[Python-3000] Parallel iteration syntax (original) (raw)

Michael Chermside mcherm at mcherm.com
Tue Mar 28 15:54:12 CEST 2006

Greg Ewing writes:

Some years ago there was a long discussion about extending the for-loop to express parallel iteration over a number of iterables [...] I'm mentioning it here again just in case anyone wants to consider it for Py3k. I still believe it would be nice to have a direct syntax for parallel iteration to avoid the overhead of using zip or iterzip.

There's one big problem I see with this. Parallel iteration is underspecified... there are several reasonable choices for what to do if the iterables are of differing length.

(1) Fill out the short iterables with "None" elements. (2) Stop as soon as any iterable runs out. (3) Raise an exception if the iterables are not all the same length.

Today, we support these with different idioms:

(1) result = map(some_func, seq_x, seq_y)

(2) for x, y in zip(seq_x, seq_y): some_func(x, y)

(3)

all right, there's no common idiom I know of for

(3), but there ought to be, because it is probably

what the programmer wants most often, since the

most common case is where you are expecting the

lists to be the same length.

Frankly, I have no interest in supporting (1)... it's not really natural. But both (2) and (3) have some good points. (2) is fundamentally more powerful (it can do things that (3) can't), while (3) is probably what programmers need most often; it has the advantage of being explicit about errors and it is difficult to simulate.

So which to support?

-- Michael Chermside

PS: To be explicit, what I mean by (3) is this:

def equizip(*iterables): iterators = [iter(x) for x in iterables] while True: try: first_value = iterators[0].next() try: other_values = [x.next() for x in iterators[1:]] except StopIteration: raise IterableLengthMismatch else: values = [first_value] + other_values yield tuple(values) except StopIteration: for iterator in iterators[1:]: try: extra_value = iterator.next() except StopIteration: pass # this is what we expect else: raise IterableLengthMismatch raise StopIteration

for x, y in equizip(seq_x, seq_y): some_func(x, y)

More information about the Python-3000 mailing list