[Python-ideas] raise EXC from None (issue #6210)

[Nick Coghlan]

The class method approach I describe in the linked issue is more likely to
be accepted - this problem shouldn't need new syntax to resolve.

--
Nick Coghlan (via Gmail on Android, so likely to be more terse than usual)
On Oct 22, 2011 4:47 PM, "Steven D'Aprano" <steve at pearwood.info> wrote:

> Jan Kaliszewski wrote:
>
>> Hello.
>>
>> Some time ago I encountered the problem described in PEP 3134 as "Open
>> Issue: Suppressing Context" ("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.").
>>
>> An idea that then appeared in my brain was:
>>
>>    raise SomeException(some, arguments) from None
>>
>> ...and I see the same idea has been proposed by Patrick Westerhoff here:
>> http://bugs.python.org/**issue6210 <http://bugs.python.org/issue6210>
>>
>
>
> I think that stating the syntax is the easy part, actually implementing may
> not be so simple.
>
>
>  I am +10, as I feel that's intuitive, elegant and status-quo-consistent.
>>
>> And what do you think?
>>
>
> I think that it's well past time to fix this wart with the new nested
> exceptions functionality.
>
> +1
>
> (Actually I'm also +10 but I don't want to start vote inflation.)
>
> The wart I'm referring to is that the common idiom of catching one
> exception and raising another is now treated as a bug in the except clause
> even when it isn't:
>
> >>> def mean(data):
> ...     try:
> ...             return sum(data)/len(data)
> ...     except ZeroDivisionError:
> ...             raise ValueError('data must be non-empty')
> ...
> >>> mean([])
> Traceback (most recent call last):
>  File "<stdin>", line 3, in mean
> ZeroDivisionError: int division or modulo by zero
>
> During handling of the above exception, another exception occurred:
>
> Traceback (most recent call last):
>  File "<stdin>", line 1, in <module>
>  File "<stdin>", line 5, in mean
> ValueError: data must be non-empty
>
>
> In this case, there is absolutely no reason to expose the fact that
> ZeroDivisionError occurred. That's an implementation detail which is
> irrelevant to the caller. But exposing it complicates the traceback for no
> benefit (since it isn't a bug that needs fixing) and possibly some harm (by
> causing some confusion to the reader).
>
> It also reflects badly on your code: it gives the impression of an
> unhandled bug when it is not.
>
> In my opinion, although the error context functionality is a big positive
> when debugging actual bugs in except clauses, the feature should never have
> been added without a way to suppress the error context.
>
>
> --
> Steven
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