spew

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 Go to latest Published: Mar 31, 2016 License: ISC, Apache-2.0 Imports: 11 Imported by: 0

Documentation

Overview

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 variables
  • 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.

Errors

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.

Index

Constants

View Source 
const (
	// UnsafeDisabled is a build-time constant which specifies whether or
	// not access to the unsafe package is available.
	UnsafeDisabled = false
)

Variables

View Source 
var Config = ConfigState{Indent: " "}

Config is the active configuration of the top-level functions. The configuration can be changed by modifying the contents of spew.Config.

Functions

func Dump 

func Dump(a ...interface{})

Dump displays the passed parameters to standard out with newlines, customizable indentation, and additional debug information such as complete types and all pointer addresses used to indirect to the final value. It provides the following features over the built-in printing facilities provided by the fmt package:

  • 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 variables
  • Byte arrays and slices are dumped like the hexdump -C command which includes offsets, byte values in hex, and ASCII output

The configuration options are controlled by an exported package global, spew.Config. See ConfigState for options documentation.

See Fdump if you would prefer dumping to an arbitrary io.Writer or Sdump to get the formatted result as a string.

func Errorf 

func Errorf(format string, a ...interface{}) (err error)

Errorf is a wrapper for fmt.Errorf that treats each argument as if it were passed with a default Formatter interface returned by NewFormatter. It returns the formatted string as a value that satisfies error. See NewFormatter for formatting details.

This function is shorthand for the following syntax: 

fmt.Errorf(format, spew.NewFormatter(a), spew.NewFormatter(b))

func Fdump 

func Fdump(w io.Writer, a ...interface{})

Fdump formats and displays the passed arguments to io.Writer w. It formats exactly the same as Dump.

func Fprint 

func Fprint(w io.Writer, a ...interface{}) (n int, err error)

Fprint is a wrapper for fmt.Fprint that treats each argument as if it were passed with a default Formatter interface returned by NewFormatter. It returns the number of bytes written and any write error encountered. See NewFormatter for formatting details.

This function is shorthand for the following syntax: 

fmt.Fprint(w, spew.NewFormatter(a), spew.NewFormatter(b))

func Fprintf 

func Fprintf(w io.Writer, format string, a ...interface{}) (n int, err error)

Fprintf is a wrapper for fmt.Fprintf that treats each argument as if it were passed with a default Formatter interface returned by NewFormatter. It returns the number of bytes written and any write error encountered. See NewFormatter for formatting details.

This function is shorthand for the following syntax: 

fmt.Fprintf(w, format, spew.NewFormatter(a), spew.NewFormatter(b))

func Fprintln 

func Fprintln(w io.Writer, a ...interface{}) (n int, err error)

Fprintln is a wrapper for fmt.Fprintln that treats each argument as if it passed with a default Formatter interface returned by NewFormatter. See NewFormatter for formatting details.

This function is shorthand for the following syntax: 

fmt.Fprintln(w, spew.NewFormatter(a), spew.NewFormatter(b))

func NewFormatter 

func NewFormatter(v interface{}) fmt.Formatter

NewFormatter returns a custom formatter that satisfies the fmt.Formatter interface. As a result, 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).

Typically this function shouldn't be called directly. It is much easier to make use of the custom formatter by calling one of the convenience functions such as Printf, Println, or Fprintf.

func Print 

func Print(a ...interface{}) (n int, err error)

Print is a wrapper for fmt.Print that treats each argument as if it were passed with a default Formatter interface returned by NewFormatter. It returns the number of bytes written and any write error encountered. See NewFormatter for formatting details.

This function is shorthand for the following syntax: 

fmt.Print(spew.NewFormatter(a), spew.NewFormatter(b))

func Printf 

func Printf(format string, a ...interface{}) (n int, err error)

Printf is a wrapper for fmt.Printf that treats each argument as if it were passed with a default Formatter interface returned by NewFormatter. It returns the number of bytes written and any write error encountered. See NewFormatter for formatting details.

This function is shorthand for the following syntax: 

fmt.Printf(format, spew.NewFormatter(a), spew.NewFormatter(b))

func Println 

func Println(a ...interface{}) (n int, err error)

Println is a wrapper for fmt.Println that treats each argument as if it were passed with a default Formatter interface returned by NewFormatter. It returns the number of bytes written and any write error encountered. See NewFormatter for formatting details.

This function is shorthand for the following syntax: 

fmt.Println(spew.NewFormatter(a), spew.NewFormatter(b))

func Sdump 

func Sdump(a ...interface{}) string

Sdump returns a string with the passed arguments formatted exactly the same as Dump.

func Sprint 

func Sprint(a ...interface{}) string

Sprint is a wrapper for fmt.Sprint that treats each argument as if it were passed with a default Formatter interface returned by NewFormatter. It returns the resulting string. See NewFormatter for formatting details.

