Consume an iterable

[Jan Kaliszewski]

Dnia 23-01-2010 o 15:19:56 Peter Otten <__peter__ at web.de> napisał(a):

>>> def consume_islice(n, items):
>>>     next(islice(items, n, n), None)
>
> One problem: the above function doesn't consume the entire iterator like  
> the original example does for n=None. Passing sys.maxint instead is not  
> pretty.

Not very pretty, but noticeably (though not dramatically) faster for  
n=None. Consider a modified version of the script from  
http://bugs.python.org/issue7764:

     import collections, sys
     from itertools import islice, repeat

     def consume0(iterator, n):  # the old one
         collections.deque(islice(iterator, n), maxlen=0)

     def consume1(iterator, n):  # similar to the primary proposal
         if n is None:
             collections.deque(iterator, maxlen=0)
         elif n != 0:
             next(islice(iterator, n-1, None), None)

     def consume2(iterator, n):  # the approved proposal (see #7764)
         if n is None:
             collections.deque(iterator, maxlen=0)
         else:
             next(islice(iterator, n, n), None)

     def consume3(iterator, n):  # with sys.maxint
         if n is None:
             n = sys.maxint      # (maybe should be sys.maxsize instead?)
         next(islice(iterator, n, n), None)

     def test(fs):
         for consume in fs:
             iterator = iter(range(10))
             consume(iterator, 3)
             rest = list(iterator)
             assert rest == range(3, 10), consume.__name__

             iterator = iter(range(10))
             consume(iterator, 0)
             rest = list(iterator)
             assert rest == range(10), consume.__name__

             iterator = iter(range(10))
             consume(iterator, None)
             rest = list(iterator)
             assert rest == [], consume.__name__

     if __name__ == "__main__":
         from timeit import Timer

         fs = (consume0, consume1,
               consume2, consume3)
         test(fs)

         iterator = repeat(None, 1000)
         for consume in fs:
             print consume.__name__
             for n in (10, 100, 1000, None):
                 print "%6s:" % n,
                 print Timer("consume(iterator, %s)" % n,
                             "import collections, sys\n"
                             "from __main__ import consume,  
iterator").timeit()
             print


Results [Python 2.6.2 (release26-maint, Apr 19 2009, 01:56:41) [GCC 4.3.3]  
on linux2 pentium4 2.4 GHz]:

consume0
     10: 2.94313001633
    100: 2.91833305359
   1000: 2.93242096901
   None: 2.90090417862

consume1
     10: 1.80793309212
    100: 1.7936270237
   1000: 1.83439803123
   None: 2.37652015686

consume2
     10: 1.58784389496
    100: 1.5890610218
   1000: 1.58557391167
   None: 2.37005710602

consume3
     10: 1.6071870327
    100: 1.61109304428
   1000: 1.60717701912
   None: 1.81885385513


Regards,
*j

-- 
Jan Kaliszewski (zuo) <zuo at chopin.edu.pl>