On Sat, Mar 16, 2013 at 2:53 PM, Peter Otten <__pete...@web.de> wrote: > Abhishek Pratap wrote: > >> I am trying to use itertools.izip_longest to read a large file in >> chunks based on the examples I was able to find on the web. However I >> am not able to understand the behaviour of the following python code. >> (contrived form of example) >> >> >> >> for x in itertools.izip_longest(*[iter([1,2,3])]*2): >> print x >> >> >> ###output: >> (1, 2) >> (3, None) >> >> >> It gives me the right answer but I am not sure how it is doing it. I >> also referred to the itertools doc but could not comprehend much. In >> essence I am trying to understand the intracacies of the following >> documentation from the itertools package. >> >> "The left-to-right evaluation order of the iterables is guaranteed. >> This makes possible an idiom for clustering a data series into >> n-length groups using izip(*[iter(s)]*n)." >> >> How is *n able to group the data and the meaning of '*' in the >> beginning just after izip. > > Break the expression into smaller chunks: > > items = [1, 2, 3] > it = iter(items) > args = [it] * 2 # same as [it, it] > chunks = itertools.izip_longest(*args) # same as izip_longest(it, it) > > As a consequence of passing the same iterator twice getting the first item > from the "first" iterator will advance the "second" iterator (which is > actually the same as the first iterator) to the second item which will in > turn advance the "first" iterator to the third item. Try to understand the > implementation given for izip() at >
Thanks Peter. I guess I missed the trick on how each iterator will be moved ahead automatically as the are basically same, replicated N times. -Abhi > http://docs.python.org/2/library/itertools.html#itertools.izip > > before you proceed to izip_longest(). > > _______________________________________________ > Tutor maillist - Tutor@python.org > To unsubscribe or change subscription options: > http://mail.python.org/mailman/listinfo/tutor _______________________________________________ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
