erf and erfc, which are +- antiderivatives of the gaussian (with a specific choice of +c) is in the python math library. So, you can easily write the CDF of normal dist using either erf or erfc.

import math

mu = 30
sig = 4
def cdf(x):
    return math.erfc((mu-x)/(sig*math.sqrt(2)))/2
print(cdf(40))
print(1 - cdf(21))
print(cdf(35)-cdf(30))