>>> from math import pi
>>>
>>> radio = 3
>>> volumen = 4.0/3.0 * pi * radio ** 3
>>>
>>> print volumen
113.097335529
>>> raw_imput = 12
>>>
>>> radio = float(raw_imput)
>>> volumen = 4.0/3.0 * pi * radio ** 3
>>>
>>> print volumen
7238.22947387