python里的π怎么输入

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()