In the previous chapter, we discussed why testing is so important and looked at a brief overview of theunittest module. We also took a cursory look at pytest's features, but barely got a taste of them.

In this chapter, we will start our journey with pytest.We will be pragmatic, so this means that we will not take an exhaustive look at all of the things it's possible to do with pytest, but instead provide you with a quick overview of the basics to make you productive quickly.We will take a look at how to write tests, how to organize them into files and directories, and how to use pytest's command line effectively.

Here's what is covered in this chapter:

• Installing pytest
• Writing and running tests
• Organizing files and packages
• Useful command-line options
• Configuration:pytest.ini file

Installing pytest is really simple, but first, let's take a moment to review good practices for Python development.

Using pytest, all you need to do to start writing tests is to create a new file namedtest_*.py and write test functions that start withtest:

    # contents of test_player_mechanics.py
    def test_player_hit():
        player = create_player()
        assert player.health == 100
        undead = create_undead()
        undead.hit(player)
        assert player.health == 80

To execute this test, simply executepytest, passing the name of the file:

λ pytest test_player_mechanics.py

If you don't pass anything, pytest will look for all of the test files from the current directory recursively and execute them automatically.

Pytest needs to import your code and test modules, and it is up to you how to organize them. Pytest supports two common test layouts, which we will discuss next.

You can place your test modules together with the code they are testing by creating atests folder next to the modules themselves:

setup.py
mylib/
    tests/
         __init__.py
         test_core.py
         test_utils.py
    __init__.py
    core.py
    utils.py

By putting the tests near the code they test, you gain the following advantages:

• It is easier to add new tests and test modules in this hierarchy and keep them in sync
• Your tests are now part of your package, so they can be deployed and run in other...

Now we will take a look at command-line options that will make you more productive in your daily work. As stated at the beginning of the chapter, this is not a complete list of all of the command-line features; just the ones that you will use (and love) the most.

Often, you don't exactly remember the full path or name of a test that you want to execute. At other times, many tests in your suite follow a similar pattern and you want to execute all of them because you just refactored a sensitive area of the code.

By using the-k <EXPRESSION> flag (fromkeyword expression), you can run tests whoseitem id loosely matches the given expression:

λ pytest -k &quot...

Users can customize some pytest behavior using a configuration file calledpytest.ini. This file is usually placed at the root of the repository and contains a number of configuration values that are applied to all test runs for that project. It is meant to be kept under version control and committed with the rest of the code.

In this chapter, we covered how to usevirtualenv andpip to install pytest. After that, we jumped into how to write tests, and the different ways to run them so that we can execute just the tests we are interested in. We had an overview of how pytest can provide rich output information for failing tests for different built-in data types. We learned how to usepytest.raises andpytest.warns to check exceptions and warnings, andpytest.approx to avoid common pitfalls when comparing floating point numbers. Then, we briefly discussed how to organize test files and modules in your projects. We also took a look at some of the more useful command-line options so that we can get productive right away. Finally, we covered howpytest.ini files are used for persistent command-line options and other configuration.

In the next chapter, we will learn how to use marks to help us skip...