How can I pretty-print JSON using Go?

Does anyone know of a simple way to pretty-print JSON output in Go?

The stock http://golang.org/pkg/encoding/json/ package does not seem to include functionality for this (EDIT: it does, see accepted answer) and a quick google doesn't turn up anything obvious.

Uses I'm looking for are both pretty-printing the result of json.Marshal and just formatting a string full of JSON from wherever, so it's easier to read for debug purposes.

By pretty-print, I assume you mean indented, like so

{
    "data": 1234
}

rather than

{"data":1234}

The easiest way to do this is with MarshalIndent, which will let you specify how you would like it indented via the indent argument. Thus, json.MarshalIndent(data, "", " ") will pretty-print using four spaces for indentation.

The accepted answer is great if you have an object you want to turn into JSON. The question also mentions pretty-printing just any JSON string, and that's what I was trying to do. I just wanted to pretty-log some JSON from a POST request (specifically a CSP violation report).

To use MarshalIndent, you would have to Unmarshal that into an object. If you need that, go for it, but I didn't. If you just need to pretty-print a byte array, plain Indent is your friend.

Here's what I ended up with:

import (
	"bytes"
	"encoding/json"
	"log"
	"net/http"
)

func HandleCSPViolationRequest(w http.ResponseWriter, req *http.Request) {
	body := App.MustReadBody(req, w)
	if body == nil {
		return
	}

	var prettyJSON bytes.Buffer
	error := json.Indent(&prettyJSON, body, "", "\t")
	if error != nil {
		log.Println("JSON parse error: ", error)
		App.BadRequest(w)
		return
	}

	log.Println("CSP Violation:", string(prettyJSON.Bytes()))
}

For better memory usage, I guess this is better:

var out io.Writer
enc := json.NewEncoder(out)
enc.SetIndent("", "    ")
if err := enc.Encode(data); err != nil {
	panic(err)
}

I was frustrated by the lack of a fast, high quality way to marshal JSON to a colorized string in Go so I wrote my own Marshaller called ColorJSON.

With it, you can easily produce output like this using very little code:

package main

import (
    "fmt"
    "encoding/json"

    "github.com/TylerBrock/colorjson"
)

func main() {
    str := `{
      "str": "foo",
      "num": 100,
      "bool": false,
      "null": null,
      "array": ["foo", "bar", "baz"],
      "obj": { "a": 1, "b": 2 }
    }`

    var obj map[string]interface{}
    json.Unmarshal([]byte(str), &obj)

    // Make a custom formatter with indent set
    f := colorjson.NewFormatter()
    f.Indent = 4

    // Marshall the Colorized JSON
    s, _ := f.Marshal(obj)
    fmt.Println(string(s))
}

I'm writing the documentation for it now but I was excited to share my solution.

Edit Looking back, this is non-idiomatic Go. Small helper functions like this add an extra step of complexity. In general, the Go philosophy prefers to include the 3 simple lines over 1 tricky line.

---

As @robyoder mentioned, json.Indent is the way to go. Thought I'd add this small prettyprint function:

package main

import (
	"bytes"
	"encoding/json"
	"fmt"
)

//dont do this, see above edit
func prettyprint(b []byte) ([]byte, error) {
	var out bytes.Buffer
	err := json.Indent(&out, b, "", "  ")
	return out.Bytes(), err
}

func main() {
	b := []byte(`{"hello": "123"}`)
	b, _ = prettyprint(b)
	fmt.Printf("%s", b)
}

https://go-sandbox.com/#/R4LWpkkHIN or http://play.golang.org/p/R4LWpkkHIN

Here's what I use. If it fails to pretty print the JSON it just returns the original string. Useful for printing HTTP responses that should contain JSON.

import (
    "encoding/json"
    "bytes"
)

func jsonPrettyPrint(in string) string {
	var out bytes.Buffer
	err := json.Indent(&out, []byte(in), "", "\t")
	if err != nil {
		return in
	}
	return out.String()
}

package cube

import (
	"encoding/json"
	"fmt"
	"github.com/magiconair/properties/assert"
	"k8s.io/api/rbac/v1beta1"
	v1 "k8s.io/apimachinery/pkg/apis/meta/v1"
	"testing"
)

func TestRole(t *testing.T)  {
	clusterRoleBind := &v1beta1.ClusterRoleBinding{
		ObjectMeta: v1.ObjectMeta{
			Name: "serviceaccounts-cluster-admin",
		},
		RoleRef: v1beta1.RoleRef{
			APIGroup: "rbac.authorization.k8s.io",
			Kind:     "ClusterRole",
			Name:     "cluster-admin",
		},
		Subjects: []v1beta1.Subject{{
			Kind:     "Group",
			APIGroup: "rbac.authorization.k8s.io",
			Name:     "system:serviceaccounts",
		},
		},
	}
	b, err := json.MarshalIndent(clusterRoleBind, "", "  ")
	assert.Equal(t, nil, err)
	fmt.Println(string(b))
}

Here is my solution:

import (
    "bytes"
    "encoding/json"
)

const (
    empty = ""
    tab   = "\t"
)

func PrettyJson(data interface{}) (string, error) {
    buffer := new(bytes.Buffer)
    encoder := json.NewEncoder(buffer)
    encoder.SetIndent(empty, tab)

	err := encoder.Encode(data)
    if err != nil {
	   return empty, err
    }
	return buffer.String(), nil
}

