1/*
2 * Copyright (c) 2013-2016 Dave Collins <dave@davec.name>
3 *
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.
7 *
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.
15 */
16
17/*
18Package spew implements a deep pretty printer for Go data structures to aid in
19debugging.
20
21A quick overview of the additional features spew provides over the built-in
22printing facilities for Go data types are as follows:
23
24 * Pointers are dereferenced and followed
25 * Circular data structures are detected and handled properly
26 * Custom Stringer/error interfaces are optionally invoked, including
27 on unexported types
28 * Custom types which only implement the Stringer/error interfaces via
29 a pointer receiver are optionally invoked when passing non-pointer
30 variables
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
33 Dump style)
34
35There are two different approaches spew allows for dumping Go data structures:
36
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
44 along to fmt
45
46Quick Start
47
48This section demonstrates how to quickly get started with spew. See the
49sections below for further details on formatting and configuration options.
50
51To dump a variable with full newlines, indentation, type, and pointer
52information use Dump, Fdump, or Sdump:
53 spew.Dump(myVar1, myVar2, ...)
54 spew.Fdump(someWriter, myVar1, myVar2, ...)
55 str := spew.Sdump(myVar1, myVar2, ...)
56
57Alternatively, if you would prefer to use format strings with a compacted inline
58printing 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)
65
66Configuration Options
67
68Configuration of spew is handled by fields in the ConfigState type. For
69convenience, all of the top-level functions use a global state available
70via the spew.Config global.
71
72It is also possible to create a ConfigState instance that provides methods
73equivalent to the top-level functions. This allows concurrent configuration
74options. See the ConfigState documentation for more details.
75
76The following configuration options are available:
77 * Indent
78 String to use for each indentation level for Dump functions.
79 It is a single space by default. A popular alternative is "\t".
80
81 * MaxDepth
82 Maximum number of levels to descend into nested data structures.
83 There is no limit by default.
84
85 * DisableMethods
86 Disables invocation of error and Stringer interface methods.
87 Method invocation is enabled by default.
88
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.
93
94 * DisablePointerAddresses
95 DisablePointerAddresses specifies whether to disable the printing of
96 pointer addresses. This is useful when diffing data structures in tests.
97
98 * DisableCapacities
99 DisableCapacities specifies whether to disable the printing of
100 capacities for arrays, slices, maps and channels. This is useful when
101 diffing data structures in tests.
102
103 * ContinueOnMethod
104 Enables recursion into types after invoking error and Stringer interface
105 methods. Recursion after method invocation is disabled by default.
106
107 * SortKeys
108 Specifies map keys should be sorted before being printed. Use
109 this to have a more deterministic, diffable output. Note that
110 only native types (bool, int, uint, floats, uintptr and string)
111 and types which implement error or Stringer interfaces are
112 supported with other types sorted according to the
113 reflect.Value.String() output which guarantees display
114 stability. Natural map order is used by default.
115
116 * SpewKeys
117 Specifies that, as a last resort attempt, map keys should be
118 spewed to strings and sorted by those strings. This is only
119 considered if SortKeys is true.
120
121Dump Usage
122
123Simply call spew.Dump with a list of variables you want to dump:
124
125 spew.Dump(myVar1, myVar2, ...)
126
127You may also call spew.Fdump if you would prefer to output to an arbitrary
128io.Writer. For example, to dump to standard error:
129
130 spew.Fdump(os.Stderr, myVar1, myVar2, ...)
131
132A third option is to call spew.Sdump to get the formatted output as a string:
133
134 str := spew.Sdump(myVar1, myVar2, ...)
135
136Sample Dump Output
137
138See the Dump example for details on the setup of the types and variables being
139shown here.
140
141 (main.Foo) {
142 unexportedField: (*main.Bar)(0xf84002e210)({
143 flag: (main.Flag) flagTwo,
144 data: (uintptr) <nil>
145 }),
146 ExportedField: (map[interface {}]interface {}) (len=1) {
147 (string) (len=3) "one": (bool) true
148 }
149 }
150
151Byte (and uint8) arrays and slices are displayed uniquely like the hexdump -C
152command as shown.
153 ([]uint8) (len=32 cap=32) {
154 00000000 11 12 13 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f 20 |............... |
155 00000010 21 22 23 24 25 26 27 28 29 2a 2b 2c 2d 2e 2f 30 |!"#$%&'()*+,-./0|
156 00000020 31 32 |12|
157 }
158
159Custom Formatter
160
161Spew provides a custom formatter that implements the fmt.Formatter interface
162so that it integrates cleanly with standard fmt package printing functions. The
163formatter is useful for inline printing of smaller data types similar to the
164standard %v format specifier.
165
166The custom formatter only responds to the %v (most compact), %+v (adds pointer
167addresses), %#v (adds types), or %#+v (adds types and pointer addresses) verb
168combinations. Any other verbs such as %x and %q will be sent to the the
169standard fmt package for formatting. In addition, the custom formatter ignores
170the width and precision arguments (however they will still work on the format
171specifiers not handled by the custom formatter).
172
173Custom Formatter Usage
174
175The simplest way to make use of the spew custom formatter is to call one of the
176convenience functions such as spew.Printf, spew.Println, or spew.Printf. The
177functions have syntax you are most likely already familiar with:
178
179 spew.Printf("myVar1: %v -- myVar2: %+v", myVar1, myVar2)
180 spew.Printf("myVar3: %#v -- myVar4: %#+v", myVar3, myVar4)
181 spew.Println(myVar, myVar2)
182 spew.Fprintf(os.Stderr, "myVar1: %v -- myVar2: %+v", myVar1, myVar2)
183 spew.Fprintf(os.Stderr, "myVar3: %#v -- myVar4: %#+v", myVar3, myVar4)
184
185See the Index for the full list convenience functions.
186
187Sample Formatter Output
188
189Double pointer to a uint8:
190 %v: <**>5
191 %+v: <**>(0xf8400420d0->0xf8400420c8)5
192 %#v: (**uint8)5
193 %#+v: (**uint8)(0xf8400420d0->0xf8400420c8)5
194
195Pointer to circular struct with a uint8 field and a pointer to itself:
196 %v: <*>{1 <*><shown>}
197 %+v: <*>(0xf84003e260){ui8:1 c:<*>(0xf84003e260)<shown>}
198 %#v: (*main.circular){ui8:(uint8)1 c:(*main.circular)<shown>}
199 %#+v: (*main.circular)(0xf84003e260){ui8:(uint8)1 c:(*main.circular)(0xf84003e260)<shown>}
200
201See the Printf example for details on the setup of variables being shown
202here.
203
204Errors
205
206Since it is possible for custom Stringer/error interfaces to panic, spew
207detects them and handles them internally by printing the panic information
208inline with the output. Since spew is intended to provide deep pretty printing
209capabilities on structures, it intentionally does not return any errors.
210*/
211package spew