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[Python-Dev] Switch statement

Nicko van Someren nicko at nicko.org
Fri Jun 16 06:26:31 CEST 2006 
On 16 Jun 2006, at 00:49, Phillip J. Eby wrote:

> At 11:45 PM 6/15/2006 +0100, Nicko van Someren wrote:
>> On 15 Jun 2006, at 11:37, Nick Coghlan wrote:
>> > ...
>> > The lack of a switch statement doesn't really bother me personally,
>> > since I
>> > tend to just write my state machine type code so that it works  
>> off a
>> > dictionary that I define elsewhere,
>>
>> Not trying to push more LISP into python or anything, but of course
>> we could converge your method and the switch statement elegantly if
>> only we could put whole suites into lamdbas rather than just single
>> expressions :-)
>
> As has already been pointed out, this
>
> 1) adds function call overhead,
> 2) doesn't allow changes to variables in the containing function, and
> 3) even if we had a rebinding operator for free variables, we would  
> have the overhead of creating closures.

Noted.  I find (2) the most compelling issue.  I was merely  
suggesting a succinct way to express the model that Nick Cohglan was  
espousing.

> The lambda syntax does nothing to fix any of these problems, and  
> you can already use a mapping of closures if you are so inclined.   
> However, you'll probably find that the cost of creating the  
> dictionary of closures exceeds the cost of a naive sequential  
> search using if/elif.

The smiley was supposed to indicate that this was not an entirely  
serious suggestion; my apologies if the signal was lost in  
transmission.  In the equivalent if/elif to a switch you're only  
comparing a single value against a set of pre-computed values, and  
expecting to only do half the tests, so it's almost certainly going  
to be quicker than sorting out the whole set of closures.  I do  
however have a bug-bear about lambdas being restricted to single  
expressions, but maybe that's just me.

	Nicko
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