169 lines
6.5 KiB
Go
169 lines
6.5 KiB
Go
/*
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* Copyright (c) 2013 Dave Collins <dave@davec.name>
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*
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* Permission to use, copy, modify, and distribute this software for any
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* purpose with or without fee is hereby granted, provided that the above
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* copyright notice and this permission notice appear in all copies.
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*
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* THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
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* WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
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* MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
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* ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
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* WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
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* ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
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* OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
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*/
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/*
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Package spew implements a deep pretty printer for Go data structures to aid in
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debugging.
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A quick overview of the additional features spew provides over the built-in
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printing facilities for Go data types are as follows:
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* Pointers are dereferenced and followed
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* Circular data structures are detected and handled properly
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* Custom Stringer/error interfaces are optionally invoked, including
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on unexported types
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* Custom types which only implement the Stringer/error interfaces via
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a pointer receiver are optionally invoked when passing non-pointer
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variables
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There are two different approaches spew allows for dumping Go data structures:
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* Dump style which prints with newlines, customizable indentation,
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and additional debug information such as types and all pointer addresses
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used to indirect to the final value
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* A custom Formatter interface that integrates cleanly with the standard fmt
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package and replaces %v, %+v, %#v, and %#+v to provide inline printing
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similar to the default %v while providing the additional functionality
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outlined above and passing unsupported format verbs such as %x and %q
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along to fmt
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Quick Start
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This section demonstrates how to quickly get started with spew. See the
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sections below for further details on formatting and configuration options.
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To dump a variable with full newlines, indentation, type, and pointer
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information use Dump or Fdump:
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spew.Dump(myVar1, myVar2, ...)
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spew.Fdump(someWriter, myVar1, myVar2, ...)
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Alternatively, if you would prefer to use format strings with a compacted inline
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printing style, use the convenience wrappers Printf, Fprintf, etc with
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%v (most compact), %+v (adds pointer addresses), %#v (adds types), or
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%#+v (adds types and pointer addresses):
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spew.Printf("myVar1: %v -- myVar2: %+v", myVar1, myVar2)
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spew.Printf("myVar3: %#v -- myVar4: %#+v", myVar3, myVar4)
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spew.Fprintf(someWriter, "myVar1: %v -- myVar2: %+v", myVar1, myVar2)
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spew.Fprintf(someWriter, "myVar3: %#v -- myVar4: %#+v", myVar3, myVar4)
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Configuration Options
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Configuration of spew is handled by fields in the ConfigState type. For
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convenience, all of the top-level functions use a global state available
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via the spew.Config global.
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It is also possible to create a ConfigState instance that provides methods
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equivalent to the top-level functions. This allows concurrent configuration
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options. See the ConfigState documentation for more details.
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The following configuration options are available:
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* Indent
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String to use for each indentation level for Dump functions.
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It is a single space by default. A popular alternative is "\t".
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* MaxDepth
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Maximum number of levels to descend into nested data structures.
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There is no limit by default.
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* DisableMethods
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Disables invocation of error and Stringer interface methods.
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Method invocation is enabled by default.
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* DisablePointerMethods
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Disables invocation of error and Stringer interface methods on types
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which only accept pointer receivers from non-pointer variables.
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Pointer method invocation is enabled by default.
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Dump Usage
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Simply call spew.Dump with a list of variables you want to dump:
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spew.Dump(myVar1, myVar2, ...)
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You may also call spew.Fdump if you would prefer to output to an arbitrary
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io.Writer. For example, to dump to standard error:
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spew.Fdump(os.Stderr, myVar1, myVar2, ...)
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Sample Dump Output
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See the Dump example for details on the setup of the types and variables being
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shown here.
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(main.Foo) {
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unexportedField: (*main.Bar)(0xf84002e210)({
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flag: (main.Flag) flagTwo,
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data: (uintptr) <nil>
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}),
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ExportedField: (map[interface {}]interface {}) {
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(string) "one": (bool) true
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}
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}
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Custom Formatter
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Spew provides a custom formatter the implements the fmt.Formatter interface
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so that it integrates cleanly with standard fmt package printing functions. The
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formatter is useful for inline printing of smaller data types similar to the
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standard %v format specifier.
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The custom formatter only responds to the %v (most compact), %+v (adds pointer
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addresses), %#v (adds types), or %#+v (adds types and pointer addresses) verb
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combinations. Any other verbs such as %x and %q will be sent to the the
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standard fmt package for formatting. In addition, the custom formatter ignores
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the width and precision arguments (however they will still work on the format
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specifiers not handled by the custom formatter).
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Custom Formatter Usage
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The simplest way to make use of the spew custom formatter is to call one of the
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convenience functions such as spew.Printf, spew.Println, or spew.Printf. The
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functions have syntax you are most likely already familiar with:
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spew.Printf("myVar1: %v -- myVar2: %+v", myVar1, myVar2)
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spew.Printf("myVar3: %#v -- myVar4: %#+v", myVar3, myVar4)
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spew.Println(myVar, myVar2)
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spew.Fprintf(os.Stderr, "myVar1: %v -- myVar2: %+v", myVar1, myVar2)
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spew.Fprintf(os.Stderr, "myVar3: %#v -- myVar4: %#+v", myVar3, myVar4)
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See the Index for the full list convenience functions.
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Sample Formatter Output
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Double pointer to a uint8:
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%v: <**>5
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%+v: <**>(0xf8400420d0->0xf8400420c8)5
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%#v: (**uint8)5
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%#+v: (**uint8)(0xf8400420d0->0xf8400420c8)5
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Pointer to circular struct with a uint8 field and a pointer to itself:
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%v: <*>{1 <*><shown>}
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%+v: <*>(0xf84003e260){ui8:1 c:<*>(0xf84003e260)<shown>}
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%#v: (*main.circular){ui8:(uint8)1 c:(*main.circular)<shown>}
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%#+v: (*main.circular)(0xf84003e260){ui8:(uint8)1 c:(*main.circular)(0xf84003e260)<shown>}
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See the Printf example for details on the setup of variables being shown
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here.
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Errors
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Since it is possible for custom Stringer/error interfaces to panic, spew
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detects them and handles them internally by printing the panic information
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inline with the output. Since spew is intended to provide deep pretty printing
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capabilities on structures, it intentionally does not return any errors.
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*/
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package spew
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