Testing Flask
Before getting to our own simple example, let's see the tests of one of the most popular Python libraries. Flask is a minimalistic framework to write web applications in Python. It is an open source project that has been around for many year.
We can install it using the regular tools, e.g. pip install flask, but what we are really interested in
is to see how a developer of Flask can check how Flask behaves on a newer version of Python, on a different operating system,
or after some changes were made to the project?
- Do they break any of the existing features of Flask?
- How things that were working earlier behave now? Do they still work?
- Are there any regressions?
For this we need to get a local copy of the development version of the source code.
We can do this by cloning the GitHub repository of Flask and running the tests locally.
Follow these instructions
The following commands were used on my Linux machine:
- Clone the project and enter the cloned folder:
git clone https://github.com/pallets/flask.git
cd flask
git switch stable
curl -LsSf https://astral.sh/uv/install.sh | sh
- Setup the virtual environment which is basically a folder called
.venv:
uv sync
- Activate the virtual environment:
. .venv/bin/activate
- Run the tests, the type-checker, and the documentation checker
pytest
mypy
tox run -e docs
If that's not enough you can also install the coverage module
uv pip install coverage
run the test using it
coverage run -m pytest
and then generate the coverage report:
coverage report -m --include=src/*
It will show you that 92% of the code has tests.
That means the developers have invested a lot of time and energy making sure their code works the same way day in, day out.