[Python-ideas] PEP-3151 pattern-matching

[Antoine Pitrou]

On Fri, 8 Apr 2011 10:11:34 -0700
Guido van Rossum <guido at python.org> wrote:
> With apologies for not reading the PEP or this thread in full, some comments:
> 
> - I really like the syntax "except <exc> [as <var>] [if <test>]:".
> This addresses a pretty common use case in my experience. I don't care
> for the alternate proposed syntax that started this thread. I'm not
> sure that the 'if' subclause makes sense without the 'as' subclause,
> since most likely you'd want to refer to the caught exception. I note
> that it is more powerful than putting "if not <test>: raise" in the
> body of the except-clause, because it will allow subsequent except
> clauses to match still. I also note that it is a much "cleaner" change
> than (again) reorganizing the exception hierarchy, since there is no
> backward compatibility to consider.

My main issue with said new syntax is that it doesn't make things much
easier to write. You still have to type an explicit condition, and
remember the appropriate errno mnemonic for the situation. The very
notion of "errno" and its plethora of abbreviated error codes is ok for
people used to C programming, but certainly alien to other programmers
(how long does it take to remember that "file not found" is spelled
"ENOENT"?).

The fact that exception messages typically indicate the errno *number*,
not mnemonic, is an additional annoyance (errno numbers are not
standardized and can vary from system to system).

> - Regarding restructuring the exception tree, I don't think this needs
> to wait for Python 4. (We've done it several times during Python 2.)
> But it is very tricky to do it without breaking existing code, and I'm
> not sure that deprecations will help that much. E.g. if two exceptions
> A and B are currently not related via inheritance, and we were to make
> A a subclass of B, then in code of the form try: ... except B: ...
> except A: ... the except A clause would become unreachable. Maybe it's
> easier to propose small changes instead of trying to refactor a huge
> section of the exception tree?

Well, I fear that small changes will not lead us very far. We have to
level the existing tree before adding new types, because otherwise it's
not obvious where these new types should be grafted, since in practice
OSError, IOError or socket.error can be used for very similar error
conditions.

> - Quite independently, I think it is time that we started documenting
> which exceptions are raised from standard APIs.

Agreed, and unit-test them too.

> (But it may well
> define a new kind of "bad data" exception, and I don't think we need a
> common base class for all "bad data" or "bad state" exceptions.)

Isn't it ValueError actually? For example, closed files raise
ValueError when you try to do an operation on them.

Regards

Antoine.