[Python-Dev] Py3k: Except clause syntax (original) (raw)
Nick Coghlan ncoghlan at gmail.com
Sat Mar 18 04:47:05 CET 2006
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Oleg Broytmann wrote:
On Fri, Mar 17, 2006 at 07:44:51PM +0000, John J Lee wrote:
Perhaps parentheses around the exception list should be mandatory for the 2-or-more exceptions case?
except NameError as e: --> fine except (NameError) as e: --> fine except (NameError,) as e: --> fine except NameError, OtherError as e: --> SyntaxError except (NameError, OtherError) as e: --> fine If parens are mandatory what is the difference from the current syntax? Why do we need "as" here? Why not stay with except (NameError, OtherError), e:
Currently the comma is used for two very different purposes within the same clause, so it's far too easy to make the mistake:
except NameError, OtherError:
and then have to deal with the fact that OtherError isn't caught (and given how often unit tests fail to cover all error cases, it wouldn't be unusual for this bug to escape a unit testing regime, too).
If this code is made to "do the right thing", then there needs to be some other syntax to indicate the name that should be used for the value of the caught exception.
"as" is already used as an embedded naming syntax in two places:
import statements use it for "locate an object or module using one name, but bind it to a different name in this module" In this case, the binding is restricted to a single name and is applied individually to each element of a comma separated list:
import foo as a, bar as b, baz as c
with statements use it for "retrieve the context manager for this context and bind the result of the manager's enter method to this name" In this case, the binding may be either a single name or a tuple of names, and the expression itself is limited to a single term - parentheses are required around both comma separated lists and unpacking to separate names is optional:
with (foo, bar, baz) as (a, b, c): pass with (foo, bar, baz) as x: pass
The proposal to use it for except clauses encounters a subtle difference - in the except clause, even if multiple exception types are listed in the clause, there is still only a single exception value to be bound to the name. The list of exception names merely limits the possible types for that exception, rather than their names.
So, as a somewhat novel approach, what about putting the "as" before the list of exceptions types? After all, the thing being bound is the exception caught, not the possible list of types:
except NameError, OtherError: # Exception value not bound
except as ex, NameError, OtherError: # Exception value bound to ex
I don't actually have a problem with simply tacking the clause on the end (with or without mandatory parentheses) but figured I'd throw this idea out as an option. I'm not sure whether the parallel with "print" and "print >> stream," is a positive or a negative, though. . .
Cheers, Nick.
-- Nick Coghlan | ncoghlan at gmail.com | Brisbane, Australia
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