startswith( prefix[, start[, end]]) Query

Bruno Desthuilliers bruno.42.desthuilliers at wtf.websiteburo.oops.com
Fri Sep 7 09:49:29 CEST 2007


Steve Holden a écrit :
> TheFlyingDutchman wrote:
>>> Else, you could as well write your own testing function:
>>>
>>> def str_starts_with(astring, *prefixes):
>>>    startswith = astring.startswith
>>>    for prefix in prefixes:
>>>      if startswith(prefix):
>>>        return true
>>>    return false
>>>
>>
>> What is the reason for
>>   startswith = astring.startswith
>>   startswith(prefix)
>>
>> instead of
>>   astring.startswith(prefix)
>>
> It's an optimization: the assigment creates a "bound method" (i.e. a 
> method associated with a specific string instance) and avoids having to 
> look up the startswith method of astring for each iteration of the inner 
> loop.
> 
> Probably not really necessary, though, and they do say that premature 
> optimization is the root of all evil ...

I wouldn't call this one "premature" optimization, since it doesn't 
change the algorithm, doesn't introduce (much) complication, and is 
proven to really save on lookup time.

Now I do agree that unless you have quite a lot of prefixes to test, it 
might not be that necessary in this particular case...



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