Running with SciPy:
#!/usr/bin/python3
import scipy.integrate as integ
import numpy as np
def normalProbabilityDensity(x):
constant = 1.0 / np.sqrt(2*np.pi)
return(constant * np.exp((-x**2) / 2.0) )
neg_percentile, _ = integ.quad(normalProbabilityDensity, np.NINF, -1.00)