Unit tests are the simplest tests to add to a project, and the standard library comes with everything needed to write some. In a project based on Flask, there usually are, alongside the views, some functions and classes, which can be unit-tested in isolation.
However, the concept of separation is quite vague for a Python project, because we don't use contracts or interfaces like in other languages, where the implementation of the class is separated from its definition.
Testing in isolation in Python usually means that you instantiate a class or call a function with specific arguments, and verify that you get the expected result. When the class or function calls another piece of code that's not built in Python or its standard library, ...
