Chapter 3: Using PyTest, MyPy and flake8 for testing

#python #testing #pytest #flake8

This chapter is part of the series:
Understanding the Python Tox Ecosystem
Namah Shrestha ・ Oct 21 '22
#python#tox#testing#pytest
Please consider reading the previous chapter (Chapter 2) before moving forward. This chapter builds upon the project structure discussed in the previous chapter (Chapter 2).
Chapter 2: The Need Of A Project Structure In Python Testing
Namah Shrestha ・ Oct 21 '22
#python#tox#testing#pytest

From the previous chapter (Chapter 2), we have the following project structure:

    - src/    - something/        - __init__.py        - app.py    - tests/        - __init__.py        - test_app.py    - .gitignore    - LICENSE    - pyproject.toml    - README.md    - requirements.txt    - setup.cfg    - setup.py

Here we are going to setuppytest,mypy andflake8.
TABLE OF CONTENTS

3.1 Adding new dependencies to requirements

Now we need to install more libraries. Namelypytest,mypy andflake8.
So ourrequirements.txt file looks something like this. We have added dependencies forpytest:
```
flake==3.9.4tox==3.24.3pytest==6.2.5pytest-cov==2.12.1mypy==0.910
```
Instead of directly overwriting therequirements.txt file, we create a new file calledrequirements_dev.txt. Since testing is required indev environment.
So we have two requirement files:
- requirements.txt which we use while running our code:
```
requests==2.26.0
```
- requirements_dev.txt which holds dev dependencies like our test, lint and type check libraries.
```
flake==3.9.4tox==3.24.3pytest==6.2.5pytest-cov==2.12.1mypy==0.910
```
While running tests we will userequirements_dev.txt file to install dependencies.

3.2 Adding to our metadata in`setup.cfg`

Oursetup.cfg file looks something like this:

    [metadata]    name = something    description = just some dummy codebase    author = Coding with Zim    license = MIT    license_file = LICENSE    platforms = unix, linux, osx, cygwin, win32    classifiers =         Programming Language :: Python :: 3        Programming Language :: Python :: 3 :: Only        Programming Language :: Python :: 3.6        Programming Language :: Python :: 3.7        Programming Language :: Python :: 3.8        Programming Language :: Python :: 3.9    [options]    packages =        something    install_requires =        requests>=2    python_requires = >=3.6    package_dir =        =src    zip_safe = no    [options.extras_require]    testing=        pytest>=6.0        pytest-cov>=2.0        mypy>=0.910        flake8>=3.9        tox>=3.24    [options.package_data]    something = py.typed    [flake8]    max-line-length = 160

options.extras_require are optional dependencies. We can read more about it here:https://setuptools.pypa.io/en/latest/userguide/dependency_management.html
options.package_data is a type ofData File Support , we can read more about it here:https://setuptools.pypa.io/en/latest/userguide/datafiles.html. We are basically using it to denote that our code istype-hinted, which means we are using types in the code.
For this to work we need to create apy.typed blank file in our something directory alongside__init__.py.
flake8 is the configuration for theflake8 linter that we will be using. Here we are only saying that the maximum line length is 160.
So ourflake8 configuration is stored in thecfg file and ourpython configuration is stored in ourpyproject.toml file.

3.3 Adding python configurations to`pyproject.toml`

Ourpyproject.toml file looks something like this:

[build-system]requires=["setuptools>=42.0", "wheel"]build-backend="setuptools.build_meta"[tool.pytest.ini_options]addopts="--cov=something"testpaths=["tests",][tool.mypy]mypy_path="src"check_untyped_defs=truedisallow_any_generics=trueignore_missing_imports=trueno_implicit_optional=trueshow_error_codes=truestrict_equality=truewarn_redundant_casts=truewarn_return_any=truewarn_unreachable=truewarn_unused_configs=trueno_implicit_reexport=true

We already discussed about thebuild-system.
tool.pytest.ini_options arepytest tool options during the execution of tests. In our case we have added the--cov=something option inaddopts so that we see the test coverage.testpaths basically denotes all the directories containing tests.
tool.mypy aremypy tool options. To learn aboutmypy:https://realpython.com/lessons/type-checking-mypy/#:~:text=“Mypy is an optional static,background on the Mypy project.

So, now we install thedev dependencies withpip:

(env)yourproject$pipinstall-r requirements_dev.txt

Finally we should be able to run our test withpytest, check linting withflake8 and type check withmypy.

$pytest# this command will test our code with default test discovery mechanism.$mypy# this command will run type checks on our code with mypy.$flake8# this command will check code linting.