Python doctest Module: Test Examples in Docstrings

doctest executes Python interactive examples embedded in docstrings and checks printed results.

Understanding the Concept

doctest executes Python interactive examples embedded in docstrings and checks printed results. It works best for small deterministic demonstrations rather than complex integration tests.

The helper returns None; use doctest.testmod for a module-wide summary.

How this fits into a real test suite

A reliable test starts with a specific expected behavior. Keep the fixture small, arrange the input, call the real code, and assert a result the reader can verify. Add one boundary or failure case so the test does not pass only for a convenient example. In a larger project, run the same test automatically after each change.

Tests are repeatable experiments. Use temporary resources and injected dependencies where possible; when the environment is part of the requirement, add a separate integration test rather than weakening the unit test.

Worked Example

Read the code from the first line to the last. Identify which values are inputs, which operation is being demonstrated and what should happen before running it.

import doctest

def square(value: int) -> int:
    """Return the square of a number.

    >>> square(4)
    16
    """
    return value * value
print(doctest.run_docstring_examples(square, globals(), verbose=False))

Walkthrough: What Each Part Does

• The imports make doctest available. These are standard-library building blocks unless the example explicitly names an external tool.
• The definition square creates reusable behavior. Its body runs when called, awaited or instantiated; defining it alone does not run the body of an ordinary function.
• The final check compares actual behavior with an explicit expectation. A successful test tells you that this particular case passed, not that every possible input works.

This demonstration focuses on one practical behavior. Before changing several lines, vary one input or one configuration choice and check whether the result changes for the reason you expected.

Example Output

None

The output above was captured by running the example in the available Python environment. The order of concurrent events, durations and platform-dependent values can legitimately differ on your computer.

Important Behavior and Edge Cases

Using unstable random, date or unordered output in doctest examples that require exact text matching.

The helper returns None; use doctest.testmod for a module-wide summary.

Practice Exercise

Add a doctest example to a simple calculation function.

Write down the result you expect before running your changed code. Check at least one normal case and one boundary or failure case. If your example starts workers or opens resources, verify how they are cleaned up.

Further reading: official documentation and API reference.

Common Beginner Mistakes

• Using unstable random, date or unordered output in doctest examples that require exact text matching.
• Writing assertions that check implementation details but fail to validate the behavior that matters to the caller.

Key Takeaways

• doctest executes Python interactive examples embedded in docstrings and checks printed results.
• The helper returns None; use doctest.testmod for a module-wide summary.
• Practice: Add a doctest example to a simple calculation function.