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To: python-list@python.org
From: Robert Kern <robert.kern@gmail.com>
Subject: Re: Slice implementation bug
Date: Thu, 12 May 2011 12:24:11 -0500
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On 5/12/11 6:06 AM, Tambet wrote:
> Hello!
>
> Let's say slice is multidimensional now - how to interpret it?
>
> I excpect these to work:
>
>     * m[0, 3] - get one element from matrix
>     * m[0:2, 0:2] - get four elements from matrix, iterate over them (I have
>       actually an rtree if it doesn't make sense to you)
>
> But it won't, because if m[0, 3] returns something, then m[0:2, 0:2] cannot
> yield anymore.

Let me try to rephrase, since you seem to be leaving out a lot of assumptions. 
You are trying to say that you want m[0:2,0:2] to return an iterator and that if 
you define __getitem__() such that m[0,3] returns a value, then you cannot 
implement __getitem__() to be a generator using a yield statement.

Okay. Fine.

But there is no reason that __getitem__() needs to be a generator function for 
you to return an iterator. You can simply return another generator. For example:

def __getitem__(self, key):
     if isinstance(key, tuple):
         if any(isinstance(x, slice) for x in key):
             return self._generate_from_slice(key)
     # The default, boring case.
     return self._get_value(key)

def _generate_from_slice(self, key):
     yield foo
     yield bar
     yield etc

> Ofcourse I could return an iterator, but this would not be so simple.

Really, it is.

-- 
Robert Kern

"I have come to believe that the whole world is an enigma, a harmless enigma
  that is made terrible by our own mad attempt to interpret it as though it had
  an underlying truth."
   -- Umberto Eco