loan_lambda.py

from __future__ import print_function

import json
from typing import Any, Optional, Union


def get_interest_per_month(interest_rate_per_year: float) -> float:
    interest_rate_per_month = (1.0 + interest_rate_per_year) ** (1.0 / 12.0)
    # we do not consider anything more than 1e-5 for rate
    interest_rate_per_month = round(interest_rate_per_month, 5)
    return interest_rate_per_month - 1


def get_monthly_payment(
    principle: float, interest_rate_per_year: float, period_in_years: int
) -> float:
    """
    Takes the interest rate per year and the number of years and calculates the monthly payment.
    The interest rate is converted to a monthly rate.
    """
    period_in_months = period_in_years * 12
    interest_rate_per_month = get_interest_per_month(interest_rate_per_year) + 1.0
    d = sum(interest_rate_per_month**i for i in range(period_in_months))
    # anything less than a penny is dropped
    return (
        int((principle * (interest_rate_per_month**period_in_months) / d) * 100) / 100
    )


def try_numeric(
    numstr: str,
    default: Optional[Union[int, float]] = 0,
    ntype: Optional[Union[int, float]] = int,
) -> Union[int, float]:
    try:
        return ntype(numstr)
    except ValueError:
        return default


def lambda_handler(event: Any, context: Any) -> Any:
    qs_params = event.get("queryStringParameters", {})
    principle = try_numeric(qs_params.get("principle", 300_000), default=300_000)
    rate_per_year = try_numeric(
        qs_params.get("rate_per_year", 0.03), default=0.03, ntype=float
    )
    period_in_years = try_numeric(qs_params.get("period_in_years", 25), default=25)
    return {
        "statusCode": 200,
        "body": json.dumps(
            {
                "principle": principle,
                "rate_per_year": rate_per_year,
                "period_in_years": period_in_years,
                "cost_per_month": get_monthly_payment(
                    principle, rate_per_year, period_in_years
                ),
            }
        ),
    }