Re: slicing, mapping types, ellipsis etc.

[Steven Bethard]

Jerry Sievers wrote:
1. what the Ellipsis object or ... syntax is used for
2. what a slice [..., j:k:l] does
My understanding is that the Ellipsis object is intended primarily for 
Numeric/numarray use.  Take a look at:

http://stsdas.stsci.edu/numarray/numarray-1.1.html/node26.html
3. how slices are applied to object of mapping types
The short answer is that they aren't.  Slices aren't hashable, so dicts 
can't handle them:

>>> d = {}
>>> d = {1:'a', 2:'b'}
>>> d[1:2]
Traceback (most recent call last):
  File "<interactive input>", line 1, in ?
TypeError: unhashable type
>>> hash(slice(1, 2))
Traceback (most recent call last):
  File "<interactive input>", line 1, in ?
TypeError: unhashable type
You can, however handle them if you write your own mapping type:
>>> class M(object):
...     def __getitem__(self, x):
...         if isinstance(x, slice):
...             return x.start, x.stop, x.step
...         else:
...             return x
...
>>>
>>> m = M()
>>> m[1]
1
>>> m[1:2]
(1, 2, None)
Hope this helps!
Steve
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