spew
Spew implements a deep pretty printer for Go data structures to aid in debugging. A comprehensive suite of tests with 100% test coverage is provided to ensure proper functionality. Spew is licensed under the liberal ISC license, so it may be used in open source or commercial projects.
If you're interested in reading about how this package came to life and some of the challenges involved in providing a deep pretty printer, there is a blog post about it here.
Documentation
Full go doc style documentation for the project can be viewed online without installing this package by using the excellent GoDoc site here: http://godoc.org/github.com/spewerspew/spew
Installation
$ go get -u github.com/spewerspew/spew
Quick Start
Add this import line to the file you're working in:
import "github.com/spewerspew/spew"
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)
Debugging a Web Application Example
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.
package main
import (
"fmt"
"html"
"net/http"
"github.com/spewerspew/spew"
)
func handler(w http.ResponseWriter, r *http.Request) {
w.Header().Set("Content-Type", "text/html")
fmt.Fprintf(w, "Hi there, %s!", r.URL.Path[1:])
fmt.Fprintf(w, "<!--\n" + html.EscapeString(spew.Sdump(w)) + "\n-->")
}
func main() {
http.HandleFunc("/", handler)
http.ListenAndServe(":8080", nil)
}
Sample Dump Output
(main.Foo) {
unexportedField: (*main.Bar)(0xf84002e210)({
flag: (main.Flag) flagTwo,
data: (uintptr) <nil>
}),
ExportedField: (map[interface {}]interface {}) {
(string) "one": (bool) true
}
}
([]uint8) {
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|
}
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>}
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.
* 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. 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 "safe" build tag specified.
Pointer method invocation is enabled by default.
* DisablePointerAddresses
DisablePointerAddresses specifies whether to disable the printing of
pointer addresses. This is useful when diffing data structures in tests.
* DisableCapacities
DisableCapacities specifies whether to disable the printing of capacities
for arrays, slices, maps and channels. This is useful when diffing data
structures in tests.
* 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
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.
Unsafe Package Dependency
This package relies on the unsafe package to perform some of the more advanced features, however it also supports a "limited" mode which allows it to work in environments where the unsafe package is not available. By default, it will operate in this mode on Google App Engine and when compiled with GopherJS. The "safe" build tag may also be specified to force the package to build without using the unsafe package.
License
Spew is licensed under the copyfree ISC License.
