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

Partial Mocking for Imported Functions

In Go, testing a function that relies on functions imported from external packages can be challenging. Consider the following example:

import x.y.z

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

Can we mock z.SomeFunc() in Go?

Refactoring for Mocking

Yes, with a simple code modification. By introducing a function-typed variable zSomeFunc and initializing it with z.SomeFunc, the package code can call this variable instead of z.SomeFunc(). This allows us to mock the imported function during testing.

var zSomeFunc = z.SomeFunc

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

Mocking in Tests

In tests, we can assign a custom function to zSomeFunc that behaves as needed for testing.

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
}

By refactoring the code, we can mock imported functions and test their impact on our code more effectively. This technique can be particularly useful when testing third-party dependencies or isolating specific pieces of functionality for targeted testing.

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