2 * Copyright (c) 2013 Dave Collins <dave@davec.name>
4 * Permission to use, copy, modify, and distribute this software for any
5 * purpose with or without fee is hereby granted, provided that the above
6 * copyright notice and this permission notice appear in all copies.
8 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
9 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
10 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
11 * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
12 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
13 * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
14 * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
18 Package spew implements a deep pretty printer for Go data structures to aid in
21 A quick overview of the additional features spew provides over the built-in
22 printing facilities for Go data types are as follows:
24 * Pointers are dereferenced and followed
25 * Circular data structures are detected and handled properly
26 * Custom Stringer/error interfaces are optionally invoked, including
28 * Custom types which only implement the Stringer/error interfaces via
29 a pointer receiver are optionally invoked when passing non-pointer
31 * Byte arrays and slices are dumped like the hexdump -C command which
32 includes offsets, byte values in hex, and ASCII output (only when using
35 There are two different approaches spew allows for dumping Go data structures:
37 * Dump style which prints with newlines, customizable indentation,
38 and additional debug information such as types and all pointer addresses
39 used to indirect to the final value
40 * A custom Formatter interface that integrates cleanly with the standard fmt
41 package and replaces %v, %+v, %#v, and %#+v to provide inline printing
42 similar to the default %v while providing the additional functionality
43 outlined above and passing unsupported format verbs such as %x and %q
48 This section demonstrates how to quickly get started with spew. See the
49 sections below for further details on formatting and configuration options.
51 To dump a variable with full newlines, indentation, type, and pointer
52 information use Dump, Fdump, or Sdump:
53 spew.Dump(myVar1, myVar2, ...)
54 spew.Fdump(someWriter, myVar1, myVar2, ...)
55 str := spew.Sdump(myVar1, myVar2, ...)
57 Alternatively, if you would prefer to use format strings with a compacted inline
58 printing style, use the convenience wrappers Printf, Fprintf, etc with
59 %v (most compact), %+v (adds pointer addresses), %#v (adds types), or
60 %#+v (adds types and pointer addresses):
61 spew.Printf("myVar1: %v -- myVar2: %+v", myVar1, myVar2)
62 spew.Printf("myVar3: %#v -- myVar4: %#+v", myVar3, myVar4)
63 spew.Fprintf(someWriter, "myVar1: %v -- myVar2: %+v", myVar1, myVar2)
64 spew.Fprintf(someWriter, "myVar3: %#v -- myVar4: %#+v", myVar3, myVar4)
68 Configuration of spew is handled by fields in the ConfigState type. For
69 convenience, all of the top-level functions use a global state available
70 via the spew.Config global.
72 It is also possible to create a ConfigState instance that provides methods
73 equivalent to the top-level functions. This allows concurrent configuration
74 options. See the ConfigState documentation for more details.
76 The following configuration options are available:
78 String to use for each indentation level for Dump functions.
79 It is a single space by default. A popular alternative is "\t".
82 Maximum number of levels to descend into nested data structures.
83 There is no limit by default.
86 Disables invocation of error and Stringer interface methods.
87 Method invocation is enabled by default.
89 * DisablePointerMethods
90 Disables invocation of error and Stringer interface methods on types
91 which only accept pointer receivers from non-pointer variables.
92 Pointer method invocation is enabled by default.
95 Enables recursion into types after invoking error and Stringer interface
96 methods. Recursion after method invocation is disabled by default.
99 Specifies map keys should be sorted before being printed. Use
100 this to have a more deterministic, diffable output. Note that
101 only native types (bool, int, uint, floats, uintptr and string)
102 and types which implement error or Stringer interfaces are
103 supported with other types sorted according to the
104 reflect.Value.String() output which guarantees display
105 stability. Natural map order is used by default.
108 Specifies that, as a last resort attempt, map keys should be
109 spewed to strings and sorted by those strings. This is only
110 considered if SortKeys is true.
114 Simply call spew.Dump with a list of variables you want to dump:
116 spew.Dump(myVar1, myVar2, ...)
118 You may also call spew.Fdump if you would prefer to output to an arbitrary
119 io.Writer. For example, to dump to standard error:
121 spew.Fdump(os.Stderr, myVar1, myVar2, ...)
123 A third option is to call spew.Sdump to get the formatted output as a string:
125 str := spew.Sdump(myVar1, myVar2, ...)
129 See the Dump example for details on the setup of the types and variables being
133 unexportedField: (*main.Bar)(0xf84002e210)({
134 flag: (main.Flag) flagTwo,
135 data: (uintptr) <nil>
137 ExportedField: (map[interface {}]interface {}) (len=1) {
138 (string) (len=3) "one": (bool) true
142 Byte (and uint8) arrays and slices are displayed uniquely like the hexdump -C
144 ([]uint8) (len=32 cap=32) {
145 00000000 11 12 13 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f 20 |............... |
146 00000010 21 22 23 24 25 26 27 28 29 2a 2b 2c 2d 2e 2f 30 |!"#$%&'()*+,-./0|
152 Spew provides a custom formatter that implements the fmt.Formatter interface
153 so that it integrates cleanly with standard fmt package printing functions. The
154 formatter is useful for inline printing of smaller data types similar to the
155 standard %v format specifier.
157 The custom formatter only responds to the %v (most compact), %+v (adds pointer
158 addresses), %#v (adds types), or %#+v (adds types and pointer addresses) verb
159 combinations. Any other verbs such as %x and %q will be sent to the the
160 standard fmt package for formatting. In addition, the custom formatter ignores
161 the width and precision arguments (however they will still work on the format
162 specifiers not handled by the custom formatter).
164 Custom Formatter Usage
166 The simplest way to make use of the spew custom formatter is to call one of the
167 convenience functions such as spew.Printf, spew.Println, or spew.Printf. The
168 functions have syntax you are most likely already familiar with:
170 spew.Printf("myVar1: %v -- myVar2: %+v", myVar1, myVar2)
171 spew.Printf("myVar3: %#v -- myVar4: %#+v", myVar3, myVar4)
172 spew.Println(myVar, myVar2)
173 spew.Fprintf(os.Stderr, "myVar1: %v -- myVar2: %+v", myVar1, myVar2)
174 spew.Fprintf(os.Stderr, "myVar3: %#v -- myVar4: %#+v", myVar3, myVar4)
176 See the Index for the full list convenience functions.
178 Sample Formatter Output
180 Double pointer to a uint8:
182 %+v: <**>(0xf8400420d0->0xf8400420c8)5
184 %#+v: (**uint8)(0xf8400420d0->0xf8400420c8)5
186 Pointer to circular struct with a uint8 field and a pointer to itself:
187 %v: <*>{1 <*><shown>}
188 %+v: <*>(0xf84003e260){ui8:1 c:<*>(0xf84003e260)<shown>}
189 %#v: (*main.circular){ui8:(uint8)1 c:(*main.circular)<shown>}
190 %#+v: (*main.circular)(0xf84003e260){ui8:(uint8)1 c:(*main.circular)(0xf84003e260)<shown>}
192 See the Printf example for details on the setup of variables being shown
197 Since it is possible for custom Stringer/error interfaces to panic, spew
198 detects them and handles them internally by printing the panic information
199 inline with the output. Since spew is intended to provide deep pretty printing
200 capabilities on structures, it intentionally does not return any errors.