math.fsum

The float type is infamous for being not as precise as you might expect. When you add 2 numbers, the result might contain a small error in precision. And the more numbers you add together, the higher the error:

sum([.9] * 1_000)
# 899.9999999999849

sum([.9] * 1_000_000)
# 900000.0000153045

If you want to minimize the error when summing together a list of floats, use math.fsum:

import math

math.fsum([.9] * 1_000_000)
# 900000.0