The `doctest` module searches a module's docstrings for text that looks
like an interactive Python session, then executes all such sessions to verify
they still work exactly as shown. Here's a complete but small example:

""" This is module example. Example supplies one function, factorial. For example, >>> factorial(5) 120 """ def factorial(n): """Return the factorial of n, an exact integer >= 0. If the result is small enough to fit in an int, return an int. Else return a long. >>> [factorial(n) for n in range(6)] [1, 1, 2, 6, 24, 120] >>> [factorial(long(n)) for n in range(6)] [1, 1, 2, 6, 24, 120] >>> factorial(30) 265252859812191058636308480000000L >>> factorial(30L) 265252859812191058636308480000000L >>> factorial(-1) Traceback (most recent call last): ... ValueError: n must be >= 0 Factorials of floats are OK, but the float must be an exact integer: >>> factorial(30.1) Traceback (most recent call last): ... ValueError: n must be exact integer >>> factorial(30.0) 265252859812191058636308480000000L It must also not be ridiculously large: >>> factorial(1e100) Traceback (most recent call last): ... OverflowError: n too large """

import math if not n >= 0: raise ValueError("n must be >= 0") if math.floor(n) != n: raise ValueError("n must be exact integer") if n+1 == n: # catch a value like 1e300 raise OverflowError("n too large") result = 1 factor = 2 while factor <= n: try: result *= factor except OverflowError: result *= long(factor) factor += 1 return result def _test(): import doctest, example return doctest.testmod(example) if __name__ == "__main__": _test()

If you run example.py directly from the command line,
`doctest` works its magic:

$ python example.py $

There's no output! That's normal, and it means all the examples
worked. Pass **-v** to the script, and `doctest`
prints a detailed log of what it's trying, and prints a summary at the
end:

$ python example.py -v Running example.__doc__ Trying: factorial(5) Expecting: 120 ok 0 of 1 examples failed in example.__doc__ Running example.factorial.__doc__ Trying: [factorial(n) for n in range(6)] Expecting: [1, 1, 2, 6, 24, 120] ok Trying: [factorial(long(n)) for n in range(6)] Expecting: [1, 1, 2, 6, 24, 120] ok Trying: factorial(30) Expecting: 265252859812191058636308480000000L ok

And so on, eventually ending with:

Trying: factorial(1e100) Expecting: Traceback (most recent call last): ... OverflowError: n too large ok 0 of 8 examples failed in example.factorial.__doc__ 2 items passed all tests: 1 tests in example 8 tests in example.factorial 9 tests in 2 items. 9 passed and 0 failed. Test passed. $

That's all you need to know to start making productive use of
`doctest`! Jump in. The docstrings in doctest.py contain
detailed information about all aspects of `doctest`, and we'll
just cover the more important points here.

See