1 """

     2 This is the "example" module.

     3 

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

     5 

     6 >>> factorial(5)

     7 120

     8 """

     9 

    10 def factorial(n):

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

    12 

    13     If the result is small enough to fit in an int, return an int.

    14     Else return a long.

    15 

    16     >>> [factorial(n) for n in range(6)]

    17     [1, 1, 2, 6, 24, 120]

    18     >>> [factorial(long(n)) for n in range(6)]

    19     [1, 1, 2, 6, 24, 120]

    20     >>> factorial(30)

    21     265252859812191058636308480000000L

    22     >>> factorial(30L)

    23     265252859812191058636308480000000L

    24     >>> factorial(-1)

    25     Traceback (most recent call last):

    26         ...

    27     ValueError: n must be >= 0

    28 

    29     Factorials of floats are OK, but the float must be an exact integer:

    30     >>> factorial(30.1)

    31     Traceback (most recent call last):

    32         ...

    33     ValueError: n must be exact integer

    34     >>> factorial(30.0)

    35     265252859812191058636308480000000L

    36 

    37     It must also not be ridiculously large:

    38     >>> factorial(1e100)

    39     Traceback (most recent call last):

    40         ...

    41     OverflowError: n too large

    42     """

    43 

    44     import math

    45     if not n >= 0:

    46         raise ValueError("n must be >= 0")

    47     if math.floor(n) != n:

    48         raise ValueError("n must be exact integer")

    49     if n+1 == n:  # catch a value like 1e300

    50         raise OverflowError("n too large")

    51     result = 1

    52     factor = 2

    53     while factor <= n:

    54         result *= factor

    55         factor += 1

    56     return result

    57 

    58 

    59 if __name__ == "__main__":

    60     import doctest

    61     doctest.testmod()

    62