[Python-Dev] RE: Python-Dev digest, Vol 1 #1917 (original) (raw)

Andy Robinson andy@reportlab.com
Wed, 27 Feb 2002 12:08:22 -0000

> I propose adding an "abstract" money base type to the standard > library, to be subclassed by real money/decimal implementations.

Why do we need this? I guess that would be Question #1... --Guido van Rossum (home page: http://www.python.org/~guido/)

I can think of 3 reasons; I've seen all these occur in real life.

Reason 1: Currency safety. Having a special type can rule out subtle programming errors. Imagine this:

x = Money(3, "USD") y = Money(4.5, "NLG") z = x + y TypeError: can't add different currencies

Likewise, if you add or subtract money you get money; if you divide money in the same currency you get a float; and just about any other operation might be an error. IMHO a basic type should just rule out operations; subclasses could do clever conversions etc. (Does anyone need Euro triagulation rules in the Python standard library?)

Reason 2: fixed decimals SQL databases and AS400s have fixed decimal data types and can do math on thousands or millions of numeric fields at C-like speeds. There would be a (very small) market for a type that could do this.

Reason (3): speed If I went for a Python "money class" with smart behaviour, I'd get a sizable speed hit compared to floats. Let's say I want to average a time series of 1000 bond prices; it will be faster on floats than on Python classes.

IMHO all these are best served by an extension package not in the core language - but having a common base for them to inherit from would get a thumbs-up from me.

Best Regards,

Andy Robinson