testing function calling using var declaration to mock the result

Question

I'm testing a function which called another function in Go. And here's what I have

package b

type b struct {...}

func (b *b) functionB(){...}

package a

import "b"

type a struct {...}

func (a *a) functionA() {
    b := b{...}
    b.functionB()
    ...
}

I want to modify the function declaration in b like this:

package b

type b struct {...}

var functionB = b.FuncInB

func (b *b) FuncInB(){...}

so that I can mock the return of functionB in a. However, I got error message in a that says b.functionB is undefined because it should be the function of b object. How can I make this work?

You should redesign and stop mocking. You should test package b and assume it is working correctly while testing package a and just call methods of b. Mocking is a bad technique, especially in Go. — Volker, Nov 19 '19 at 08:24
@Volker I'm creating unit testing for my function and need to do this. I see another post about mocking function like this https://stackoverflow.com/questions/19167970/mock-functions-in-go — cainam, Nov 19 '19 at 08:31
It is not possible to call `b.functionB`, whether it's a function or a method, from `a` because it is *not* [exported](https://golang.org/ref/spec#Exported_identifiers). — mkopriva, Nov 19 '19 at 08:44
"[I] need to do this". That's why I recommend to redesign because you should not do this. — Volker, Nov 19 '19 at 09:07

mbuechmann · Answer 1 · 2019-11-19T08:29:30.897

If you want to use a definition of one package in another package you have to export it. This is done by letting the definition start with a capital letter. Your type should be defined like this:

type B struct {...}

You cannot access a function via its type. This line

var functionB = b.FuncInB

will not work, as you can refer to the function only from an existing struct. You shoule remove this line and call it differently (see below).

Your package a has now two minor issues. When you want to use struct B from package b you have to either refer to it like this b.B

import "b"

b.B{...}

or make a dot import for package b:

import . "b"

B{...}

The last thing is, that you shadow the package name when assigning a variable with the same name as the package. In that, you should use a different variable name:

func (a *a) functionA() {
    myB := b{...}
    myB.FunctionB()
    ...
}

So in the end, your packages should look like this:

package b

type B struct {
    // ...
}

func (b *B) FuncInB() {
    // ...
}

package a

import "b"

type a struct {
    // ...
}

func (a *a) functionA () {
    myB := B {
        // ...
    }
    myB.functionB()
    // ...
}

thank you for your reply but my problem is about the declaration using var in `b` — cainam, Nov 19 '19 at 08:35
Your are welcome. It is difficult to answer, because you state your problem in a very abstract way. If you could provide more details of what you actually want to achieve, it would be easier to answer. — mbuechmann, Nov 19 '19 at 08:38

testing function calling using var declaration to mock the result

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