[Python-3000] New builtin exceptions classes?

[Frank Benkstein]

Hi,

I'm new to this list and interested in Python 3000 development.

The following is not really a proposal that is ready to be implemented.
I'm merely interested if this is a problem others want to be solved,
too, and if it would be possible to get such a thing into Python 3000.

IMHO Exceptions should carry more information about why and where the
exception happened. A good example of an exception class carrying that
kind of information is IOError with its 'filename' and 'errno'
attributes (although these could be documented better). It would be
nice if all builtin Exceptions had those kind of attributes. As an
example KeyError could have a 'key' and a 'mapping' attribute,
IndexError could have a 'index' and an 'sequence' attribute and so on.
It is arguable if the second ones ('mapping' and 'sequence') are really
needed, but I think it would be really nice to have those as it would
make debugging a lot easier. Additionally they shouldn't really hurt
because as per PEP 3110 exception instances are deleted when the
exception handler is finished.

A further addition to this proposal would be to discourage raising
exceptions as wrappers around diagnostical messages. Currently it is
very hard to programmatically find out what happened here:

>>> int([])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: int() argument must be a string or a number, not 'list'

It would be nice if the recommended way would be something like the
following, with only __str__ formatting the message: 

raise TypeError(
    '%(target)s() argument must be one of %(allowed)s, not %(given)s',
    target=int,
    allowed=(str, int, float, long),
    given=type(...))

Maybe the the most common messages could be predefined in the Exception
class instead of constructed ad-hoc to make processing easier but
having them constructed as shown would already help a lot.

Best regards
Frank Benkstein.

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