Dia do Pi

#!/usr/bin/env python
from __future__ import with_statement
import decimal
 
def pi_gauss_legendre():
     D = decimal.Decimal
     with decimal.localcontext() as ctx:
          ctx.prec += 2
          pi_fun = lambda a,b,t: (a + b)**2 / (4 * t)
          a, b, t, p = 1, 1/D(2).sqrt(), 1/D(4), 1
          pi, piold = pi_fun(a, b, t), None
          while pi != piold:
               piold = pi
               an = (a + b) / 2
               b = (a * b).sqrt()
               t -= p * (a - an)**2
               a, p = an, 2*p
               pi = pi_fun(a, b, t)
               return +pi
 
decimal.getcontext().prec = 100
print pi_gauss_legendre()