[Python-Dev] Switch statement

[Josiah Carlson]

"Phillip J. Eby" <pje at telecommunity.com> wrote:
> At 06:56 PM 6/18/2006 -0700, Josiah Carlson wrote:
> >The non-fast version couldn't actually work if it referenced any names,
> >given current Python semantics for arbitrary name binding replacements.
> 
> Actually, one could consider "case" expressions to be computed at function 
> definition time, the way function defaults are.  That would solve the 
> problem of symbolic constants, or indeed any sort of expressions.

Using if/elif/else optimization precludes any non-literal constants, so
we would necessarily have to go with a switch/case for this semantic. It
seems as though it would work well, and wouldn't be fraught with
any of the gotchas that catch users like:
    def fcn(..., dflt={}, dflt2=[]):
...


> An alternate possibility would be to have them computed at first use and 
> cached thereafter.
> 
> Either way would work, and both would allow multiple versions of the same 
> switch statement to be spun off as closures without losing their "constant" 
> nature or expressiveness.  It's just a question of which one is easier to 
> explain.  Python already has both types of one-time initialization: 
> function defaults are computed at definition time, and modules are only 
> loaded once, the first time you import them.

I would go with the former rather than the latter, if only for
flexibility.

 - Josiah