Exception Handling in Python 3

MRAB python at mrabarnett.plus.com
Fri Oct 29 19:41:20 CEST 2010

On 24/10/2010 13:28, Steve Holden wrote:
> On 10/24/2010 4:48 AM, Martin v. Loewis wrote:
>> Am 24.10.2010 07:01, schrieb Steve Holden:
>>> I was somewhat surprised to discover that Python 3 no longer allows an
>>> exception to be raised in an except clause (or rather that it reports it
>>> as a separate exception that occurred during the handling of the first).
>> I think you are misinterpreting what you are seeing. The exception being
>> raised actually *is* an attribute error, and it actually is the
>> attribute error that gets reported. It's only that reporting an
>> exception that has a __context__ first reports the context, then reports
>> the actual exception.
> I don't believe I *am* misinterpreting it. The fact of the matter is
> that the context is irrelevant to the user, and there should be some way
> to suppress it to avoid over-complicating the traceback.
> This behavior is quite reasonable during testing, but I would prefer to
> exclude an explicit raise directly in the except handler since that is
> hardly to be construed as accidental (whereas an exception in a function
> called in the handler perhaps *should* be reported).
>> You may now wonder whether it is possible to set __context__ to None
>> somehow. See PEP 3134:
>> Open Issue: Suppressing Context
>>      As written, this PEP makes it impossible to suppress '__context__',
>>      since setting exc.__context__ to None in an 'except' or 'finally'
>>      clause will only result in it being set again when exc is raised.
> I have already read that. Peter Otten has separately explained how to
> suppress the behavior using sys.excepthook, which appears to be a
> halfway satisfactory solution.
Suggestion: an explicit 'raise' in the exception handler excludes the 
context, but if you want to include it then 'raise with'. For example:

# Exclude the context
except KeyError:
     raise CommandError("Unknown command")

# Include the context
except KeyError:
     raise with CommandError("Unknown command")

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