[Python-Dev] Switch statement

[M.-A. Lemburg]

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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