Doctest: putting your tests in your documentation

Python has a popular and wonderful approach to testing: put tests into your documentation. The doctest library puts this principle into practice. Here’s an example:

The docstring here just has a sample interaction at the Python REPL pasted into it. The doctest library will find these pasted in interactions, try the code, and warn you if the answer the REPL gives is different from the one you suggest. Here’s a log:

Trying:
    fact(5)
Expecting:
    120
ok
Trying:
    fact(0)
Expecting:
    1
ok
Trying:
    fact(-1)
Expecting:
    1
ok
1 items had no tests:
    fact
1 items passed all tests:
3 tests in fact.fact
3 tests in 2 items.
3 passed and 0 failed.
Test passed.

To run this test, you would run python -m doctest -v fact.py. (There are other ways to run the tests, but this is the simplest.)