Is it possible to mock a function imported from a package in golang?

Question

I have the following method to test, which uses a function imported from a package.

import x.y.z

func abc() {
    ...
    v := z.SomeFunc()
    ... 
}


        

Answer 1

Yes, with a simple refactoring. Create a zSomeFunc variable of function type, initialized with z.SomeFunc, and have your package call that instead of z.SomeFunc():

var zSomeFunc = z.SomeFunc

func abc() {
    // ...
    v := zSomeFunc()
    // ...
}

In tests you may assign another function to zSomeFunc, one that is defined in tests, and does whatever the test wants it to.

For example:

func TestAbc(t *testing.T) {
    // Save current function and restore at the end:
    old := zSomeFunc
    defer func() { zSomeFunc = old }()

    zSomeFunc = func() int {
        // This will be called, do whatever you want to,
        // return whatever you want to
        return 1
    }

    // Call the tested function
    abc()

    // Check expected behavior
}

See related / possible duplicate: Testing os.Exit scenarios in Go with coverage information (coveralls.io/Goveralls)

Answer 2

Having function pointer and monkey patching it is the one of doing it. But then when you multiple functions to mock, you will have a number function pointers and unnecessarily you will have to call function using the function pointer only.

The better and the recommended idea to have an interface and make your function part of the struct implementing the interface. Once done that, you can generate mocks using some nice tools available for go.

I have been using this:

mockgen -source myModule.go -package myPackage -destination myModuleMock.go

You can install it by:

got get github.com/golang/mock

Answer 3

One thing you can do is this:

import "x/y/z"

var someFunc = z.SomeFunc

func abc() {
    ...
    v := someFunc()
    ... 
}

And in your test file you would do this.

func Test_abc() {
    someFunc = mockFunc
    abc()
}

But make sure that you do this in a concurrent manner, if you have multiple TestXxx functions calling abc or setting someFunc you may be better of using a struct with a someFunc field.