mocker.spy

Learn how to use Pytest's mocker.spy for robust website testing

March 12, 2025 by cryan.com

Testing interactions with external services or complex internal functions can be tricky. You want to ensure your website behaves correctly without relying on the actual implementation, which might be slow, unreliable, or have side effects. That's where pytest-mock's spy comes in!

What is mocker.spy?

mocker.spy lets you wrap any callable (function, method, etc.) and record its calls. You can then assert how many times it was called, what arguments it received, and what values it returned. This is incredibly useful for verifying interactions without actually mocking the underlying implementation.

Why is it cool for website testing?

Imagine you have a website that:

• Logs user activity to an external analytics service.
• Sends emails for password resets.
• Interacts with a third-party API for data retrieval.

Instead of actually sending data to analytics, sending real emails, or hitting the live API, you can use mocker.spy to verify that these interactions occurred as expected.

A Practical Example: Tracking Analytics Events

Let's say your website has a function that logs user interactions to an analytics service:

# website/analytics.py
import requests
def track_event(user_id, event_name, event_data):
    try:
        requests.post("https://analytics.example.com/track", json={
            "user_id": user_id,
            "event_name": event_name,
            "event_data": event_data,
        })
    except requests.exceptions.RequestException as e:
        print(f"Error tracking event: {e}")
   

And your website's view function calls this:

# website/views.py
from website.analytics import track_event
def process_user_action(user_id, action_data):
    # ... process user action ...
    track_event(user_id, "user_action", action_data)
    # ... more logic ...

Here's how you can test it with mocker.spy:

# tests/test_views.py
from website.views import process_user_action
from website.analytics import track_event
def test_process_user_action_tracks_event(mocker):
    spy = mocker.spy(track_event)
    user_id = 123
    action_data = {"item_id": 456}
    process_user_action(user_id, action_data)
    spy.assert_called_once_with(user_id, "user_action", action_data)

By incorporating mocker.spy into your website testing strategy, you can create robust and reliable tests that give you confidence in your website's functionality. Happy testing!