doctest_example.py

""" This is the "example" module.

The example module supplies one function, factorial(). For example,

factorial(5) 120 """

def factorial(n): """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
"""

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:
    result *= factor
    factor += 1
return result

if name == "main": import doctest # doctest.testmod() # doctest.testfile("doctest_example.txt")