[Python-ideas] except expression

[Nick Coghlan]

On 19 Feb 2014 07:48, "Chris Angelico" <rosuav at gmail.com> wrote:
>
> On Wed, Feb 19, 2014 at 8:38 AM, Greg Ewing <greg.ewing at canterbury.ac.nz>
wrote:
> > What *might* be useful is to allow the exception type to
> > be omitted, and have it default to LookupError. Then the
> > most common anticipated use cases would be very concise:
> >
> >    things[i] except: default_value
>
> While that does seem very tempting, I'm strongly against having a
> dramatic-yet-likely-unnoticed difference between these two:
>
> _ = things[i] except: default_value
>
> and
>
> try:
>     _ = things[i]
> except:
>     _ = default_value
>
> By your suggestion, the first one is equivalent to:
>
> _ = things[i] except LookupError: default_value
>
> But by current rules of Python, the second is equivalent to:
>
> _ = things[i] except BaseException: default_value
>
> and that's really REALLY bad to do unexpectedly. Suppose the lookup
> into things[i] takes a long time (maybe the system's low on memory and
> has to page stuff back in), and the user hits Ctrl-C while it's doing
> it. Catching BaseException means getting back the default_value there;
> catching LookupError means having KeyboardInterrupt propagate upward.
> Same goes for typoing 'things', or any other sort of error. I want to
> be able to explain the exception-expression in terms of a
> similarly-spelled exception-statement, which means that every piece of
> common syntax should have parallel semantics - just as lambda and def
> do. That means having a bare except either cause an error, or do the
> same thing a bare except does in the statement form.

+1000 from me for disallowing bare except in the expression form -
suppressing SystemExit and KeyError is bad, and if an explicit exception is
required, people are more likely to spell "catch everything" as "except
Exception:" rather than "except BaseException:" .

Cheers,
Nick.
>
> ChrisA
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