doctest.
Understanding the Concept
doctest.testfile() runs interactive examples stored in a separate text file. The file contents are treated as doctest examples rather than ordinary Python source.
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
import tempfile
from pathlib import Path
with tempfile.TemporaryDirectory() as temporary_directory:
example_path = Path(temporary_directory) / 'examples.txt'
example_path.write_text('>>> 2 + 5\n7\n', encoding='utf-8')
result = doctest.testfile(str(example_path), module_relative=False, encoding='utf-8')
print(result.failed, result.attempted)Walkthrough: What Each Part Does
- The central operation is doctest.testfile(str(example_path), module_relative=False, encoding='utf-8'). 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, tempfile, pathlib available. These are standard-library building blocks unless the example explicitly names an external tool.
- The with or async with scope controls lifetime. Setup takes place on entry, and cleanup runs on exit even when an exception interrupts the body.
- 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.
The example uses terminal, platform, external tool or process behavior that was not exercised automatically. Run it only in a suitable environment after reviewing the shown permissions and paths.
Important Behavior and Edge Cases
Writing an ordinary script in a doctest text file without >>> prompts or expecting arbitrary markdown to be executable.
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 temporary .txt file with a doctest and test it.
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
- Writing an ordinary script in a doctest text file without >>> prompts or expecting arbitrary markdown to be executable.
- Writing assertions that check implementation details but fail to validate the behavior that matters to the caller.
Key Takeaways
- doctest.
- doctest.testfile() runs interactive examples stored in a separate text file. The file contents are treated as doctest examples rather than ordinary Python source.
- Practice: Create a temporary .txt file with a doctest and test it.