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The <a href="http://docs.python.org/library/itertools.html?highlight=itertools#itertools.islice" rel="nofollow"><code>itertools.islice</code></a> function can produce a slice from any iterable. The statement: <pre><code>itertools.islice(lst, 0, None, 2) </code></pre> In simplified terms means "return an iterator which, when evaluated, will return every other element in lst starting with the element at 0". The <code>0</code> arg is the starting point and <code>2</code> is the step value (how many to skip to get the next value) It would be better to use a 6-element list to illustrate, since looking at <code>[(1, 2), (1, 5)]</code> it may be unclear which function produces the inner tuples (they are not produced until the final izip). <pre><code>>>> lst = [1, 2, 3, 4, 5, 6] >>> list(itertools.islice(lst, 0, None, 2)) [1, 3, 5] >>> list(itertools.islice(lst, 0, None, 2)) [2, 4, 6] </code></pre> Be careful using this with a generator; the first call will traverse the whole sequence, draining the generator: <pre><code>>>> def foo(): ... for i in range(6): ... yield i + 1 >>> >>> gen = foo() >>> list(itertools.islice(gen, 0, None, 2) [1, 3, 5] >>> list(itertools.islice(gen, 1, None, 2) [] </code></pre> Your function needs to produce 2 sequences, odds and evens. This is where the list comprehension comes in: <pre><code>>>> [itertools.islice(lst, i, None, 2) for i in range(2)] [<itertools.islice object at 0x7f958a79eaf8>, <itertools.islice object at 0x7f958a79eaa0>] </code></pre> Now you have 2 islice objects ready to be interleaved. To do this we could use <a href="http://docs.python.org/library/functions.html?highlight=zip#zip" rel="nofollow"><code>zip</code></a>, but <a href="http://docs.python.org/library/itertools.html?highlight=itertools#itertools.izip" rel="nofollow"><code>itertools.izip</code></a> is more efficient because it returns an iterator: <pre><code>>>> list(zip([1, 3, 5], [2, 4, 6])) [(1, 2), (3, 4), (5, 6)] >>> tmp = itertools.izip(*[itertools.islice(lst, i, None, 2) for i in range(2)]) >>> tmp <itertools.izip object at 0x7f958a7b6cf8> >>> list(tmp) [(1, 2), (3, 4), (5, 6)] </code></pre> Hope that helps clarify the steps.
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1. COvery clear, thanks.
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