[Python-Dev] Switch statement

Fri Jun 16 10:20:20 CEST 2006

Raymond Hettinger wrote:
>>> The optimisation of the if-elif case would then simply be to say that the
>>> compiler can recognise if-elif chains like the one above where the RHS
>>> of the comparisons are all hashable literals and collapse them to switch
>>> statements.
>>>>>>> Right (constants are usually hashable :-).
>>>> The LHS is more challenging. Given:
>> if x == 1: p_one()
> elif x == 2: p_two()
> elif x == 3: p_three()
> else: p_catchall()
>> There is no guarantee that x is hashable. For example:
>> class X:
> def __init__(self, value):
> self.value = value
> def __cmp__(self, y):
> return self.value == y
> x = X(2)

That's a good point.
The PEP already addresses this by retricting the type of x to a
few builtin immutable and hashable types:
 ...the switching variable is one of the builtin
 immutable types: int, float, string, unicode, etc. (not
 subtypes, since it's not clear whether these are still
 immutable or not).
-- 
Marc-Andre Lemburg
eGenix.com
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