To test our application, we use the excellent pytest framework, which is the gold standard in test runners for Python applications.
Basically, pytest has a lot of plugins and add-ons to deal with a lot of situations. We will be using pytest-flask, which helps with running tests for Flask applications.
To run all the tests, just call pytest in the command line:
$ pytest
============== test session starts ==============
....
==== 17 passed, 177 warnings in 1.50 seconds =====
Note that pytest has a lot of features available to deal with a lot of situations while testing. Things running a subset of matched tests (the -k option), running the last failed tests (--lf), or stopping after the first failure (-x) are incredibly useful when working with tests. I highly recommend checking its full documentation (https://docs.pytest.org/en/latest/) and discovering all its possibilities...