[EMAIL PROTECTED] wrote: > 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."
This is in place to support multidimensional arrays, such as in numpy. If, for instance, you have a 9x9 array A, then A[3:6,3:6] will extract a 3x3 block from the center of it. A[3:6,3:6] is equivalent to A[(slice(3,6,None), slice(3,6,None))] and the resulting tuple gets passed through the mapping interface, but it is not a mapping in any real sense. I don't think there's anything in core Python that uses this and it's not really relevant to this thread. -tim -- http://mail.python.org/mailman/listinfo/python-list
