zip(*[iter1, iter2, iter3])
where I want to receive tuples of (item1, item2, item3) from the iterables. But it doesn't work well for a situation like:
zip(*tuple_iter)
where tuple_iter is an iterator to tuples of the form
(item1, item2, item3) and I want to receive three iterators, one to the item1s, one to the item2s and one to the item3s. I don't think this is too unreasonable of a desire as the current zip, in a situation like:
zip(*tuple_list)
where tuple_list is a list of tuples of the form (item1, item2, item3), returns a list of three tuples, one of the item1s, one of the item2s and one of the item3s.
Of course, the reason this doesn't work currently is that the fn(*itr) notation converts 'itr' into a tuple, exhausting the iterator:
>>> def g(x): ... for i in xrange(x): ... yield (i, i+1, i+2) ... print "exhausted" ... >>> zip(*g(4)) exhausted [(0, 1, 2, 3), (1, 2, 3, 4), (2, 3, 4, 5)] >>> it.izip(*g(4)) exhausted <itertools.izip object at 0x01157710> >>> x, y, z = it.izip(*g(4)) exhausted >>> x, y, z ((0, 1, 2, 3), (1, 2, 3, 4), (2, 3, 4, 5))
What I would prefer is something like:
>>> zip(*g(4)) <iterator object at ...> >>> x, y, z = zip(*g(4)) >>> x, y, z (<iterator object at ...>, <iterator object at ..., <iterator object at ...)
Of course, I can write a separate function that will do what I want here[1] -- my question is if Python's builtin zip will support this in Python 3000. It's certainly not a trivial change -- it requires some pretty substantially backwards incompatible changes in how *args is parsed for a function call -- namely that fn(*itr) only extracts as many of the items in the iterable as necessary, e.g.
>>> def h(x, y, *args): ... print x, y, args ... print list(it.islice(args, 4)) ... >>> h(*it.count()) 0 1 count(2) [2, 3, 4, 5]
So I guess my real question is, should I expect Python 3000 to play nicely with *args and iterators? Are there reasons (besides backwards incompatibility) that parsing *args this way would be bad?
Steve
[1] In fact, with the help of the folks from this list, I did: http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/302325 -- http://mail.python.org/mailman/listinfo/python-list
