Mock functions in Go

Solution 1:

Personally, I don't use gomock (or any mocking framework for that matter; mocking in Go is very easy without it). I would either pass a dependency to the downloader() function as a parameter, or I would make downloader() a method on a type, and the type can hold the get_page dependency:

Method 1: Pass get_page() as a parameter of downloader()

type PageGetter func(url string) string

func downloader(pageGetterFunc PageGetter) {
    // ...
    content := pageGetterFunc(BASE_URL)
    // ...
}

Main:

func get_page(url string) string { /* ... */ }

func main() {
    downloader(get_page)
}

Test:

func mock_get_page(url string) string {
    // mock your 'get_page()' function here
}

func TestDownloader(t *testing.T) {
    downloader(mock_get_page)
}

Method2: Make download() a method of a type Downloader:

If you don't want to pass the dependency as a parameter, you could also make get_page() a member of a type, and make download() a method of that type, which can then use get_page:

type PageGetter func(url string) string

type Downloader struct {
    get_page PageGetter
}

func NewDownloader(pg PageGetter) *Downloader {
    return &Downloader{get_page: pg}
}

func (d *Downloader) download() {
    //...
    content := d.get_page(BASE_URL)
    //...
}

Main:

func get_page(url string) string { /* ... */ }

func main() {
    d := NewDownloader(get_page)
    d.download()
}

Test:

func mock_get_page(url string) string {
    // mock your 'get_page()' function here
}

func TestDownloader() {
    d := NewDownloader(mock_get_page)
    d.download()
}

Solution 2:

If you change your function definition to use a variable instead:

var get_page = func(url string) string {
    ...
}

You can override it in your tests:

func TestDownloader(t *testing.T) {
    get_page = func(url string) string {
        if url != "expected" {
            t.Fatal("good message")
        }
        return "something"
    }
    downloader()
}

Careful though, your other tests might fail if they test the functionality of the function you override!

The Go authors use this pattern in the Go standard library to insert test hooks into code to make things easier to test:

• https://golang.org/src/net/hook.go
• https://golang.org/src/net/dial.go#L248
• https://golang.org/src/net/dial_test.go#L701