports a "limited" mode which allows it to work in
194environments where the unsafe package is not available. By default, it will
195operate in this mode on Google App Engine and when compiled with GopherJS. The
196"safe" build tag may also be specified to force the package to build without
197using the unsafe package.
198
199## License
200
201Go-spew is licensed under the [copyfree](http://copyfree.org) ISC License.
View as plain text