Nested loops is strangely slow, totally at a loss.

[Steven D'Aprano]

On Wed, 10 Dec 2014 17:53:05 +1100, Chris Angelico wrote:

> On Wed, Dec 10, 2014 at 5:44 PM, Steven D'Aprano <steve at pearwood.info>
> wrote:
>> It would be nice if product iterators behaved like xrange() objects and
>> could perform "in" tests without exhausting the iterator, but they
>> don't. That's sad.
> 
> It'd be very difficult to do that in the general sense. But it should be
> possible to have a multi-dimensional range object that behaves the way
> Py3's range object does, including membership tests and stuff. (Might
> already exist, for all I know.) That would do what the OP wants, I
> think.

Oh yes. If the product object has n-ordinates, then you could implement 
it something vaguely like this:

def contains(product_obj, item):
    if not isinstance(item, tuple):
        return False
    if len(item) != number_of_ordinates:
        return False
    for i, x in enumerate(item):
        if x not in ordinate[i]:
            return False
    return True


but note that this too might exhaust the product iterator, if it is 
constructed from iterators! But for the case whether it is constructed 
from xrange objects, lists, tuples, etc. it would work fine.

Strings are another funny case: consider what happens here:

("bc", "x", "1") in product("abcd", "wxyz", "1234")

It should return False, but since string containment tests operate on 
substrings not just single characters, it will wrongly return True.


-- 
Steven