An example on how to test a client which calls out to an external API, without requiring the API server to be up and running. From Testing Techniques video by Andrew Gerrand at Google I/O 2014.

Go has a standard library net/http/httptest which you can use to create a test HTTP server, similar to Go’s normal HTTP server. The test server will create a server that listens locally on a random port.

A test consists of

  • Create test server
  • Setup server to return what you want
  • Pass server URL to your test

Here’s a simple example from my fetcher library which tests retrieving a URL with a GET parameter.

I setup my test server to just echo back whatever parameter gets passed in, so then I just need to confirm what I pass in, is what I get back.

Create test fetcher_test.go and run from my package dir using go test

    // This tests GET request with passing in a parameter.
    func TestGetParams(t *testing.T) {

        // echoHandler, passes back form parameter p
        echoHandler := func( w http.ResponseWriter, r *http.Request) {
            fmt.Fprint(w, r.FormValue("p"))

        // create test server with handler
        ts := httptest.NewServer(http.HandlerFunc(echoHandler))
        defer ts.Close()

        // call library I want to test, using test server ts
        f := fetcher.NewFetcher()
        f.Params.Add("p", "hello")
        result, err := f.Fetch(ts.URL,"GET")
        if err != nil {
            t.Errorf("Error: %v", err)

        // confirm result
        if result != "hello" {
            t.Errorf("Unexpected result: %v", result)

That’s all there is to it, check out the video for a ton more useful information around testing in Go. Plus read the testing and httptest package documentation.