Package spew implements a deep pretty printer for Go data structures to aid in debugging.

A quick overview of the additional features spew provides over the built-in printing facilities for Go data types are as follows:

* Pointers are dereferenced and followed
* Circular data structures are detected and handled properly
* Custom Stringer/error interfaces are optionally invoked, including
  on unexported types
* Custom types which only implement the Stringer/error interfaces via
  a pointer receiver are optionally invoked when passing non-pointer
* Byte arrays and slices are dumped like the hexdump -C command which
  includes offsets, byte values in hex, and ASCII output (only when using
  Dump style)

There are two different approaches spew allows for dumping Go data structures:

* Dump style which prints with newlines, customizable indentation,
  and additional debug information such as types and all pointer addresses
  used to indirect to the final value
* A custom Formatter interface that integrates cleanly with the standard fmt
  package and replaces %v, %+v, %#v, and %#+v to provide inline printing
  similar to the default %v while providing the additional functionality
  outlined above and passing unsupported format verbs such as %x and %q
  along to fmt

Quick Start

This section demonstrates how to quickly get started with spew. See the sections below for further details on formatting and configuration options.

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)

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.

The following configuration options are available:

* 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.
	Pointer method invocation is enabled by default.

* 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
	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.

Dump Usage

Simply call spew.Dump with a list of variables you want to dump:

spew.Dump(myVar1, myVar2, ...)

You may also call spew.Fdump if you would prefer to output to an arbitrary io.Writer. For example, to dump to standard error:

spew.Fdump(os.Stderr, myVar1, myVar2, ...)

A third option is to call spew.Sdump to get the formatted output as a string:

str := spew.Sdump(myVar1, myVar2, ...)

Sample Dump Output

See the Dump example for details on the setup of the types and variables being shown here.

(main.Foo) {
 unexportedField: (*main.Bar)(0xf84002e210)({
  flag: (main.Flag) flagTwo,
  data: (uintptr) <nil>
 ExportedField: (map[interface {}]interface {}) (len=1) {
  (string) (len=3) "one": (bool) true

Byte (and uint8) arrays and slices are displayed uniquely like the hexdump -C command as shown.

([]uint8) (len=32 cap=32) {
 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|

Custom Formatter

Spew provides a custom formatter that implements the fmt.Formatter interface so that it integrates cleanly with standard fmt package printing functions. The formatter is useful for inline printing of smaller data types similar to the standard %v format specifier.

The custom formatter only responds to the %v (most compact), %+v (adds pointer addresses), %#v (adds types), or %#+v (adds types and pointer addresses) verb combinations. Any other verbs such as %x and %q will be sent to the the standard fmt package for formatting. In addition, the custom formatter ignores the width and precision arguments (however they will still work on the format specifiers not handled by the custom formatter).

Custom Formatter Usage

The simplest way to make use of the spew custom formatter is to call one of the convenience functions such as spew.Printf, spew.Println, or spew.Printf. The functions have syntax you are most likely already familiar with:

spew.Printf("myVar1: %v -- myVar2: %+v", myVar1, myVar2)
spew.Printf("myVar3: %#v -- myVar4: %#+v", myVar3, myVar4)
spew.Println(myVar, myVar2)
spew.Fprintf(os.Stderr, "myVar1: %v -- myVar2: %+v", myVar1, myVar2)
spew.Fprintf(os.Stderr, "myVar3: %#v -- myVar4: %#+v", myVar3, myVar4)

See the Index for the full list convenience functions.

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

See the Printf example for details on the setup of variables being shown here.


Since it is possible for custom Stringer/error interfaces to panic, spew detects them and handles them internally by printing the panic information inline with the output. Since spew is intended to provide deep pretty printing capabilities on structures, it intentionally does not return any errors.

spew is referenced in 1 repository