Python doctest.testmod() Function: Usage and Examples

doctest.

Understanding the Concept

doctest.testmod() finds and tests interactive examples within module docstrings. It returns a result with attempted and failed counts for reproducible examples.

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 triple(value: int) -> int:
    """Triple a value.

    >>> triple(3)
    9
    """
    return value * 3
import types
example_module = types.ModuleType('example_doctest_module')
example_module.triple = triple
result = doctest.testmod(example_module)
print(result.failed, result.attempted)

Walkthrough: What Each Part Does

• The central operation is doctest.testmod(example_module). Read its actual arguments: those values determine what is tested, logged, measured or scheduled. Then locate the line that inspects its result or handles a failure.
• The imports make doctest, types available. These are standard-library building blocks unless the example explicitly names an external tool.
• The definition triple 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

0 0

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

Forgetting that doctest compares exact displayed output and whitespace.

Pay particular attention to the documented return values, errors and resource-lifetime rules. A convenient result under normal inputs does not guarantee correct behavior under cancellation, missing data or environmental failure.

Practice Exercise

Create a deterministic docstring and run doctest.testmod().

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

• Forgetting that doctest compares exact displayed output and whitespace.
• Writing assertions that check implementation details but fail to validate the behavior that matters to the caller.

Key Takeaways

• doctest.
• doctest.testmod() finds and tests interactive examples within module docstrings. It returns a result with attempted and failed counts for reproducible examples.
• Practice: Create a deterministic docstring and run doctest.testmod().