Good tests are always executed considering a predefined, reproducible application state; that is, whenever you run a test in the chosen state, the result will always be equivalent. Usually, this is achieved by setting your database data yourself and clearing your cache and any temporary files (if you make use of external services, you should mock them) for each test. Clearing cache and temporary files is not hard, while setting your database data, on the other hand, is.

If you're using Flask-SQLAlchemy to hold your data, you would need to hardcode, somewhere in your tests as follows:

attributes = { … }
model = MyModel(**attributes)
db.session.add(model)
db.session.commit()

This approach does not scale as it is not easily reusable (when you define this as a function and a method, define it for each test). There are two ways to populate your database for testing: fixtures and pseudo-random data.

Using pseudo-random data is usually library-specific and produces better test data as the...