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Re: except clause syntax question

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On Jan 31, 2012, at 8:24 AM, Mel Wilson wrote:

> Charles Yeomans wrote:
> 
>> To catch more than one exception type in an except block, one writes
>> 
>> except (A, B, C) as e:
>> 
>> I'm wondering why it was decided to match tuples, but not lists:
>> 
>> except [A, B, C] as e:
>> 
>> 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".
> 
> On reflection, it seems to hint at a style that Python extensions were made 
> in.  (IIRC) the first operand in an `except` statement was originally just 
> an arbitrary marker to identify the exception.  Unique string values were 
> customary, although the Python library defined things with standard 
> exception names.  Using a string means that general exceptions weren't to be 
> collected in general sequences; `except "Serious Error"` was never meant to 
> catch `raise "r"`.  If only tuples were used for collections, it would 
> create havoc for fewer of any weirdos who had used strange markers of their 
> own devising.
> 
> It looks like tuples were chosen as the most "lightweight", or maybe least 
> intrusive, sequence type to require to denote a collection of exceptions.
> 
> You see a similar decision, with the opposite emphasis, with the string 
> modulo operator.  The second operand is supposed to be a tuple, but if the 
> template string needs only one value, then the rules are relaxed and any 
> single non-tuple value is used as-is.
> 

Compatilbility; that makes sense.  I came to python well after strings were used for exceptions.  Thanks.


Charles Yeomans

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except clause syntax question Charles Yeomans <charles@declareSub.com> - 2012-01-30 12:41 -0500
  Re: except clause syntax question Mel Wilson <mwilson@the-wire.com> - 2012-01-30 14:15 -0500
  Re: except clause syntax question Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-01-31 00:00 +0000
    Re: except clause syntax question Devin Jeanpierre <jeanpierreda@gmail.com> - 2012-01-30 19:25 -0500
    Re: except clause syntax question Charles Yeomans <charles@declareSub.com> - 2012-01-31 08:57 -0500
      Re: except clause syntax question Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-01-31 14:51 +0000
        Re: except clause syntax question Charles Yeomans <charles@declareSub.com> - 2012-01-31 11:23 -0500
        Re: except clause syntax question Devin Jeanpierre <jeanpierreda@gmail.com> - 2012-01-31 11:38 -0500
        Re: except clause syntax question Charles Yeomans <charles@declaresub.com> - 2012-01-31 12:29 -0500
        Re: except clause syntax question Ethan Furman <ethan@stoneleaf.us> - 2012-01-31 08:56 -0800
    Re: except clause syntax question Terry Reedy <tjreedy@udel.edu> - 2012-01-31 19:12 -0500
    Re: except clause syntax question Charles Yeomans <charles@declareSub.com> - 2012-01-31 19:27 -0500
  Re: except clause syntax question Mel Wilson <mwilson@the-wire.com> - 2012-01-31 08:24 -0500
    Re: except clause syntax question Charles Yeomans <charles@declareSub.com> - 2012-01-31 13:04 -0500
  Re: except clause syntax question Duncan Booth <duncan.booth@invalid.invalid> - 2012-01-31 22:03 +0000
    Re: except clause syntax question Chris Angelico <rosuav@gmail.com> - 2012-02-01 11:53 +1100
      Re: except clause syntax question Mel Wilson <mwilson@the-wire.com> - 2012-01-31 23:39 -0500
    Re: except clause syntax question Ian Kelly <ian.g.kelly@gmail.com> - 2012-01-31 18:09 -0700
    Re: except clause syntax question Ian Kelly <ian.g.kelly@gmail.com> - 2012-01-31 18:12 -0700
    Re: except clause syntax question Chris Angelico <rosuav@gmail.com> - 2012-02-01 13:46 +1100

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