except clause syntax question

[Duncan Booth]

Charles Yeomans <charles at declareSub.com> 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". 
> 
It may not be the only reason but the code would have to be slower and much 
more complex to handle lists.

If you wanted you can write:

   except ((A,), ((B,), C)) as e:

or other such complicated expression with nested tuples. If lists were 
allowed in a similarly nested structure there would be a danger that you 
could pass in a recursive list structure so the code would have to detect 
and avoid infinite loops.

    exceptions = [A, B, C]
    exceptions[1:1] = exceptions,
    ...
    except exceptions as e: # argh!

Abitrarily nested tuples of exceptions cannot contain loops so the code 
simply needs to walk through the tuples until it finds a match.

-- 
Duncan Booth http://kupuguy.blogspot.com