[Python-Dev] decimal API

[mcherm2 at mcherm.com]

Facundo writes:
> I'm +1 to enable some by default: DivisionByZero, DivisionUndefined,
> Overflow, InvalidOperation, ConversionSyntax and
> DivisionImpossible are my
> proposal, but it's my taste and know nothing about numbers
> usability...

Here are my thoughts, just as untutored as yours. I'm working
primarily from hursley's spec here...

Signal              Should it raise by default? And why?
------              ------------------------------------
Clamped             Yes - well, maybe. When could we ever generate this?
ConversionSyntax    Yes - it prevents lots of possible user errors
DivisionByZero      Yes - other numeric types raise for this in Python
DivisionImpossible  Yes - I don't fully understand it but it seems bad
DivisionUndefined   Yes - other numeric types raise for this in Python
Inexact              No - this is perfectly normal
InsufficientStorage N/A - this doesn't exist in the Python implementation
InvalidContext      Yes - I don't understand how this could happen, but
                          surely we should complain about it, right?
InvalidOperation    Yes - Users want exceptions when doing invalid math
Overflow            Yes - overflows lose data, so we should raise
Rounded              No - this is perfectly normal
Subnormal            No - this is perfectly normal (subnormal?)
Underflow            No - since Inexact, Rounded, and Subnormal are all
                          unreported, this should be also!

Then there are these, which I don't recognize:
    DecimalException, LostDigits


To sum things up, I would suggest that *by default* errors should never
pass silently, and that "errors" is defined as broadly as possible
without triggering on normal arithmatic. Obviously, it's easy to
change the settings, but it's much easier to DISABLE an exception when
your program starts generating it, than to ENABLE it, because if it's
disabled, you don't necessarily know that it is happening. Besides,
it seems Pythonic to be pedantic.

-- Michael Chermside