problem with money datatype based calculations in python

[John Machin]

On May 3, 12:09 am, Dave Angel <da... at ieee.org> wrote:
> Krishnakant wrote:
> > hello all,
> > I am using postgresql as a database server for my db application.
>
> > The database is related to accounts and point of sales and many
> > calculations involve money datatype.
>
> > The application logic is totally done in python.  Now the problem is
> > that there is a situation where we calculate tax on % of the total
> > amount on the invoice.  The  percentage is in terms of float but the
> > eventual amount has to be in a money datatype.
>
> > The product cost comes in money datatype from postgresql and the tax %
> > comes in float.  So I would like to know if python supports some thing
> > similar to money datatype so that I can cast my total amount (in money )
> > divided by tax% (in float ) so money divided by float should be the
> > result and that result should be in money datatype.
> > For example a product x has the cost Rs. 100 which is stored in the
> > tabel as money type.  and in the tax table for that product the VAT is
> > 5% and this 5% is stored in float datatype.  So after using both these
> > values the resulting total 105Rs. should be in money type and not float.
> > I awaite some reply,
> > Thanks in advice.
> > happy hacking.
> > Krishnakant.
>
> Tax rate isn't divided, it's multiplied.  But probably that's just a
> typo in your message.
>
> Short answer is use your money class to convert the float into money.  
> And then test, to see that's really what you wanted.  Be sure and check
> the edge cases, because float is a binary system, and you can get both
> quantization and rounding problems converting between them.
>
> Is the money you're dealing with decimal-based?  So maybe money values
> are integers plus two more digits of fractional precision?
>
> It's going to be tough for somebody with no access to that "money type"
> to guess what your problem is.  What operations does it currently
> support?  If it supports multiply, then just change the tax rate to be a
> money type.

Aarrgghh!! If the "money type" supports multiplying money by money,
the author should be put in the stocks and pelted with rotten
vegetables.

(IMHO)