This function is shorthand for the following syntax: 

fmt.Sprint(spew.NewFormatter(a), spew.NewFormatter(b))

func Sprintf 

func Sprintf(format string, a ...interface{}) string

Sprintf is a wrapper for fmt.Sprintf that treats each argument as if it were passed with a default Formatter interface returned by NewFormatter. It returns the resulting string. See NewFormatter for formatting details.

This function is shorthand for the following syntax: 

fmt.Sprintf(format, spew.NewFormatter(a), spew.NewFormatter(b))

func Sprintln 

func Sprintln(a ...interface{}) string

Sprintln is a wrapper for fmt.Sprintln that treats each argument as if it were passed with a default Formatter interface returned by NewFormatter. It returns the resulting string. See NewFormatter for formatting details.

This function is shorthand for the following syntax: 

fmt.Sprintln(spew.NewFormatter(a), spew.NewFormatter(b))

Types

type ConfigState 

type ConfigState struct {
	// Indent specifies the string to use for each indentation level.  The
	// global config instance that all top-level functions use set this to a
	// single space by default.  If you would like more indentation, you might
	// set this to a tab with "\t" or perhaps two spaces with "  ".
	Indent string

	// MaxDepth controls the maximum number of levels to descend into nested
	// data structures.  The default, 0, means there is no limit.
	//
	// NOTE: Circular data structures are properly detected, so it is not
	// necessary to set this value unless you specifically want to limit deeply
	// nested data structures.
	MaxDepth int

	// DisableMethods specifies whether or not error and Stringer interfaces are
	// invoked for types that implement them.
	DisableMethods bool

	// DisablePointerMethods specifies whether or not to check for and invoke
	// error and Stringer interfaces on types which only accept a pointer
	// receiver when the current type is not a pointer.
	//
	// NOTE: This might be an unsafe action since calling one of these methods
	// with a pointer receiver could technically mutate the value, however,
	// in practice, types which choose to satisify an error or Stringer
	// interface with a pointer receiver should not be mutating their state
	// inside these interface methods.  As a result, 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 "disableunsafe" build tag specified.
	DisablePointerMethods bool

	// ContinueOnMethod specifies whether or not recursion should continue once
	// a custom error or Stringer interface is invoked.  The default, false,
	// means it will print the results of invoking the custom error or Stringer
	// interface and return immediately instead of continuing to recurse into
	// the internals of the data type.
	//
	// NOTE: This flag does not have any effect if method invocation is disabled
	// via the DisableMethods or DisablePointerMethods options.
	ContinueOnMethod bool

	// 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
	// that support the error or Stringer interfaces (if methods are
	// enabled) are supported, with other types sorted according to the
	// reflect.Value.String() output which guarantees display stability.
	SortKeys bool

	// 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.
	SpewKeys bool
}

ConfigState houses the configuration options used by spew to format and display values. There is a global instance, Config, that is used to control all top-level Formatter and Dump functionality. Each ConfigState instance provides methods equivalent to the top-level functions.

The zero value for ConfigState provides no indentation. You would typically want to set it to a space or a tab.

Alternatively, you can use NewDefaultConfig to get a ConfigState instance with default settings. See the documentation of NewDefaultConfig for default values.

func NewDefaultConfig 

func NewDefaultConfig() *ConfigState

NewDefaultConfig returns a ConfigState with the following default settings. 

Indent: " "
MaxDepth: 0
DisableMethods: false
DisablePointerMethods: false
ContinueOnMethod: false
SortKeys: false

func (*ConfigState) Dump 

func (c *ConfigState) Dump(a ...interface{})

Dump displays the passed parameters to standard out with newlines, customizable indentation, and additional debug information such as complete types and all pointer addresses used to indirect to the final value. It provides the following features over the built-in printing facilities provided by the fmt package:

  • 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 variables
  • Byte arrays and slices are dumped like the hexdump -C command which includes offsets, byte values in hex, and ASCII output

The configuration options are controlled by modifying the public members of c. See ConfigState for options documentation.

See Fdump if you would prefer dumping to an arbitrary io.Writer or Sdump to get the formatted result as a string.

func (*ConfigState) Errorf 

func (c *ConfigState) Errorf(format string, a ...interface{}) (err error)

Errorf is a wrapper for fmt.Errorf that treats each argument as if it were passed with a Formatter interface returned by c.NewFormatter. It returns the formatted string as a value that satisfies error. See NewFormatter for formatting details.

