On 10/7/14 2:10 AM, Gelonida N wrote:
Disadvantage of itertools.product() is, that it makes a copy in memory. Reason ist, that itertools also makes products of generators (meaning of objects, that one can't iterate several times through) There are two use cases, that I occasionaly stumble over: One is making the product over lists(), product( list_of_lists ) ex: product( [ [1,2,3], ['A','B'], ['a', 'b', 'c'] ] ) the other one making a product over a list of functions, which will create generators ex: product( [ lambda: [ 'A', 'B' ], lambda: xrange(3) ] ) I personally would also be interested in a fast generic solution that can iterate through an N-dimensional array and which does not duplicate the memory or iterate through a list of generator-factories or however this would be called.
itertools.product makes a copy of the sequences passed in, but it is a shallow copy. It doesn't copy the objects in the sequences. It also doesn't store the entire product.
If you are calling product(j, k, l, m, n), where len(j)==J, the extra memory is J+K+L+M+N, which is much smaller than the number of iterations product will produce. Are you sure that much extra memory use is a problem? How large are your lists that you are product'ing together?
I don't understand your point about a list of functions that create generators? What is the problem there?
-- Ned Batchelder, http://nedbatchelder.com -- https://mail.python.org/mailman/listinfo/python-list
