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

The recommended practice for installing dependencies is to create a virtualenv. A virtualenv (https://packaging.python.org/guides/installing-using-pip-and-virtualenv/) acts like a complete separate Python installation from the one that comes with your operating system, making it safe to install the packages required by your application without risk of breaking your system Python or tools.

Now we will learn how to create a virtual environment and install pytest using pip. If...

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

    # 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 execute pytest, 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 a tests 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 (from keyword expression), you can run tests whose item id loosely matches the given expression:

λ pytest -k &quot...

Users can customize some pytest behavior using a configuration file called pytest.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.

The format follows a simple ini-style format with all pytest-related options under a [pytest] section. For more details, go to:https://docs.python.org/3/library/configparser.html.

[pytest]

The location of this file also defines what pytest calls the root directory (rootdir): if present, the directory that contains the configuration file is considered the root directory.

The root directory is used for the following:

• To create the tests node IDs
• As a stable location to store information about the project (by pytest plugins and features...

In this chapter, we covered how to use virtualenv and pip 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 use pytest.raises and pytest.warns to check exceptions and warnings, and pytest.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 how pytest.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...