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[Python-Dev] Switch statement

Guido van Rossum guido at python.org
Wed Jun 21 18:47:21 CEST 2006 
On 6/21/06, Fredrik Lundh <fredrik at pythonware.com> wrote:
> Guido van Rossum wrote:
>
> > (Note how I've switched to the switch-for-efficiency camp, since it
> > seems better to have clear semantics and a clear reason for the syntax
> > to be different from if/elif chains.)
>
> if you're now in the efficiency camp, why not just solve this on the
> code generator level ?  given
>
>      var = some expression
>      if var == constant:
>          ...
>      elif var == constant:
>          ...
>
> let the compiler use a dispatch table, if it can and wants to.

But in most cases the 'constant' is actually an expression involving a
global, often even a global in another module. (E.g. sre_compile.py)
The compiler will have a hard time proving that this is really a
constant, so it won't optimize the code.

The proposed switch semantics (create the table when the containing
function is defined) get around this by "defining" what it means by
"constant".

BTW I would like references to locals shadowing globals to be flagged
as errors (or at least warnings) so that users who deduced the wrong
mental model for a switch statement are caught out sooner.

-- 
--Guido van Rossum (home page: http://www.python.org/~guido/)
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