We recently encountered an issue with some unit tests at work that I spent some time investigating, and a colleague suggested I write it up as a blog post. I think it serves as a good example of a principle I’m constantly espousing to coworkers–don’t be afraid to dig down into the libraries and frameworks your application uses!
The issue showed up in some unit tests we have for a Python web application that uses the Django framework. We’re using PyMySQL as our database backend and pytest as our test harness with the pytest-django plugin to facilitate things. Some functionality in the application relies on checking the current date against data in the database so to make our tests straightforward my colleague added the freezegun Python library which allows mocking Python’s datetime module: