[Python-ideas] Yet Another Switch-Case Syntax Proposal

[Guido van Rossum]

I don't want to discourage you too much, but I think that adding a switch
statement comes *very* low on the list of improvements we would like to
make in Python 3.5.

We should probably focus on speed (or aspects of it, like startup),
language features that help porting Python 2 code to it (e.g. bytes
formatting), and things that improve the user experience of getting started
with Python on a new machine (e.g. pip/venv). Or perhaps stdlib issues like
an asyncio-infused variation of WSGI.

I've probably missed a few focus areas, but I still very much doubt we'll
be adding a switch statement -- it's a "sexy" language design issue (like
anonymous functions) but that's not what will help Python compete.




On Thu, Apr 17, 2014 at 12:14 PM, Lucas Malor <7vsfeu4pxg at snkmail.com>wrote:

> Hello everybody. I firstly proposed this syntax in the python-list. As I
> already wrote there, I read PEP 3103 and I'm not completely satisfied by
> any of the proposed solutions.
>
> My first idea was a little different, this is my final proposal after a
> short but good brainstorm:
>
> switch_stmt ::=  "switch" switch_expr "case" case_expr ":" suite
>     ("case" | "elcase" case_expr ":" suite)*
>     ["else" ":" suite]
> switch_expr ::= expression
> case_expr ::= expression_list
>
>  - if case_expr is a tuple, the case suite will be executed if switch_expr
> is a member of the tuple
>  - if case_expr is not a tuple, the case suite will be executed if
> switch_expr == case_expr
>  - if a case_expr is checked, any subsequent elcase statements are
> skipped, and the next case statement is performed, of there's one. This is
> completely identical to if - elif.
>
> Example:
>
> briefing_days = ("Tue", "Thu")
> normal_days = ("Mon", "Wed", "Fri")
>
> switch day case normal_days + briefing_days:
>     go_to_work = True
>     day_type = "weekday"
> case normal_days:
>     lunch_time = datetime.time(12)
>     meeting_time = datetime.time(14)
> elcase briefing_days:
>     lunch_time = datetime.time(11, 30)
>     meeting_time = datetime.time(12, 30)
> else:
>     go_to_work = False
>     day_type = "festive"
>     lunch_time = None
>     meeting_time =None
>
> A simpler example:
>
> switch tarot case 0:
>     card = "Fool"
> elcase 1:
>     card = "Alan Moore"
> elcase 2:
>     card = "High Priestess"
> <etc....>
>
> Some remarks:
>
> 1. switch is on the same line of the first case. This is because
> alternatives in PEP 3103 seems unpythonic to me
> 2. I decided to not use already existing keywords like "if" or "in", since
> this will be misleading
> 3. I preferred case / elcase instead of using break or continue keyword
> because:
>     a. break / continue can confuse, since they are used in loop
> statements, and this is not a loop statement, as Ethan Furman pointed out
> in the other mailing list
>     b. break / continue is useful only if it can be used not only as final
> command, so you can skip the rest of the current case suite. If it must be
> the final command, it's the same of case - elcase. IMHO there's no much
> real need to allow this
>     c. case - elcase syntax is much more readable and less confusing of a
> break / continue
>     d. case - elcase syntax is identical to if - elif, so people will be
> more familiar with it
> 4. notice that you can put a "case" statement after an "elcase" one. For
> example:
>
> switch cpu case "A6-6400K", "A8-6600K"
>     clock = 3.9
> elcase "A10-6790K", "A10-6800B"
>     clock = 4.1
> case "A6-6400K"
>     tdp = 65
> elcase "A8-6600K", "A10-6790K", "A10-6800B"
>     tdp = 100
>
> is equivalent to
>
> if cpu in ("A6-6400K", "A8-6600K")
>     clock = 3.9
> elif cpu in ("A10-6790K", "A10-6800B")
>     clock = 4.1
>
> if cpu == "A6-6400K"
>     tdp = 65
> elif cpu in ("A8-6600K", "A10-6790K", "A10-6800B")
>     tdp = 100
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>



-- 
--Guido van Rossum (python.org/~guido)
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