[Python-Dev] exception chaining

Ethan Furman ethan at stoneleaf.us
Fri Jan 20 21:05:12 CET 2012

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.

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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