Script that generates monthly plain text accounting (https://plaintextaccounting.org) interest records for fixed-rate savings accounts. Amounts are faked.

generate-interest-posts.py

#!/bin/python

from collections import namedtuple
from datetime import date, timedelta
import itertools
import logging


logging.basicConfig(level=logging.DEBUG)


RiskFreeInvestment = namedtuple('RiskFreeInvestment', ['name', 'start_date', 'duration_months', 'amount', 'yearly_interest'])
MonthlyValue = namedtuple('MonthlyValue', ['date', 'investment', 'value', 'days', 'status'])


TAX = 0.3
INVESTMENTS = [
        RiskFreeInvestment(
            'nordax',
            date(2022, 3, 24),
            12,
            100000,
            0.0125,
            ),
        RiskFreeInvestment(
            'aros kapital',
            date(2022, 3, 24),
            12,
            100000,
            0.012,
            ),
        RiskFreeInvestment(
            'bluestep',
            date(2022, 3, 23),
            12,
            100000,
            0.0125,
            ),
        ]


record = '''{payout.date}{payout.status} Interest payout after {payout.days} days
    assets:banks:restricted:{payout.investment.name}   = {payout.value:.2f} SEK
    income:interest:other
'''


def main():
    nested_dividends = [generate_monthly_interests(i) for i in INVESTMENTS]
    dividends = list(itertools.chain.from_iterable(nested_dividends))   # https://stackoverflow.com/a/953097/260805
    dividends.sort()   # sort to print them in date order.
    for d in dividends:
        print(record.format(payout=d).replace('.', ','))


def generate_monthly_interests(investment):
    start_date_truncated_to_first = date(investment.start_date.year, investment.start_date.month, 1)

    # Using start_date_truncated_to_first instead of investment.start_date
    # below since increment_month can't handle overflowing days for certain
    # months.
    investment_end = increment_month(start_date_truncated_to_first, investment.duration_months)
    logging.debug("%s %s %s", investment.start_date, start_date_truncated_to_first, investment_end)

    interests = []
    d = increment_month(start_date_truncated_to_first)
    while d <= investment_end:
        time_since_investment = d - investment.start_date
        logging.debug("%s %s %s", investment.start_date, d, time_since_investment.days)
        dividend = (1-TAX) * investment.amount * ((1+investment.yearly_interest) ** (1.0*time_since_investment.days/365) - 1)
        inv = MonthlyValue(d, investment, investment.amount + dividend, time_since_investment.days, ' *' if d < date.today() else ' ')
        interests.append(inv)
        d = increment_month(d)

    return interests


def increment_month(d, months=1):
    """Increment a month for a date.

    >>> increment_month(date(2022, 1, 1))
    datetime.date(2022, 2, 1)
    >>> increment_month(date(2022, 1, 1), 2)
    datetime.date(2022, 3, 1)
    >>> increment_month(date(2022, 12, 1))
    datetime.date(2023, 1, 1)
    """
    if months > 1:
        for i in range(months):
            d = increment_month(d)
        return d
    # BUG: This will fail if d.day doesn't exist for the new month. Not needed at the moment.
    return date(
            d.year + 1 if d.month==12 else d.year,
            1 if d.month==12 else d.month+1,
            d.day)


if __name__=='__main__':
    main()

Execute the tests with python3 -m doctest generate-interest-posts.py.