dated-money — dmon.money.Money

Manipulate monetary values, each consisting in an amount (stored as a Decimal in cents) and a currency, with control on the date on which currency conversions take place.

It differs from the money package in that it treats the date as a first class element.

It downloads and saves today's conversion rates from https://www.exchangerate-api.com/ using your API key (see below). If your account is free currency exchanges will only work for today, or for dates for which a conversion rate json file is found. If your account is paid all dates will work.

Examples

from decimal import Decimal as Dec
from dmon.money import Money, Currency

# Compute in eur with today's conversion rates (default)
Eur = Money(Currency.EUR)

# Compute in gbp with today's conversion rates
Gbp = Money('£')

assert Eur(40).cents() == Dec('4000')
assert Eur(40).to('$').cents() == Dec('4020.100502512562832012897042')
assert Eur(20) < Gbp(20)
assert Eur(20, '£') == Eur(20, 'gbp') == Eur(20, Currency.GBP) == Gbp(20)

# We can define the default date for conversions when creating the class
OldUsd = Money('$', on='2022-01-07')
assert str(OldUsd(20, 'eur').to('$')) == '$22.61'
assert str(Eur(20, 'eur').to('$')) == '$20.13'  # With today's rates

# Computations are done in the default currency of the class Eur
assert str(Eur(20, 'aud') + Eur(20, 'gbp')) == '€37.00'

# If we move to another date the values differ
Eur.to_date('2022-01-07')
assert str(Eur(20, 'aud') + Eur(20, 'gbp')) == '€36.65'

# If we use two classes with different dates the first one wins
assert str(Eur(20, 'aud') + Gbp(20, 'gbp')) == '€36.23'
assert str(Gbp(20) + Eur(20, 'aud')) == '£30.61'

assert Eur(20) / Gbp(20) == Dec('0.8500402576489532855181620688')
assert Eur(20) / 2 == Eur(10)
assert str(1.2 * Eur(20)) == '€24.00'
assert str(1.2 * Eur(20).to(Currency.CAD)) == 'C$31.53'

paid = Gbp(10)
amount, currency = paid.as_tuple()
assert (amount, currency) == (Dec('1000'), 'gbp')
assert Gbp((amount, currency)) == paid

Installation

: pip install dated-money

Usage

The metaclass Money returns a class that knows its currency and the date for which currency transformations should be done. When the on argument is None (default) the date is set to today's. If it is a date string, like '2021-10-29', it will do the conversions with the date's exchange rate (assuming that it can find the json file with the rates, otherwise it will fail).

Important All computations are done with cents stored as Decimal, but comparisons are are rounded to the second decimal. So, for example,

paid = Gbp(10)
paid_usd = paid.to('$')
paid_usd.cents()  # Decimal('1183.992422448496305219877141')
str(paid_usd)     # '$11.84'

# If we initialize with a string instead of a float it will be an exact Decimal.
native_usd = Gbp('11.84', 'usd')
native_usd.cents()  # Decimal('1184.00')
assert native_usd == paid_usd
assert native_usd.cents() != paid_usd.cents()
native_usd.cents() - paid_usd.cents()  # Decimal('0.007577551503694780122859')

Environment variables

The MONEY_RATES_API_KEY environment variable contains your API key for [https://your_exchangerate-api.com](the exchange rates provider).

export MONEY_RATES_API_KEY=your_exchangerate-api.com_key

The exchange rates will only be downloaded once a day, and cached in the directory pointed at by MONEY_RES, if defined, or the current directory otherwise.

export MONEY_RES=a/resources/directory