Mortgage Calculator — Python source

"""
mortgage-calculator - amortized monthly payment.

Display snippet: ports the core monthly_payment() formula from the
TypeScript lib (src/lib/mortgage.ts). The full amortization schedule
(integer-cent tracking, extra payments, payoff dates) lives in the
TypeScript/Go sources.

Standard amortization formula: P*r*(1+r)**n / ((1+r)**n - 1) with
r = rate_pct/100/12 and n = years*12. A zero rate divides the principal
evenly. Returns the payment rounded to cents, or None on invalid input.
Standard library only.
"""

import math


def monthly_payment(principal, rate_pct, years):
    if not all(math.isfinite(v) for v in (principal, rate_pct)):
        return None
    if principal <= 0 or years <= 0 or not isinstance(years, int):
        return None

    r = rate_pct / 100 / 12
    n = years * 12
    if r == 0:
        payment = principal / n
    else:
        growth = (1 + r) ** n
        payment = principal * r * growth / (growth - 1)
    return math.floor(payment * 100 + 0.5) / 100


# Example:
#   monthly_payment(300000, 6, 30) -> 1798.65