Summation (sigma) notation in python 3

sigma.py

#! /usr/bin/env python3
#
#  Σ i
# i=s

from functools import reduce

# the sequence to sum, you can use range(blah) as well
s = [1, 2, 3, 5]

# alter this lambda for the right side of the sigma
right_side_of_sigma = lambda a, x: a + x

result = reduce(right_side_of_sigma, [0] + s)
print(result)

# another example:
result = reduce(lambda a, x: a + 3.14*x*x, [0]+[2,3,5])
assert result == ((3.14*2*2) + (3.14*3*3) + (3.14*5*5))