[ x for x in xrange(10) when p(x) ]

[Steve Holden]

George Sakkis wrote:
> "bonono at gmail.com" wrote:
> 
> 
>>Alex Martelli wrote:
>>
>>>This becomes a valid list comprehension by writing 'if' instead of
>>>'when'.
>>
>>valid, yes. efficient, I am not sure.
>>
>>[ x for x in xrange(10000000) if p(x) ]
>>
>>means I need to go through the whole range even if p = lambda x: x < 2
> 
> 
> Itertools is your friend in this case:
> 
>>>>from itertools import takewhile
>>>>list(takewhile(p, xrange(10000000)))
> 
> [0, 1]

Maybe, but the code also implies an esoteric knowledge that the trught 
value of the predicate is monotonically decreasing (in a Boolean sense). 
This would not be true if (e.g.) p = lambda x: x % 2 == 0. So while 
itertools.takewhile can save you unnecessary computations, such savings 
rely on provable conditions of the predicate which are frequently false.

regards
  Steve
-- 
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