python - 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 works in python2 but not in python3, where it goes into 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

Looks like change 61361 was the problem, as it merged changes from python 2.6 without verifying that they were correct for python3.

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/.