Question about idioms for clearing a list

[bonono at gmail.com]

Fredrik Lundh wrote:
> > Python now has, what, three built-in mutable collections types:
> > lists, dictionaries, and sets. Dicts and sets both have a clear()
> > method and lists do not.
>
> dicts and sets are mappings, and lists are not.  mappings don't
> support slicing.  lists do.

I am confused. Could you explain this ? I was under the impression said
above(mapping don't support slicing), until after I read the language
reference. I don't think it is slicing as in the list slicing sense but
it does use the term "extend slicing".

http://www.python.org/doc/2.4.2/ref/slicings.html

"The semantics for an extended slicing are as follows. The primary must
evaluate to a mapping object, and it is indexed with a key that is
constructed from the slice list, as follows. If the slice list contains
at least one comma, the key is a tuple containing the conversion of the
slice items; otherwise, the conversion of the lone slice item is the
key. The conversion of a slice item that is an expression is that
expression. The conversion of an ellipsis slice item is the built-in
Ellipsis object. The conversion of a proper slice is a slice object
(see section 3.2) whose start, stop and step attributes are the values
of the expressions given as lower bound, upper bound and stride,
respectively, substituting None for missing expressions."