Running Specific Tests with Pytest from a Configuration File

Introduction

Pytest is a powerful testing framework for Python that provides various ways to run tests selectively. This flexibility is particularly useful when you need to execute specific test cases based on different criteria, such as test names or directory structures. In this tutorial, we will explore methods to select and run pytest tests using configuration files and command-line options.

Understanding Pytest Test Identification

Before diving into running selective tests, it’s essential to understand how pytest identifies tests:

Node IDs: Each test in pytest is assigned a unique nodeid consisting of the module filename, class names, function names, and parameters from parametrization. This structure allows precise identification of tests.
Test Discovery: Pytest automatically discovers tests by identifying files prefixed with test_ or suffixed with _test.py, and functions prefixed with test.

Running Tests Using Configuration Files

To run specific pytest tests listed in a configuration file, such as foo.txt, you can use the following approach:

Step 1: Prepare Your Configuration File

Create a text file (foo.txt) that lists the full node IDs of the tests you wish to execute. For example:

tests_directory/foo.py::test_001
tests_directory/bar.py::test_some_other_test

This format specifies the exact path and name of each test function.

Step 2: Execute Tests from the Configuration File

Use the following command to run tests specified in foo.txt:

pytest -v @foo.txt

-v: Verbose mode, which provides detailed output about which tests are being executed.
@filename: Tells pytest to read test identifiers from the specified file.

This method allows you to maintain a list of tests and execute them as needed without modifying your command line each time.

Running Tests Using Keywords

If you prefer using keyword expressions or patterns, pytest provides several options:

Using `-k` for Keyword Expressions

The -k option enables running tests based on their names with logical operators like and, or, and not. For example:

pytest -v -k 'test_001 or test_some_other_test'

This command runs any test containing the specified substrings in its name. Be cautious, as it may include unintended tests if names are similar.

Running Tests from Specific Directories

To run all tests within a particular directory:

pytest tests_directory/

Running Tests by Node ID

For precise control, specify tests using their node IDs:

pytest test_mod.py::TestClass::test_method

This command runs the test_method function in TestClass within test_mod.py.

Tips and Best Practices

Organize Your Test Files: Keep your test files well-organized by grouping related tests together. This organization simplifies running specific groups of tests.
Use Markers for Categorization: Decorate tests with markers (e.g., @pytest.mark.slow) to categorize them, making it easier to run subsets based on these categories:
```
pytest -m slow
```
Combine Methods: You can combine different methods, such as using a configuration file and keyword expressions, to tailor your test execution strategy.

Conclusion

Pytest offers versatile options for running specific tests through configuration files or command-line options. By leveraging node IDs, keyword expressions, and markers, you can efficiently manage and execute targeted tests in your projects. Whether you prefer maintaining lists in configuration files or using logical operators for dynamic selection, pytest adapts to your testing needs.