How to mock a function to get a different return value in golang

Question

I'm writing unit tests for a function that I developed in golang. Here is the code snippet:

func myFunc () {
    //there is some code
    err := json.Unmarshal (a, &b)
    if err != nil {
        //handle error    
    }

    //there is some more code

    err := json.Unmarshal (c, &d)
    if err != nil {
        //handle error    
    }
}

Now I want to mock first unmarshal to return success and second unmarshal to return failure. In python, I see one similar post: Python mock multiple return values

But I cannot find one for golang. Can anyone please help me on this.

Unfortunately this is not that simple in Golang. The best bet would be to refactor your function so that `json.Unmarshal` can be forcefully made to return an error, e.g. when calling `json.Unmarshal(a, &b)` and a is not in json format. — Yerken, Jun 14 '17 at 23:15

score 2 · Answer 1 · answered Jun 15 '17 at 00:00

What I would do in this situation is have an exported package variable Unmarshaller and set it to json.Unmarshal in the init() method.

var Unmarshaller func(data []byte, v interface{}) error;

func init() {
    Unmarshaller = json.Unmarshal
}

Then, when you want to force an error, you can just do

mypackage.Unmarshaller = func(data[] byte, v interface{}) error {
    return errors.New("It broke!")
}

Then, in your code, instead of calling json.Unmarshal directly, you call your package level `Unmarshaller, so

err =: json.Unmarshal(jsonBytes, &test)

would become

err =: Unmarshaller(jsonBytes, &test)

As a short example: https://play.golang.org/p/CinLmprtp5

How to mock a function to get a different return value in golang

1 Answers1