immoney 0.0.3

Immutable money types for Python

immoney

Design goals

There are a few core design aspects of this library that each eliminate entire classes of bugs:

• Exposed and internal data types are either immutable or faux immutable.
• Invalid amounts of money cannot be represented. There is no such thing as 0.001 US dollars, and there is no such thing as negative money.
• Builtin operations never implicitly lose precision.
• Built from the ground-up with support for static type checking in mind. This means that bugs that attempt to mix currencies can be found by a static type checker.

Features

Safe division

In real life we cannot split the subunit of a currency, and so for our abstractions to safely reflect reality, we shouldn't be able to do that in code either. Therefore instead of defining division to return a value with precision loss, the implementation of division for Money returns a tuple of new instances with the value split up as even as possible.

>>> Money("0.11", SEK) / 3
(Money('0.04', SEK), Money('0.04', SEK), Money('0.03', SEK))

This method of division will always be safe, as it has the guaranteed property that the sum of the instances returned by the operation always equal the original numerator.

Subunit fractions

Sometimes we do need to represent fractions of monetary values that are smaller than the subunit of a currency, for instance as a partial result of a larger equation. For that purpose, this library exposes a SubunitFraction type. This type is used as return type for Money.__floordiv__.

>>> SEK(13) // 3
SubunitFraction('1300/3', SEK)

Because there is no guarantee that a SubunitFraction is a whole subunit (by definition ...), converting back to Money can only be done with precision loss.

>>> (SEK(13) // 3).round_money(Round.DOWN)
Money('4.33', SEK)

Overdraft

Again referring to real life, there is no such thing as negative money. Following in the same vein as for not allowing subunits to be split, the value of a Money instance cannot be negative. Instead, to represent for instance a negative balance on an account, this library exposes an Overdraft class that is used as return type of Money.__sub__ when the computed value would have been negative.

>>> balance = SEK(5)
>>> balance - SEK(4)
Money('1.00', SEK)
>>> balance - SEK(5)
Money('0.00', SEK)
>>> balance - SEK("6.50")
Overdraft('1.50', SEK)
>>> balance - SEK("6.50") + SEK("1.50")
Money('0.00', SEK)

Because negative values are encoded as its own type in this way, situations where negative values can result from arithmetic but aren't logically expected, such as for the price of an item in a store, can be discovered with a static type checker.

Type-safe comparison

Instances of Money do not support direct comparison with numeric scalar values. For convenience an exception is made for integer zero, which is always unambiguous.

Immediate and full instantiation

"2 dollars" is represented exactly the same as "2.00 dollars", in every aspect. This means that normalization of values happen at instantiation time.

Instantiating normalizes precision to the number of subunits of the instantiated currency.

>>> EUR(2)
Money('2.00', EUR)
>>> EUR("2.000")
Money('2.00', EUR)

Trying to instantiate with a value that would result in precision loss raises a runtime error.

>>> EUR("2.001")
Traceback (most recent call last):
  ...
immoney.errors.MoneyParseError: Cannot interpret value as Money of currency EUR ...

Instance cache

Since instances of Money and Currency are immutable it's safe to reuse existing instances instead of instantiating new ones. This happens transparently when instantiating a new Money instance and can lead to faster code and less consumed memory.