205 lines
7 KiB
Markdown
205 lines
7 KiB
Markdown
go-spew
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=======
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[]
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(https://travis-ci.org/davecgh/go-spew) [![ISC License]
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(http://img.shields.io/badge/license-ISC-blue.svg)](http://copyfree.org) [![Coverage Status]
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(https://img.shields.io/coveralls/davecgh/go-spew.svg)]
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(https://coveralls.io/r/davecgh/go-spew?branch=master)
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Go-spew implements a deep pretty printer for Go data structures to aid in
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debugging. A comprehensive suite of tests with 100% test coverage is provided
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to ensure proper functionality. See `test_coverage.txt` for the gocov coverage
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report. Go-spew is licensed under the liberal ISC license, so it may be used in
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open source or commercial projects.
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If you're interested in reading about how this package came to life and some
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of the challenges involved in providing a deep pretty printer, there is a blog
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post about it
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[here](https://web.archive.org/web/20160304013555/https://blog.cyphertite.com/go-spew-a-journey-into-dumping-go-data-structures/).
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## Documentation
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[]
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(http://godoc.org/github.com/davecgh/go-spew/spew)
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Full `go doc` style documentation for the project can be viewed online without
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installing this package by using the excellent GoDoc site here:
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http://godoc.org/github.com/davecgh/go-spew/spew
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You can also view the documentation locally once the package is installed with
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the `godoc` tool by running `godoc -http=":6060"` and pointing your browser to
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http://localhost:6060/pkg/github.com/davecgh/go-spew/spew
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## Installation
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```bash
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$ go get -u github.com/davecgh/go-spew/spew
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```
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## Quick Start
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Add this import line to the file you're working in:
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```Go
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import "github.com/davecgh/go-spew/spew"
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```
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To dump a variable with full newlines, indentation, type, and pointer
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information use Dump, Fdump, or Sdump:
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```Go
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spew.Dump(myVar1, myVar2, ...)
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spew.Fdump(someWriter, myVar1, myVar2, ...)
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str := spew.Sdump(myVar1, myVar2, ...)
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```
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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 %v (most
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compact), %+v (adds pointer addresses), %#v (adds types), or %#+v (adds types
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and pointer addresses):
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```Go
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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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```
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## Debugging a Web Application Example
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Here is an example of how you can use `spew.Sdump()` to help debug a web application. Please be sure to wrap your output using the `html.EscapeString()` function for safety reasons. You should also only use this debugging technique in a development environment, never in production.
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```Go
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package main
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import (
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"fmt"
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"html"
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"net/http"
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"github.com/davecgh/go-spew/spew"
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)
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func handler(w http.ResponseWriter, r *http.Request) {
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w.Header().Set("Content-Type", "text/html")
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fmt.Fprintf(w, "Hi there, %s!", r.URL.Path[1:])
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fmt.Fprintf(w, "<!--\n" + html.EscapeString(spew.Sdump(w)) + "\n-->")
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}
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func main() {
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http.HandleFunc("/", handler)
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http.ListenAndServe(":8080", nil)
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}
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```
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## Sample Dump Output
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```
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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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([]uint8) {
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00000000 11 12 13 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f 20 |............... |
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00000010 21 22 23 24 25 26 27 28 29 2a 2b 2c 2d 2e 2f 30 |!"#$%&'()*+,-./0|
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00000020 31 32 |12|
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}
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```
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## Sample Formatter Output
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Double pointer to a uint8:
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```
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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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```
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Pointer to circular struct with a uint8 field and a pointer to itself:
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```
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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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```
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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 via the
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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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```
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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. This option
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relies on access to the unsafe package, so it will not have any effect when
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running in environments without access to the unsafe package such as Google
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App Engine or with the "safe" build tag specified.
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Pointer method invocation is enabled by default.
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* DisablePointerAddresses
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DisablePointerAddresses specifies whether to disable the printing of
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pointer addresses. This is useful when diffing data structures in tests.
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* DisableCapacities
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DisableCapacities specifies whether to disable the printing of capacities
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for arrays, slices, maps and channels. This is useful when diffing data
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structures in tests.
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* ContinueOnMethod
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Enables recursion into types after invoking error and Stringer interface
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methods. Recursion after method invocation is disabled by default.
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* SortKeys
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Specifies map keys should be sorted before being printed. Use
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this to have a more deterministic, diffable output. Note that
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only native types (bool, int, uint, floats, uintptr and string)
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and types which implement error or Stringer interfaces are supported,
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with other types sorted according to the reflect.Value.String() output
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which guarantees display stability. Natural map order is used by
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default.
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* SpewKeys
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SpewKeys specifies that, as a last resort attempt, map keys should be
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spewed to strings and sorted by those strings. This is only considered
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if SortKeys is true.
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```
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## Unsafe Package Dependency
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This package relies on the unsafe package to perform some of the more advanced
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features, however it also supports a "limited" mode which allows it to work in
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environments where the unsafe package is not available. By default, it will
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operate in this mode on Google App Engine and when compiled with GopherJS. The
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"safe" build tag may also be specified to force the package to build without
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using the unsafe package.
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## License
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Go-spew is licensed under the [copyfree](http://copyfree.org) ISC License.
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