StopIteration in generators - python-3.x

Question

I'm learning python's generators, iterators, iterables, and I can't explain why the following is not working. I want to create, as an exercise, a simple version of the function zip. Here's what i did:
def myzip(*collections):
iterables = tuple(iter(collection) for collection in collections)
yield tuple(next(iterable) for iterable in iterables)
test = myzip([1,2,3],(4,5,6),{7,8,9})
print(next(test))
print(next(test))
print(next(test))
What I do is:
I have collections which is a tuple of some collections
I create a new tuple iterables where, for each collection (which is iterable), I get the iterator using iter
Then, I create a new tuple where, on each iterable, I call next. This tuple is then yield.
So I expect that at the first execution the object iterables is created (and stored). Then in each iteration (including the first one) I call next on every iterable stored before and return it.
However this is what I get:
(1, 4, 8)
---------------------------------------------------------------------------
StopIteration Traceback (most recent call last)
<ipython-input-108-424963a58e58> in <module>()
8
9 print(next(test))
---> 10 print(next(test))
StopIteration:
So I see that the first iteration is fine and the result is correct. However, the second iteration raise a StopIteration exception and I don't understand why: each iterable still has some values, so none of the nexts return StopIteration. In fact, this works:
def myziptest(*collections):
iterables = tuple(iter(collection) for collection in collections)
for _ in range(3):
print(tuple(next(iterable) for iterable in iterables))
test = myziptest([1,2,3],(4,5,6),{7,8,9})
Output:
(1, 4, 8)
(2, 5, 9)
(3, 6, 7)
So what is going on?
Thanks a lot

Answer 1

Here's a working solution
def myzip(*collections):
iterables = tuple(iter(collection) for collection in collections)
while True:
try:
yield tuple([next(iterable) for iterable in iterables])
except StopIteration:
# one of the iterables has no more left.
break
test = myzip([1,2,3],(4,5,6),{7,8,9})
print(next(test))
print(next(test))
print(next(test))
The difference between this code and yours is that your code only yields one result. Meaning, calling next more than once will give you a StopIteration.
Think of yield x as putting x into a queue, and next as popping from that queue. And when you try to pop from an empty queue, you get the Stopiteration. You can pop only as many as you put.