understanding zip function

Question

All discussion is about python 3.1.2; see Python docs for the source of my question.

I know what zip does; I just don't understand why it can be implemented like this:

def zip(*iterables):
    # zip('ABCD', 'xy') --> Ax By
    iterables = map(iter, iterables)
    while iterables:
        yield tuple(map(next, iterables))

Let's say I call zip(c1, c2, c3). If I understand correctly, iterables is initially the tuple (c1, c2, c3).

The line iterables = map(iter, iterables) converts it to an iterator that would return iter(c1), iter(c2), iter(c3) if iterated through.

Inside the loop, map(next, iterables) is an iterator that would return next(iter(c1)), next(iter(c2)), and next(iter(c3)) if iterated through. The tuple call converts it to (next(iter(c1)), next(iter(c2)), next(iter(c3)), exhausting its argument (iterables) on the very first call as far as I can tell. I don't understand how the while loop manages to continue given that it checks iterables; and if it does continue why the tuple call doesn't return empty tuple (the iterator being exhausted).

I'm sure I'm missing something very simple..

Answer 1

It looks like it's a bug in the documentation. The 'equivalent' code working in python2, but not in python3, where it has an infinite loop.

And the latest version of the documentation has the same problem: http://docs.python.org/release/3.1.2/library/functions.html

Look like change 61361 was the problem, as it merged changes from python 2.6 without verifying that they were correct for python 3.

It looks like the issue doesn't exist on the trunk documentation set, but you probably should report a bug about it at http://bugs.python.org/.

Answer 2

It seems like this code is supposed to be read as python-2.x code. It doesn't even run properly in py3k.

What happens in python-2.x is that map return a list of iterators, when next is called it returns an element of iterator, those elements combined into tuple. So, given

>>> zip('ABCD', 'xy')

iterables is a list of 2 iterators, on each iteration within the while loop, next (first remaining) element of iterator is consumed (''A' and 'x', etc), and yielded as an element of a tuple, then after last elements are yielded, (on 3rd iteration) raised StopIteration stops the generator. while iterables always remains True.