Prime number testing

[Rahul]

HI.
Python , with its support of arbit precision integers, can be great
for number theory. So i tried writing a program for testing whether a
number is prime or not. But then found my function painfully slow.Here
is the function :

from math import sqrt
def isPrime(x):
    if x%2==0 or x%3==0:
       return 0
    i=5
    sqrt=sqrt(x)
    sqrt=sqrt(x)+1
    while i<sqrt:
       if x%i==0:
          return 0
       i=i+2
       if x%i==0:
          return 0
       i=i+4
    return 1

Try testing some number like 2**61-1.Its very slow.In comparision
Mathematica not only tests but finds factors in an instant by
Divisors[x].

Does anybody have a faster way to test for primality.(and not just
pseudoprime).

rahul