Python pi calculation?

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It seems you are losing precision in this line:

pi = pi * Decimal(12)/Decimal(640320**(1.5))

Try using:

pi = pi * Decimal(12)/Decimal(640320**Decimal(1.5))

This happens because even though Python can handle arbitrary scale integers, it doesn’t do so well with floats.

Bonus

A single line implementation using another algorithm (the BBP formula):

from decimal import Decimal, getcontext
getcontext().prec=100
print sum(1/Decimal(16)**k * 
          (Decimal(4)/(8*k+1) - 
           Decimal(2)/(8*k+4) - 
           Decimal(1)/(8*k+5) -
           Decimal(1)/(8*k+6)) for k in range(100))

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