The following calculates $\pi$ to 10 decimal places.

>>> import math
>>> pi = 0
>>> for k in range(20):
...     pi += pow(-3, -k) / (2*k+1)
...
>>> pi *= math.sqrt(12)
>>> print('pi = ', pi)
pi =  3.1415926535714034
>>> print('error = ', abs(pi - math.pi))
error =  1.8389734179891093e-11

Note that the built-in pow(x, j) is equivalent to (x)**j.