How to specify default values when parsing JSON in Go

Question

I want to parse a JSON object in Go, but want to specify default values for fields that are not given. For example, I have the struct type:

type Test struct {
    A string
    B string
    C string
}

The default values for A, B, and C, are "a", "b", and "c" respectively. This means that when I parse the json:

{"A": "1", "C": 3}

I want to get the struct:

Test{A: "1", B: "b", C: "3"}

Is this possible using the built-in package encoding/json? Otherwise, is there any Go library that has this functionality?

rustyMagnet · Accepted Answer · 2022-03-29 08:23:56Z

102

This is possible using encoding/json: when calling json.Unmarshal, you do not need to give it an empty struct, you can give it one with default values.

For your example:

var example []byte = []byte(`{"A": "1", "C": "3"}`)

out := Test{
    A: "default a",
    B: "default b",
    // default for C will be "", the empty value for a string
}
err := json.Unmarshal(example, &out) // <--
if err != nil {
    panic(err)
}
fmt.Printf("%+v", out)

Running this example returns {A:1 B:default b C:3}.

As you can see, json.Unmarshal(example, &out) unmarshals the JSON into out, overwriting the values specified in the JSON, but leaving the other fields unchanged.

edited Mar 29, 2022 at 8:23

rustyMagnet

4,2853 gold badges42 silver badges50 bronze badges

answered May 25, 2015 at 20:18

JW.

3,1062 gold badges28 silver badges26 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Donald Over a year ago

what if out is an array and you don't know how many elements are there in json.

Christian · Accepted Answer · 2021-08-03 07:22:30Z

33

In case u have a list or map of Test structs the accepted answer is not possible anymore but it can easily be extended with a UnmarshalJSON method:

func (t *Test) UnmarshalJSON(data []byte) error {
  type testAlias Test
  test := &testAlias{
    B: "default B",
  }

  err := json.Unmarshal(data, test)
  if err != nil {
      return err
  }

  *t = Test(*test)
  return nil
}

The testAlias is needed to prevent recursive calls to UnmarshalJSON. This works because a new type has no methods defined.

https://play.golang.org/p/qiGyjRbNHg

edited Aug 3, 2021 at 7:22

answered Dec 12, 2016 at 14:23

Christian

3,7311 gold badge32 silver badges26 bronze badges

Comments

Elviss Strazdins · Accepted Answer · 2022-09-22 04:14:10Z

5

You can also implement the UnmarshalJSON interface and set the default values in it.

func (test *Test) UnmarshalJSON(data []byte) error {
    test.A = "1"
    test.B = "2"
    test.C = "3"

    type tempTest Test
    return json.Unmarshal(data, (*tempTest)(test))
}

The type tempTest is needed so that UnmarshalJSON is not called recursively.

edited Sep 22, 2022 at 4:14

answered Sep 22, 2022 at 4:08

Elviss Strazdins

1,50416 silver badges31 bronze badges

Comments

zangw · Accepted Answer · 2022-07-06 05:50:18Z

The json.Unmarshal with a default value is simple and clean like the answers given by Christian and JW., but it has some downsides.

First, it strongly ties the default values of fields with the parsing logic. It's conceivable that we want to let user code down the line set its defaults; right now, the defaults have to be set before unmarshaling.
The second downside is that it only works in simple cases. If our Options struct has a slice or map of other structs, we can't populate defaults this way.

Another option is Default values with pointer fields

    type Test struct {
        A *string
        B *string
        C *string
    }

    js := []byte(`{"A": "1", "C": "3"}`)

    var t Test
    if err := json.Unmarshal(js, &t); err != nil {
        fmt.Println(err)
    }

    if t.B == nil {
        var defaultB = "B"
        t.B = &defaultB
    }

Collectives™ on Stack Overflow

How to specify default values when parsing JSON in Go

4 Answers 4

1 Comment

Comments

Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

1 Comment

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related