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

[Lucas Malor]

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