[Python-Dev] Py3k: Except clause syntax

[Guido van Rossum]

On 3/18/06, skip at pobox.com <skip at pobox.com> wrote:
> The comma and "as" have different precedence in your proposed except clause
> than they currently do in the import statement.  I think that can lead to
> confusion.  In particular, if someone is used to
>
>     from foo import bar, baz as flurp
>
> I think they might expect that in this try/except statement:
>
>     try:
>         something()
>     except E1, E2 as e
>         print "something bad happened"
>
> if E1 is raised, e will not be (re)bound, but if E2 is raised, then it will
> be.  I know Python is not as heavily an operator-oriented language as C or
> Perl, but I think it makes sense to maintain the same relative binding
> precedence between operators in different contexts wherever possible.

This is as good a point as any to explain that 'as' is not an
operator. It's a piece of syntax. It behaves different in each context
where it is used -- because the context determines what it does. The
parallel you suggest simply doesn't make sense: 'as' in the import
context causes a simple renaming, and the name should be different for
each renamed module, of course. But 'as' in an except clause is not a
renaming -- it simply separates the exception "matching pattern" from
the variable that is to receive the exception *value*. Having
different exceptions bind different variables doesn't make sense --
how would the code know which variable to use?

> There seem to be other places where Python is beginning to require parens
> even though they aren't strictly necessary to resolve syntactic ambiguity.

In the style guide only, I hope. The parens that are mandatory in a
few places *are* necessary to resolve ambiguity.

> This might be one of them.

No, the original proposal (which was already in PEP 3000 anyway) is fine.

--
--Guido van Rossum (home page: http://www.python.org/~guido/)