Dr. Phillip M. Feldman wrote: > > It is supposed to be possible to generate a list representation of any > iterator that produces a sequence of finite length, but this doesn't > always work. Here's a case where it does work: > > Input: > > from itertools import combinations > list(combinations(range(4),2)) > > Output: > > [(0, 1), (0, 2), (0, 3), (1, 2), (1, 3), (2, 3)] > > When I define my own classes that produce iterators, conversion to a list > works for some of these classes but not for others. Here's a case where it > doesn't work: > > In: list(balls_in_numbered_boxes(2, [3,3,3])) > > Out: > > [array([0, 0, 1]), > array([0, 0, 1]), > array([0, 0, 1]), > array([0, 0, 1]), > array([0, 0, 1])]
But it does work -- it generates a list, exactly as expected. The problem is not that you can't generate a list. The problem is that the list you generate is not what you expect. What you have here is a list containing the same array, repeated three times. When you print the permutations one at a time, you don't notice, but by collecting them all at once, you see clearly that they are the same array object. Try this: combos = list(balls_in_numbered_boxes(2, [3,3,3])) [id(a) for a in combos] I expect you will see something like [123456, 123456, 123456]. The simplest fix is to fix the next() method so that it returns a copy of self.distribution instead of the array itself. -- Steven -- http://mail.python.org/mailman/listinfo/python-list
