How Can I Mock Imported Package Functions in Go for Effective Testing?

Mocking Functions Imported from Packages in Go

When testing Go methods that utilize functions imported from external packages, it may be necessary to mock these external functions for effective testing. One approach is to refactor the code and introduce an intermediate function call instead of directly calling the external function.

In the example provided:

import x.y.z

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

Create a new variable zSomeFunc of type func and initialize it with the external function z.SomeFunc. Then, have your package call zSomeFunc instead of z.SomeFunc.

var zSomeFunc = z.SomeFunc

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

Now, in your tests, you can assign a different function to zSomeFunc, one that is defined in the test and returns a predefined value or performs specific actions. This allows you to mock the behavior of the external function for testing purposes.

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

    zSomeFunc = func() int {
        // Do whatever you want to do and return whatever value you need.
        return 1
    }

    // Call the tested function.
    abc()

    // Test the expected behavior.
}

Alternatively, you can create a mock implementation of the x.y.z package and use the Go Mocking Framework to mock the SomeFunc() function specifically.

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