How can I pretty-print JSON using Go?

Question

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 functiona

Answer 1

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.

Answer 2

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.

Answer 3

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))
}

Answer 4

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"

Answer 5

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