How do I mock a function from another package without using dependency injection?

Question

Somewhat of a golang beginner, but I've worked with testing frameworks before. How do I go about mocking out and faking what a dependent method returns without injecting the dependency? The reason why I don't want to use dependency injection is because there are many external package methods that are being used and injecting all of the methods in the constructor is unwieldy.

I've searched for this online/stackoverflow and the solution is to always use dependency injection. Sometimes that is not a viable option.

Here's what I'm trying to do code-wise:

b/b_test.go

package b

func TestResults(t *testing.T) {
    t.Run("Test", func(t *testing.T) {
        b := NewB()
        // How do I mock out and fake a.DoSomething() to be
        // "complete" instead of whats in the code right now?
        result = b.Results()
        assert.Equal(t, "complete", result)            
    }
}

b/b.go

package b

import "a"

type B struct {}

func NewB() B {
    return &B{}
}

func (b B) Results() {
    return a.DoSomething()
}

a/a.go

package a

func DoSomething() {
    return "done"
}

Thanks!

Answer 1

You can use conditional compilation with build tags

a/a.go

// +build !mock

package a
func DoSomething() {
    return "done"
}

a/a_mock.go

// +build mock

package a
func DoSomething() {  // Insert fake implementation here
    return "complete"
}

$ go test -tags mock