[Python-ideas] Having a messageless exception show its docstring

[cool-RR]

I propose that when raising an exception without providing a message, the
first line of the docstring will be shown. (I mean shown in the traceback
where the message would normally be.

Example session:

####################################

>>> class MyExcpetion(Exception):
'''Trying to fit a square piece into a round hole.'''

>>> raise MyExcpetion('Problem bla bla')

Traceback (most recent call last):
  File "<pyshell#3>", line 1, in <module>
    raise MyExcpetion('Problem bla bla')
MyExcpetion: Problem bla bla

>>> raise MyExcpetion()

Traceback (most recent call last):
  File "<pyshell#4>", line 1, in <module>
    raise MyExcpetion()
MyExcpetion: Trying to fit a square piece into a round hole.

####################################

This will be useful for me because I often raise exceptions and have nothing
to say besides repeating the exception's docstring, and I have many places
in the code where I'd want to conditionally raise it. I'm faced with two
choices: Type the exception's docstring in every place, which is lame, and
breaks DRY. Or just raise the exception without any message, which may
potentially confuse users.

This is the motivation for this suggestion, so I could be succinct without
confusing users.

What do you think?

Ram.
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