//You can do it with json.MarshalIndent(data, "", "  ")

package main

import(
  "fmt"
  "encoding/json" //Import package
)

//Create struct
type Users struct {
    ID   int
    NAME string
}

//Asign struct
var user []Users
func main() {
 //Append data to variable user
 user = append(user, Users{1, "Saturn Rings"})
 //Use json package the blank spaces are for the indent
 data, _ := json.MarshalIndent(user, "", "  ")
 //Print json formatted
 fmt.Println(string(data))
}

Another example with http.ResponseWriter.

import (
	"encoding/json"
	"net/http"
)

func main() {
	var w http.ResponseWriter

	type About struct {
		ProgName string
		Version string
	}
	goObj := About{ProgName: "demo", Version: "0.0.0"}
	beautifulJsonByte, err := json.MarshalIndent(goObj, "", "  ")
	if err != nil {
		panic(err)
	}
	_, _ = w.Write(beautifulJsonByte)
}

output

{
  "ProgName": "demo",
  "Version": "0.0.0"
}

A simple off the shelf pretty printer in Go. One can compile it to a binary through:

go build -o jsonformat jsonformat.go

It reads from standard input, writes to standard output and allow to set indentation:

package main

import (
	"bytes"
	"encoding/json"
	"flag"
	"fmt"
	"io/ioutil"
	"os"
)

func main() {
	indent := flag.String("indent", "  ", "indentation string/character for formatter")
	flag.Parse()
	src, err := ioutil.ReadAll(os.Stdin)
	if err != nil {
		fmt.Fprintf(os.Stderr, "problem reading: %s", err)
		os.Exit(1)
	}

	dst := &bytes.Buffer{}
	if err := json.Indent(dst, src, "", *indent); err != nil {
		fmt.Fprintf(os.Stderr, "problem formatting: %s", err)
		os.Exit(1)
	}
	if _, err = dst.WriteTo(os.Stdout); err != nil {
		fmt.Fprintf(os.Stderr, "problem writing: %s", err)
		os.Exit(1)
	}
}

It allows to run a bash commands like:

cat myfile | jsonformat | grep "key"

If you want to create a commandline utility to pretty print JSON

package main

import ("fmt"
  "encoding/json"
  "os"
  "bufio"
  "bytes"
)


func main(){

	var out bytes.Buffer

	reader := bufio.NewReader(os.Stdin)
	text, _ := reader.ReadString('\n')

	err := json.Indent(&out, []byte(text), "", "  ")
	if err != nil {
	  fmt.Println(err)
	}

	fmt.Println(string(out.Bytes()))
}

echo "{\"boo\":\"moo\"}" | go run main.go 

will produce the following output :

{
  "boo": "moo"
}

feel free to build a binary

go build main.go

and drop it in /usr/local/bin

i am sort of new to go, but this is what i gathered up so far:

package srf

import (
	"bytes"
	"encoding/json"
	"os"
)

func WriteDataToFileAsJSON(data interface{}, filedir string) (int, error) {
	//write data as buffer to json encoder
	buffer := new(bytes.Buffer)
	encoder := json.NewEncoder(buffer)
	encoder.SetIndent("", "\t")

	err := encoder.Encode(data)
	if err != nil {
		return 0, err
	}
	file, err := os.OpenFile(filedir, os.O_RDWR|os.O_CREATE, 0755)
	if err != nil {
		return 0, err
	}
	n, err := file.Write(buffer.Bytes())
	if err != nil {
		return 0, err
	}
	return n, nil
}

This is the execution of the function, and just standard

b, _ := json.MarshalIndent(SomeType, "", "\t")

Code:

package main

import (
	"encoding/json"
	"fmt"
	"io/ioutil"
	"log"

	minerals "./minerals"
	srf "./srf"
)

func main() {

	//array of Test struct
	var SomeType [10]minerals.Test

	//Create 10 units of some random data to write
	for a := 0; a < 10; a++ {
		SomeType[a] = minerals.Test{
			Name:   "Rand",
			Id:     123,
			A:      "desc",
			Num:    999,
			Link:   "somelink",
			People: []string{"John Doe", "Aby Daby"},
		}
	}

	//writes aditional data to existing file, or creates a new file
	n, err := srf.WriteDataToFileAsJSON(SomeType, "test2.json")
	if err != nil {
		log.Fatal(err)
	}
	fmt.Println("srf printed ", n, " bytes to ", "test2.json")
	
	//overrides previous file
	b, _ := json.MarshalIndent(SomeType, "", "\t")
	ioutil.WriteFile("test.json", b, 0644)

}

Use json.MarshalIndent with string

This easyPrint function accepts argument data (any type of data) to print it into the intended (pretty) JSON format.

import (
  "encoding/json"
  "log"
)

func easyPrint(data interface{}) {
  manifestJson, _ := json.MarshalIndent(data, "", "  ")

  log.Println(string(manifestJson))
}

With name argument.

TODO: make argument name optional.

func easyPrint(data interface{}, name string) {
  manifestJson, _ := json.MarshalIndent(data, "", "  ")

  log.Println(name + " ->", string(manifestJson))
}