Test-Driven Development with Python, 3rd Edition

Chapter 20. Using Mocks to Test External Dependencies

In this chapter, we’ll start testing the parts of our code that send emails—i.e., the second item on our scratchpad:

• Token model with email and UID

• View to create token and send login email incl. url w/ token UID

• Custom user model with USERNAME_FIELD=email

• Authentication backend with authenticate() and get_user() functions

• Registering auth backend in settings.py

• Login view calls authenticate() and login() from django.contrib.auth

• Logout view calls django.contrib.auth.logout

In the functional test (FT), you saw that Django gives us a way of retrieving any emails it sends by using the mail.outbox attribute. But in this chapter, I want to demonstrate a widespread testing technique called mocking. So, for the purpose of these unit tests, we’ll pretend that this nice Django shortcut doesn’t exist.

Am I telling you not to use Django’s mail.outbox? No—use it; it’s a neat helper. But I want to teach you about mocks because they’re a useful general-purpose tool for unit testing external dependencies. You may not always be using Django! And even if you are, you may not be sending email—​any interaction with a third-party API is a place you might find yourself wanting to test with mocks.

To Mock or Not to Mock?

I once gave a talk called “Stop Using Mocks”; it’s entirely possible to find ways to write tests for external dependencies without using mocks at all.

I’m covering mocking in this book because it’s such ...