gistfile1.py

>>> import random
>>> ''.join(random.sample('abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789', 4))
'eKq1'
>>> # 62!/(62-4)! = 13 388 280
>>> # (62 !) / ((62 - 3) !) = 226 920
>>> # (62 !) / ((62 - 4) !) = 13 388 280
>>> def factorial(n):
...     return reduce(lambda x,y: x*y, range(1,n+1))
...     
... 
>>> def p(m,n):
...     """m!/(m-n)!"""
...     return factorial(m)/factorial(m-n)
...     
... 
>>> p(62,4)
13388280L
>>> p(62,3)
226920L