[Python-ideas] Match statement brainstorm

[Michael Selik]

On Thu, May 19, 2016 at 10:38 PM Nick Coghlan <ncoghlan at gmail.com> wrote:

> On 19 May 2016 at 14:15, Guido van Rossum <guido at python.org> wrote:
> > The attribute idea would be similar to a duck-type check, though more
> > emphasizing data attributes. It would be nice if we could write a
> > match that said "if it has attributes x and y, assign those to local
> > variables p and q, and ignore other attributes".
>


> If we went down that path, then the "assign if you can, execute this
> case if you succeed" options would presumably need an explicit prefix
> to indicate they're not normal expressions, perhaps something like
> "?=":
>
>     switch expr as arg:
>         case ?= (.x as p, .y as q): print('x=', p, 'y=', q)
>

If you don't mind adding a new operator, then an easier way to handle
several of these situations would be to make ``?=`` an assignment
expression that evaluates to True/False whether the assignment succeeded:

    def foo(obj):
        return a ?= obj.a

Could be equivalent to:

    def foo(obj):
        try:
            a = obj.a
        except Exception:
            return False
        else:
            return True


The use-cases of are somewhat overlapping with the idea of an inline
try/except as in PEP 463 (https://www.python.org/dev/peps/pep-0463/).


Which would then have the further implication that it might also make
> sense to support attribute unpacking as the LHS of normal assignment
> statements:
>
>     (.x as p, .y as q) = expr
>

The ``as`` syntax flips the familiar variable-on-the-left and makes this
one tough for me to read. I'd rather force a little repetition:

    o = expr
    p, q = o.x, o.y

Using a temporary variable like ``o`` makes it even fewer characters than
the proposed parens and ``as`` keyword.
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