[Python-Dev] Switch statement (original) (raw)

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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