[Python-Dev] exception chaining

Benjamin Peterson benjamin at python.org
Fri Jan 20 21:56:27 CET 2012

2012/1/20 Ethan Furman <ethan at stoneleaf.us>:
> Benjamin Peterson wrote:
>> 2012/1/20 Ethan Furman <ethan at stoneleaf.us>:
>>> Summary:
>>> Exception Chaining is cool, unless you are writing libraries that want to
>>> transform from Exception X to Exception Y as the the previous exception
>>> context is unnecessary, potentially confusing, and cluttery (yup, just
>>> made
>>> that word up!).
>>> For all the gory details, see http://bugs.python.org/issue6210.
>>> I'm going to attempt a patch implementing MRAB's suggestion:
>>> try:
>>>   some_op
>>> except ValueError:
>>>   raise as OtherError() # `raise` keeps context, `raise as` does not
>> I dislike this syntax. Raise what as OtherError()? I think the "raise
>> x from None" idea is preferable, since it indicates you are nulling
>> the context. The optimal solution would be to have "raise X
>> nocontext", but that would obviously require another keyword...
> Raise 'the error' as OtherError.

Where 'the error' is? Aren't you trying to override the current error?

> The problem I have with 'raise x from None' is it puts 'from None' clear at
> the end of line -- not a big deal on this very short example, but when you
> have actual text it's not as obvious:
> except SomeError():
>    raise SomeOtherError('explanatory text with actual %data to help track
> down the problem' % data) from None
> Of course, I suppose that same issue exists with the 'raise x from exc'
> syntax, and 'from None' certainly matches that better...



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