Nested loops is strangely slow, totally at a loss.

[Peter Otten]

Ian Kelly wrote:

> On Tue, Dec 9, 2014 at 11:30 PM, Chris Angelico <rosuav at gmail.com> wrote:
>> Are you sure it isn't? Your 'space' is an iterable cubic
>> cross-product. Your first loop checks (0,0,0) which is the first
>> element returned, and is thus fast... but it also *consumes* that
>> first element. The next time you test it, the entire space gets
>> consumed, looking for another (0,0,0), which won't exist. That means
>> iterating over 580**3 == 195112000 (two hundred million) tuples, and
>> that *is* going to be slow.
> 
> Huh, I wasn't even aware that membership tests worked on iterables with no
> __contains__ method. Seems odd to me that 'x in y' should be supported but
> not 'len(y)'.

To me

def contains(iterable, value):
    for item in iterable:
        if item == value:
            return True
    return False

seems to be a perfectly natural default behaviour. I don't see why

>>> 42 in itertools.count()
True

should require that len(count()) must succeed.