How Can I Achieve Monkey Patching-like Behavior in Go for Easier Testing?

Monkey Patching in Go: A Way to Modify Objects at Runtime

In Go, when working with a heavily interconnected code base that lacks interfaces or dependency injection, testing or benchmarking can become challenging due to the inability to mock or swap out components. However, there are techniques that resemble monkey patching in scripting languages like Python, enabling you to modify objects at runtime in Go.

One approach is to create a custom interface that wraps the original object and allows for mocking in tests. For instance, if you have a struct named Concrete that depends on a package called somepackage:

type Concrete struct {
    client *somepackage.Client
}

You can define your own interface MyInterface with the desired methods:

type MyInterface interface {
    DoSomething(i int) error
    DoSomethingElse() ([]int, error)
}

Then, implement this interface in a mock object:

type MockConcrete struct {
    DoSomethingCalled bool
    DoSomethingElseCalled bool
}

func (m *MockConcrete) DoSomething(i int) error {
    m.DoSomethingCalled = true
    return nil
}

func (m *MockConcrete) DoSomethingElse() ([]int, error) {
    m.DoSomethingElseCalled = true
    return []int{}, nil
}

In your tests, you can inject the mock object into Concrete and verify its behavior:

func TestDoSomething(t *testing.T) {
    mockConcrete := &MockConcrete{}
    c := &Concrete{client: mockConcrete}

    c.DoSomething(42)

    if !mockConcrete.DoSomethingCalled {
        t.Error("DoSomething was not called")
    }
}

Another technique is embedding the type you want to mock into your own struct. This allows you to override the desired methods for mocking while retaining access to the original object's other methods. For example:

type Concrete struct {
    *somepackage.Client
}

With this approach, you can directly call non-overridden methods like DoSomethingNotNeedingMocking on Concrete without having to mock them out.

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