The best way is to use simple math
>>> a = 8
>>> a**(1./3.)
2.0
EDIT
For Negative numbers
>>> a = -8
>>> -(-a)**(1./3.)
-2.0
Complete Program for all the requirements as specified
x = int(input("Enter an integer: "))
if x>0:
ans = x**(1./3.)
if ans ** 3 != abs(x):
print x, 'is not a perfect cube!'
else:
ans = -((-x)**(1./3.))
if ans ** 3 != -abs(x):
print x, 'is not a perfect cube!'
print 'Cube root of ' + str(x) + ' is ' + str(ans)