Golang: Intercepting and Mocking an HTTP Response with httptest - go

Question

I've looked into various different tools that can be used for mock testing in golang, but I'm trying to accomplish this task using httptest. In particular, I have a function as such:
type contact struct {
username string
number int
}
func getResponse(c contact) string {
url := fmt.Sprintf("https://mywebsite/%s", c.username)
req, err := http.NewRequest(http.MethodGet, url, nil)
// error checking
resp, err := http.DefaultClient.Do(req)
// error checking
return response
}
A lot of the documentation I've read seems to require creating a client interface or a custom transport. Is there no way to mock a response in a test file without changing this main code at all? I want to keep my client, response, and all the related details within the getResponse function. I could have the wrong idea, but I'm trying to find a way to intercept the http.DefaultClient.Do(req) call and return a custom response, is that possible?

Answer 1

https://pkg.go.dev/net/http/httptest#example-Server is a good example for your use case with a small refactoring of your code.
You just have to change the getResponse() by getResponse(url string) to be able to give the server mock url.

Answer 2

I've read seems to require creating a client interface
without changing this main code at all
Keeping your code clean is a good practice and you'll finally get used to it, a testable code is cleaner and a cleaner code is more testable, so don't worry to change your code (using interfaces) so it can accept mock objects.
Your code in its simplest form can be like this:
package main
import (
"fmt"
"net/http"
)
type contact struct {
username string
number int
}
type Client interface {
Do(req *http.Request) (*http.Response, error)
}
func main() {
getResponse(http.DefaultClient, contact{})
}
func getResponse(client Client, c contact) string {
url := fmt.Sprintf("https://mywebsite/%s", c.username)
req, _ := http.NewRequest(http.MethodGet, url, nil)
// error checking
resp, _ := http.DefaultClient.Do(req)
// error checking and response processing
return response
}
And your test can be like this:
package main
import (
"net/http"
"testing"
)
type mockClient struct {
}
// Do function will cause mockClient to implement the Client interface
func (tc mockClient) Do(req *http.Request) (*http.Response, error) {
return &http.Response{}, nil
}
func TestGetResponse(t *testing.T) {
client := new(mockClient)
getResponse(client, contact{})
}
But if you prefer to use httptest:
package main
import (
"fmt"
"io"
"net/http"
"net/http/httptest"
)
type contact struct {
username string
number int
}
func main() {
fmt.Println(getResponse(contact{}))
}
func getResponse(c contact) string {
// Make a test server
ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
fmt.Fprintln(w, "your response")
}))
defer ts.Close()
// You should still set your base url
base_url := ts.URL
url := fmt.Sprintf("%s/%s", base_url, c.username)
req, _ := http.NewRequest(http.MethodGet, url, nil)
// Use ts.Client() instead of http.DefaultClient in your tests.
resp, _ := ts.Client().Do(req)
// Processing the response
response, _ := io.ReadAll(resp.Body)
resp.Body.Close()
return string(response)
}