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[Python-Dev] PEP 3103: A Switch/Case Statement

Fredrik Lundh fredrik at pythonware.com
Tue Jun 27 07:28:31 CEST 2006 
Guido van Rossum wrote:

> Most school I proponents (perhaps you're the only exception) have
> claimed that optimization is desirable, but added that it would be
> easy to add hash-based optimization. IMO it's not so easy in the light
> of various failure modes of hash(). (A possible solution would be to
> only use hashing if the expression's type is one of a small set of
> trusted builtins, and not a subclass; we can trust int.__hash__,
> str.__hash__ and a few others.)

that's the obvious approach for the optimize-under-the-hood school -- 
only optimize if you can do that cleanly (to enable optimization, all 
case values must be either literals or statics, have the same type, and 
belong to a set of trusted types).  this gives a speedup in all main use 
cases, and clear semantics in all cases.

another approach would be to treat switch/case and switch/case-static as 
slightly different constructs; if all cases are static, put the case 
values in a dictionary, and do the lookup as usual (propagating any errors).

</F>
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