Testing Clients to an HTTP API in Go

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

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.