[Python-Dev] Revised decimal type PEP

Michael McLay mclay@nist.gov
Tue, 31 Jul 2001 03:27:11 -0400

On Tuesday 31 July 2001 12:36 pm, M.-A. Lemburg wrote:

> I'd suggest to follow the rules for the SQL definitions
> of DECIMAL(,).

> Well, there are several options. I support that the IBM paper
> on decimal types has good hints as to what the type should do.
> Again, SQL is probably a good source for inspiration too, since
> it deals with decimals a lot.

Ok, I know about the IBM paper.  is there online document on the SQL 
semantics that can be referenced in the PEP?

> I see, the small 'b' still looks funny to me though. Wouldn't
> 1.23f and 25i be more intuitive ?

I originally used 'f' for both the integer and float.    The use of 'b' was 
suggested by Guido. There were two reasons not to use 'i' for integers.  The 
first has to do with how the tokenizer works.  It doesn't distringuish 
between float and int when the token string is passed to parsenumber().  Both 
float and int are processed by the same function.  I could have got around 
this problem by having the switch statement in parsenumber recognize both 'i' 
and 'f', but there is another problem with using 'i'.  The 25i would be 
confusing for someone if they was trying to use an imaginary numbers If they 
accidentally typed 25i instead of 25j they would get an integer instead of an 
imaginary number.  The error might not be detected since 3.0 + 4i would 
evaluate properly.

> > > I'd rather have this explicit in the sense that you define which
> > > assumptions will be made and what issues arise (rounding, truncation,
> > > loss of precision, etc.).
> >
> > Can you give an example of how this might be implemented.
> You would typically first coerce the types to the "larger"
> type, e.g. float + decimal -> float + float -> float, so
> you'd only have to document how the conversion is done and
> which accuracy to expect.

I would be concerned about the float + decimal automatically generating a 
float.  Would it generate an error message if the pedantic flag was set?  
Would it generate a warning in safe mode?

Also, why do you consider a float to be a "larger" value type than decimal?  
Do you mean that a float is less precise?