doctest

A doctest is a docstring containing what looks like interactive Python sessions. ^{Python Docs}

"""
Return the factorial of n, an exact integer >= 0.

>>> [factorial(n) for n in range(6)]
[1, 1, 2, 6, 24, 120]
>>> factorial(30)
265252859812191058636308480000000
>>> 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)
265252859812191058636308480000000

It must also not be ridiculously large:
>>> factorial(1e100)
Traceback (most recent call last):
    ...
OverflowError: n too large
"""

This can then be run

if __name__ == '__main__':
  import doctest
  doctest.testmod()