[Python-Dev] RE: Python-Dev digest, Vol 1 #1917 - 14 msgs

[Andy Robinson]

> > 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