number ranges (was Re: Matlab page on scipy wiki)

[bonono at gmail.com]

Tim Hochberg wrote:
> Colin J. Williams wrote:
>
> >>
> >>It would be good if the range and slice could be merged in some way,
> >>although the extended slice is rather complicated - I don't understand it.
> >>
> >>   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 section 4.2 The standard type
> >>   hierarchy <http://www.network-theory.co.uk/docs/pylang/ref_30.html>)
> >>   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.
> >>
> >>[source: http://www.network-theory.co.uk/docs/pylang/ref_60.html]
> >>
> >>The seems to be a bit of a problem with slicing that needs sorting out.
> >>
> >>The syntax for a slice list appears to allow multiple slices in a list:
> >>
> >>   extended_slicing     ::=     primary <primaries.html#tok-primary> "["
> >>   slice_list <slicings.html#tok-slice_list> "]"
> >>   slice_list     ::=     slice_item <slicings.html#tok-slice_item> (","
> >>   slice_item <slicings.html#tok-slice_item>)* [","]
> >>
> >>but the current interpreter reports an error:
> >>
> >>    >>> a= range(20)
> >>    >>> a[slice(3, 9, 2)]
> >>   [3, 5, 7]
> >>    >>> a[slice(3, 9, 2), slice(5, 10)]
> >>   Traceback (most recent call last):
> >>     File "<interactive input>", line 1, in ?
> >>   TypeError: list indices must be integers
> >>    >>>
> >>
>
> Extended slicing has nothing to do with lists. All that gobbeldy gook is
> trying to tell you is what the interpreter does with O[1:2:3, 4:5:6]
> where O is some arbitrary object. What it does is:
>
> O[1:2:3, 4:5:6] == O[slice(1,2,3), slice(4,5,6)]
>                  == O.__getitem__((slice(1,2,3), slice(4,5,6)))
>
> In the case of python lists, __getitem__ doesn't support multiple slice
> arguments. However, you are free to define your own types which do
> support multiple slices.
>
That explains it but the current documentation in the reference manual
doesn't(I also tried what Colin did when reading that part). And as I
mentioned in another post, the current documentation suggests that
extended slicing is not supported for sequence type. As it said the
primary must be evaluated to mapping object for extended slicing.
Though it seems it doesn't matter as all python does now is turn
[start:stop:stride] into slice object and pass that to __getitem__, no
matter what object it is.