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Date: Mon, 7 Apr 2014 04:07:02 +1000
Subject: Re: Yet Another Switch-Case Syntax Proposal
From: Chris Angelico <rosuav@gmail.com>
Cc: Python <python-list@python.org>
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On Mon, Apr 7, 2014 at 3:49 AM, Marco S. <elbarbun@gmail.com> wrote:
> switch day case in briefing_days:
>
>     lunch_time = datetime.time(11, 30)
>     meeting_time = datetime.time(12, 30)
> case not in briefing_days + festive_days:
>
>     lunch_time = datetime.time(12)
>     meeting_time = datetime.time(14)
> case in festive_days:
>
>     go_to_work = False
>     day_type = "festive"
> else:
>
>     go_to_work = True
>     day_type = "ferial"
>
> The if-else equivalent will be:
>
> if day in briefing_days:
>
>     lunch_time = datetime.time(11, 30)
>     meeting_time = datetime.time(12, 30)
> if day not in briefing_days + festive_days:
>
>     lunch_time = datetime.time(12)
>     meeting_time = datetime.time(14)
> if day in festive_days:
>
>     go_to_work = False
>     day_type = "festive"
> else:
>
>     go_to_work = True
>     day_type = "ferial"

Here's a simpler form of the proposal, which might cover what you
need. It's basically a short-hand if/elif tree.

case expression comp_op expression:
    suite
case [comp_op] expression:
    suite
...
else:
    suite

This has a slight oddity of parsing (in that an expression can
normally have a comparison in it); if you really want to use the
result of a comparison inside a case block, you'd have to parenthesize
it. But it's easy enough to explain to a human.

case day in briefing_days:
    lunch_time = datetime.time(11, 30)
    meeting_time = datetime.time(12, 30)
case not in briefing_days + festive_days:
    lunch_time = datetime.time(12)
    meeting_time = datetime.time(14)
case in festive_days:
    go_to_work = False
    day_type = "festive"
else:
    go_to_work = True
    day_type = "ferial"

A case statement that opens with a comparison operator takes the value
from the previous case (without re-evaluating it); a case statement
that lacks a comparison altogether assumes == and does the above. In
either case (pardon the pun), the check will be done only if the
preceding case was false. An 'else' clause is effectively equivalent
to a 'case' that's always true.

Adds only one keyword to the language ("switch" is gone), and adds an
edge case to parsing that's unlikely to come up in non-contrived code.

ChrisA