except clause syntax question

[Charles Yeomans]

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