[Python-Dev] Switch statement

[Phillip J. Eby]

At 09:16 AM 6/21/2006 -0700, Guido van Rossum wrote:
>After thinking about it a bit I think that if it's not immediately
>contained in a function, it should be implemented as alternative
>syntax for an if/elif chain.

That worries me a little.  Suppose I write a one-off script like this:

for line in sys.stdin:
     words = line.split()
     if words:
         switch words[0]:
             case "foo": blah
             case words[-1]: print "mirror image!"

Then, if I later move the switch into a function, it's not going to mean 
the same thing any more.  If the values are frozen at first use or 
definition time (which are the same thing for module-level code), then I'll 
find the lurking bug sooner.

OTOH, breaking it sooner doesn't seem like such a great idea either; seems 
like a recipe for a newbie-FAQ, actually.  ISTM that the only sane way to 
deal with this would be to ban the switch statement at module level, which 
then seems to be an argument for not including the switch statement at all.  :(

I suppose the other possibility would be to require at compilation time 
that a case expression include only non-local variables.  That would mean 
that you couldn't use *any* variables in a case expression at module-level 
switch, but wording the error message for that to not be misleading might 
be tricky.

I suppose an error message for the above could simply point to the fact 
that 'words' is being rebound in the current scope, and thus can't be 
considered a constant.  This is only an error at the top-level if the 
switch appears in a loop, and the variable is rebound somewhere within that 
loop or is rebound more than once in the module as a whole (including 
'global' assignments in functions).