[Python-ideas] Match statement brainstorm

[Michael Selik]

On Mon, May 23, 2016 at 3:58 PM Guido van Rossum <guido at python.org> wrote:

> On Mon, May 23, 2016 at 11:45 AM, Joao S. O. Bueno
> <jsbueno at python.org.br> wrote:
> > I still fail to see what justifies violating The One Obvious Way to Do
> It which uses an if/elif sequence
>
> Honestly I'm not at all convinced either! If it was easy to do all
> this with a sequence of if/elif clauses we wouldn't need it. The
> problem is that there are some types of matches that aren't so easy to
> write that way, e.g. combining an attempted tuple unpack with a guard,
> or "instance unpack" (check whether specific attributes exist)
> possibly combined with a guard. (The tricky thing is that the guard
> expression often needs to reference to the result of the unpacking.)
>

Wouldn't an exception-catching expression (PEP 463) enable most of what you
want with the existing if/elif/else chain?

    def demo(arg):
        if ((arg.x, arg.y) except AttributeError: False):
            print('x=', arg.x, 'y=', arg.y)
        elif ((arg[0], arg[1]) except IndexError: False):
            a, b, *_ = arg
            print('a=', a, 'b=', b)
        else:
            print('Too bad')


To get everything you want, we could have an exception-catching assignment
expression. While a plain ``=`` assignment would be problematic as an
expression due to the common ``==`` typographical error, a less error-prone
operator might be OK. I'd suggested ``?=`` earlier in the thread, but
perhaps I didn't write out a good example at that time. Here's one that's
closer to the original demo example.

    def demo(arg):
        if p, q ?= arg.x, arg.y: print('x=', p, 'y=', q)
        elif a, b, *_ ?= arg: print('a=', a, 'b=', b)
        else: print('Too bad')


I figure it's better to solve the category of problems --
exception-catching expressions -- rather than the single problem of
catching exceptions in an if/elif/else chain. Or do you think this is too
much temptation for unreadable code golf?
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