My function returns None. I have checked to make sure all the operations are correct, and that I have a return statement for each function.

def parameter_function(principal, annual_interest_rate, duration):n = float(duration * 12)if annual_interest_rate == 0:r = float(principal / n)else:r = float(annual_interest_rate / 1200)p = principalreturn (p, r, n)def monthly_payment_function(p, r, n):monthly_payment = p * ((r * ((1 + r) ** n)) / (((1 + r) ** n) - 1))result = monthly_payment_function(p, r, n)return result

monthly_payment_function does not return anything. Replace monthly_payment= with return (that's 'return' followed by a space).

Also you have an unconditional return before def monthly_payment_function, meaning it never gets called (strictly speaking, it never even gets defined).

Also you are pretty randomly mixing units, and your variable names could use some help:

from __future__ import division    # Python 2.x: int/int gives floatMONTHS_PER_YEAR = 12def monthly_payment(principal, pct_per_year, years):months = years * MONTHS_PER_YEARif pct_per_year == 0:return principal / monthselse:rate_per_year   = pct_per_year / 100.rate_per_month  = rate_per_year / MONTHS_PER_YEARrate_compounded = (1. + rate_per_month) ** months - 1.return principal * rate_per_month * (1. + rate_compounded) / rate_compounded