This function is shorthand for the following syntax: 

fmt.Errorf(format, c.NewFormatter(a), c.NewFormatter(b))

func (*ConfigState) Fdump 

func (c *ConfigState) Fdump(w io.Writer, a ...interface{})

Fdump formats and displays the passed arguments to io.Writer w. It formats exactly the same as Dump.

func (*ConfigState) Fprint 

func (c *ConfigState) Fprint(w io.Writer, a ...interface{}) (n int, err error)

Fprint is a wrapper for fmt.Fprint that treats each argument as if it were passed with a Formatter interface returned by c.NewFormatter. It returns the number of bytes written and any write error encountered. See NewFormatter for formatting details.

This function is shorthand for the following syntax: 

fmt.Fprint(w, c.NewFormatter(a), c.NewFormatter(b))

func (*ConfigState) Fprintf 

func (c *ConfigState) Fprintf(w io.Writer, format string, a ...interface{}) (n int, err error)

Fprintf is a wrapper for fmt.Fprintf that treats each argument as if it were passed with a Formatter interface returned by c.NewFormatter. It returns the number of bytes written and any write error encountered. See NewFormatter for formatting details.

This function is shorthand for the following syntax: 

fmt.Fprintf(w, format, c.NewFormatter(a), c.NewFormatter(b))

func (*ConfigState) Fprintln 

func (c *ConfigState) Fprintln(w io.Writer, a ...interface{}) (n int, err error)

Fprintln is a wrapper for fmt.Fprintln that treats each argument as if it passed with a Formatter interface returned by c.NewFormatter. See NewFormatter for formatting details.

This function is shorthand for the following syntax: 

fmt.Fprintln(w, c.NewFormatter(a), c.NewFormatter(b))

func (*ConfigState) NewFormatter 

func (c *ConfigState) NewFormatter(v interface{}) fmt.Formatter

NewFormatter returns a custom formatter that satisfies the fmt.Formatter interface. As a result, 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), and %#+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).

Typically this function shouldn't be called directly. It is much easier to make use of the custom formatter by calling one of the convenience functions such as c.Printf, c.Println, or c.Printf.

func (*ConfigState) Print 

func (c *ConfigState) Print(a ...interface{}) (n int, err error)

Print is a wrapper for fmt.Print that treats each argument as if it were passed with a Formatter interface returned by c.NewFormatter. It returns the number of bytes written and any write error encountered. See NewFormatter for formatting details.

This function is shorthand for the following syntax: 

fmt.Print(c.NewFormatter(a), c.NewFormatter(b))

func (*ConfigState) Printf 

func (c *ConfigState) Printf(format string, a ...interface{}) (n int, err error)

Printf is a wrapper for fmt.Printf that treats each argument as if it were passed with a Formatter interface returned by c.NewFormatter. It returns the number of bytes written and any write error encountered. See NewFormatter for formatting details.

This function is shorthand for the following syntax: 

fmt.Printf(format, c.NewFormatter(a), c.NewFormatter(b))

func (*ConfigState) Println 

func (c *ConfigState) Println(a ...interface{}) (n int, err error)

Println is a wrapper for fmt.Println that treats each argument as if it were passed with a Formatter interface returned by c.NewFormatter. It returns the number of bytes written and any write error encountered. See NewFormatter for formatting details.

This function is shorthand for the following syntax: 

fmt.Println(c.NewFormatter(a), c.NewFormatter(b))

func (*ConfigState) Sdump 

func (c *ConfigState) Sdump(a ...interface{}) string

Sdump returns a string with the passed arguments formatted exactly the same as Dump.

func (*ConfigState) Sprint 

func (c *ConfigState) Sprint(a ...interface{}) string

Sprint is a wrapper for fmt.Sprint that treats each argument as if it were passed with a Formatter interface returned by c.NewFormatter. It returns the resulting string. See NewFormatter for formatting details.

This function is shorthand for the following syntax: 

fmt.Sprint(c.NewFormatter(a), c.NewFormatter(b))

func (*ConfigState) Sprintf 

func (c *ConfigState) Sprintf(format string, a ...interface{}) string

Sprintf is a wrapper for fmt.Sprintf that treats each argument as if it were passed with a Formatter interface returned by c.NewFormatter. It returns the resulting string. See NewFormatter for formatting details.

This function is shorthand for the following syntax: 

fmt.Sprintf(format, c.NewFormatter(a), c.NewFormatter(b))

func (*ConfigState) Sprintln 

func (c *ConfigState) Sprintln(a ...interface{}) string

Sprintln is a wrapper for fmt.Sprintln that treats each argument as if it were passed with a Formatter interface returned by c.NewFormatter. It returns the resulting string. See NewFormatter for formatting details.

This function is shorthand for the following syntax: 

fmt.Sprintln(c.NewFormatter(a), c.NewFormatter(b))

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