Writing tests for RESTful APIs in Python using requests – part 4: mocking responses

In this short series of blog posts, I want to explore the Python requests library and how it can be used for writing tests for RESTful APIs. This is the fourth blog post in the series, in which we will cover working mocking responses for unit testing purposes. Previous blog posts in this series talked about getting started with requests and pytest, about creating data driven tests and about working with XML-based APIs.

One thing that has been keeping me busy in the last couple of months is improving my software development skills. I’m currently working on a Python development project, and one of the tasks of a developer is writing good unit tests. When I was writing these tests, I ran into a challenge when I wanted to test a method that involves communicating with a REST API using the requests library.

Obviously, I don’t want to have to invoke the API itself in my unit tests, so I was looking for a way to mock out that dependency instead. One thing I considered was writing mocks for the API myself, until I stumbled upon the responses library (PyPI, GitHub). According to their homepage, this is ‘A utility library for mocking out the requests Python library’. Exactly what I was looking for.

So, what can you do with the responses library, and how can you use to your advantage when you’re writing unit tests? Let’s look at a couple of examples that involve creating mock responses for the Zippopotam.us API.

Creating a mock response
Let’s say that in our unit test, we want to test that our code handles an HTTP 404 returned by a REST API dependency as expected. This implies we need a way to ‘override’ the actual API response with a response that contains an HTTP 404 status code, and (maybe) a response body with an error message.

To use the responses library to create such a mock response, you’ll first have to add the @responses.activate decorator to your test method. In the test method body, you can then add a new mock response as follows:

def test_simulate_data_cannot_be_found():
        json={"error": "No data exists for US zip code 90210"},

When you use the requests library to perform an HTTP GET to http://api.zippopotam.us/us/90210, instead of the response from the live API (which will return an HTTP 200), you’ll receive the mock response, instead, which we can confirm like this:

response = requests.get("http://api.zippopotam.us/us/90210")
assert response.status_code == 404
response_body = response.json()
assert response_body["error"] == "No data exists for US zip code 90210"

You can add any number of mock responses in this way.

Unmapped responses
If, during testing, you accidentally hit an endpoint that does not have an associated mock response, you’ll get a ConnectionError:

def test_unmatched_endpoint_raises_connectionerror():
    with pytest.raises(ConnectionError):

Simulating an exception being thrown
If you want to test how your code handles an exception being thrown when you perform an API call using requests, you can do that using responses, too:

def test_responses_can_raise_error_on_demand():
        body=RuntimeError("A runtime error occurred")

You can confirm that this works as expected by asserting on the behaviour in a test:

with pytest.raises(RuntimeError) as re:
assert str(re.value) == "A runtime error occurred"

Creating dynamic responses
If you want to generate more complex and/or dynamic responses, you can do that by creating a callback and using that in your mock. This callback should return a tuple containing the response status code (an integer), the headers (a dictionary) and the response (in a string format).

In this example, I want to parse the request URL, extract the path parameters from it and then use those values in a message I return in the response body:

def test_using_a_callback_for_dynamic_responses():

    def request_callback(request):
        request_url = request.url
        resp_body = {"value": generate_response_from(request_url)}
        return 200, {}, json.dumps(resp_body)

        responses.GET, "http://api.zippopotam.us/us/55555",

    def generate_response_from(url):
        parsed_url = urlparse(url).path
        split_url = parsed_url.split("/")
        return f"You requested data for {split_url[1].upper()} zip code {split_url[2]}"

Again, writing a test confirms that this works as expected:

response = requests.get("http://api.zippopotam.us/us/55555")
assert response.json() == {"value": "You requested data for US zip code 55555"}

Plus, responses retains all calls made to the callback and the responses it returned, which is very useful when you want to verify that your code made the correct (number of) calls:

assert len(responses.calls) == 1
assert responses.calls[0].request.url == "http://api.zippopotam.us/us/55555"
assert responses.calls[0].response.text == "{"value": "You requested data for US zip code 55555"}"

Using the examples for yourself
The code examples I have used in this blog post can be found on my GitHub page. If you download the project and (given you have installed Python properly) run

pip install -r requirements.txt

from the root of the python-requests project to install the required libraries, you should be able to run the tests for yourself. See you next time!