[Python-Dev] PEP 3103: A Switch/Case Statement (original) (raw)

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).

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