[Python-Dev] Why not make frames? [was: Alternative forms [was: PEP 463: Exception-catching expressions]] (original) (raw)

R. David Murray rdmurray at bitdance.com
Wed Mar 12 18:27:09 CET 2014

On Mon, 10 Mar 2014 14:26:14 +1100, Chris Angelico <rosuav at gmail.com> wrote:

On Mon, Mar 10, 2014 at 1:16 PM, Jim J. Jewett <jimjjewett at gmail.com> wrote: > I don't claim that syntax is perfect. I do think it is less flawed > than the no-parentheses (or external parentheses) versions: > > (expr1 except expr3 if expr2) > expr1 except expr3 if expr2 > > because the tigher parentheses correctly indicate that expr2 and expr3 > should be considered as a (what-to-do-in-case-of-error) group, which > interacts (as a single unit) with the main expression.

But it doesn't, really. The entire set of three expressions is a single unit. You can't break out the bit inside the parens and give that a name, like you can in most places where something "acts as a single unit" to interact with something else. (Yes, there are special cases, like the syntax for constructing slice objects that works only inside square brackets. And you can't break out a function's arguments, as a unit, into a single object (the nearest is *args,**kw). I said most places, and I don't want to add more to the special-case set.)

Actually, function arguments almost aren't a special case any more:

import inspect def a(a, b=2): ... print(a, b) ... def b(c, d=3): ... print(c, d) ... sa = inspect.signature(a) print(sa) (a, b=2) ba = sa.bind(1, 2) b(*ba.args, **ba.kwargs) 1 2

Note: I said almost :) But the point is that we found that the fact that we couldn't give this thing in parens a name bothersome enough to partially fix it.

--David

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