python怎么调用math函数库求π值？

import math

def main():

    a=1.0

    b=1.0/math.sqrt(2)

    t=1.0/4.0

    p=1.0

    for i in range(1000):

        at=(a+b)/2

        bt=math.sqrt(a*b)

        tt=t-p*(a-at)**2

        pt=2*p

        a=at;b=bt;t=tt;p=pt

    my_pi=(a+b)**2/(4*t)

    print("Pi is approximately: "+str(my_pi))

if __name__== "__main__":

main()