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Subject: Re: except clause syntax question
From: Charles Yeomans <charles@declareSub.com>
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Date: Tue, 31 Jan 2012 13:04:22 -0500
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On Jan 31, 2012, at 8:24 AM, Mel Wilson wrote:

> Charles Yeomans wrote:
>=20
>> To catch more than one exception type in an except block, one writes
>>=20
>> except (A, B, C) as e:
>>=20
>> I'm wondering why it was decided to match tuples, but not lists:
>>=20
>> except [A, B, C] as e:
>>=20
>> The latter makes more sense semantically to me -- "catch all =
exception
>> types in a list" as opposed to "catch this single thing composed of =
three
>> exception types".
>=20
> On reflection, it seems to hint at a style that Python extensions were =
made=20
> in.  (IIRC) the first operand in an `except` statement was originally =
just=20
> an arbitrary marker to identify the exception.  Unique string values =
were=20
> customary, although the Python library defined things with standard=20
> exception names.  Using a string means that general exceptions weren't =
to be=20
> collected in general sequences; `except "Serious Error"` was never =
meant to=20
> catch `raise "r"`.  If only tuples were used for collections, it would=20=

> create havoc for fewer of any weirdos who had used strange markers of =
their=20
> own devising.
>=20
> It looks like tuples were chosen as the most "lightweight", or maybe =
least=20
> intrusive, sequence type to require to denote a collection of =
exceptions.
>=20
> You see a similar decision, with the opposite emphasis, with the =
string=20
> modulo operator.  The second operand is supposed to be a tuple, but if =
the=20
> template string needs only one value, then the rules are relaxed and =
any=20
> single non-tuple value is used as-is.
>=20

Compatilbility; that makes sense.  I came to python well after strings =
were used for exceptions.  Thanks.


Charles Yeomans