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

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

import x.y.z

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

Is it possible to mock SomeFunc() in Go?

Solution

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
}