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To: python-list@python.org
From: Terry Reedy <tjreedy@udel.edu>
Subject: Re: Nested loops is strangely slow, totally at a loss.
Date: Wed, 10 Dec 2014 02:16:37 -0500
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On 12/10/2014 1:53 AM, Chris Angelico wrote:
> On Wed, Dec 10, 2014 at 5:44 PM, Steven D'Aprano <steve@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.

Itertools are general tools for building specialized objects. 
itertools.product provides the iter method.

class ReitProd():  # untested
     def __init__(self, reiterable, n):
         self.reit = reiterable  # must support 'in'
         self.n = n
     def __iter__(self):
         return itertools.product(self.reit, repeat=self.n)
     def __contains__(self, seq):
	if len(seq) != self.n:
             return False
         for i, item in enumerate(it):
             if item not in self.reit:
                 return False
         return True

-- 
Terry Jan Reedy