[Python-ideas] while conditional in list comprehension ?? (original) (raw)

Shane Green shane at umbrellacode.com
Wed Jan 30 20:02:51 CET 2013

Although it's a bit of a cheat, if you create a wrapper of the thing you're iterating, or don't mind closing it (it's probably best to wrap it unless you know what it is), both generators and list comprehensions can be "while iterated" using this approach:

[item for item in items if condition or items.close()]

When I tested it earlier with a 1000 entries 5 times and had forgotten the parens on close(), it made it really obvious there would be times when the wrapping overhead wasn't a problem:

On Jan 30, 2013, at 9:02 AM, Shane Green <shane.green at me.com> wrote:

Nice catch. New times,

>>> timeit.timeit(var1) 8.533167123794556 >>> timeit.timeit(var2) 9.067211151123047 >>> timeit.timeit(var3) 12.966150999069214 >>> timeit.timeit(var4) And I accidentally ran this (without parens), so it was a regular comprehension: def var5(count=1000): seq = (i for i in xrange(count)) return [i for i in seq if i < 50 or seq.close]_ _>>> timeit.timeit(var5) 212.26763486862183 Then fixed it: >>> timeit.timeit(var5) 10.280441045761108 >>>

Shane Green 805-452-9666 | shane.green at me.com Begin forwarded message: From: Wolfgang Maier <wolfgang.maier at biologie.uni-freiburg.de> Subject: RE: [Python-ideas] while conditional in list comprehension ?? Date: January 30, 2013 8:40:51 AM PST To: 'Shane Green' <shane.green at me.com> Careful! You’re using range() in the slow ones, but xrange() in the fast ones. With the input seq being much longer than the output, differences in the time it takes to produce the range object may be important. From: Shane Green [mailto:shane.green at me.com] Sent: Wednesday, January 30, 2013 5:37 PM To: Wolfgang Maier Subject: Re: [Python-ideas] while conditional in list comprehension ?? >>> def var1(count=1000): ... def gen(): ... for i in range(count): ... if i > 50: break ... yield i ... return list(gen()) ... >>> def var2(count=1000): ... def stop(): ... raise StopIteration ... return list(i for i in range(count) if i <= 50 or stop())_ _..._ _>>> def var3(count=1000): ... return [i for i in itertools.takewhile(lambda n: n <= 50, range(count))]_ _..._ _>>> def var4(count=1000): ... return [i for i in itertools.takewhile(functools.partial(operator.lt, 50) ... >>> def var5(count=1000): ... seq = (i for i in xrange(count)) ... return [i for i in seq if i < 50 or seq.close()]_ _>>> timeit.timeit(var1) 19.118155002593994 >>> timeit.timeit(var2) 19.217869997024536 >>> timeit.timeit(var5) 10.251838207244873 >>>

Shane Green 805-452-9666 | shane.green at me.com On Jan 30, 2013, at 8:17 AM, Wolfgang Maier <wolfgang.maier at biologie.uni-freiburg.de> wrote: list(i for i in a if i < 5000 or a.close())

Shane Green www.umbrellacode.com 408-692-4666 | shane at umbrellacode.com

On Jan 30, 2013, at 10:05 AM, Oscar Benjamin <oscar.j.benjamin at gmail.com> wrote:

On 30 January 2013 17:56, Yuriy Taraday <yorik.sar at gmail.com> wrote:

On Wed, Jan 30, 2013 at 1:46 PM, Wolfgang Maier <wolfgang.maier at biologie.uni-freiburg.de> wrote:

your condition is 'partial(lt,50)', but this is not met to begin with and results in an empty list at least for me. Have you two actually checked the output of the code or have you just timed it? Yeah. Shame on me. You're right. My belief in partial and operator module has been shaken. This is why I prefer this stop() idea to any of the takewhile() versions: regardless of performance it leads to clearer code, that can be understood more easily. Oscar

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