[Tutor] iterators

[eryksun]

On Sat, Jan 18, 2014 at 4:50 AM, Peter Otten <__peter__ at web.de> wrote:
>
> PS: There is an odd difference in the behaviour of list-comps and generator
> expressions. The latter swallow Stopiterations which is why the above
> myzip() needs the len() test:

A comprehension is building a list in a `for` loop, which won't
swallow a `StopIteration` from the body of the loop itself.

For a generator, an unhandled `StopIteration` propagates to the frame
that called `__next__`. In this case the tuple constructor swallows
the `StopIteration`, as it rightly should. There's no way for it know
the original source of the `StopIteration` exception. You'd have to
use a generator that translates `StopIteration` to a custom exception.
Then use that exception to `break` the `while` loop. But no one sane
would go that far.

>>>> def myzip(*iterables):
> ...     iterators = [iter(it) for it in iterables]
> ...     while True:
> ...         t = [next(it) for it in iterators]
> ...         yield tuple(t)

The C implementation for `zip.__next__` calls `PyTuple_SET_ITEM` in a
loop over the iterators. It reuses the same tuple so long as the
reference count is only 1. That can't be duplicated in pure Python.
You'd have to use a `list`.