[Tutor] What exactly is [::-1]?

[Bob Gailer]

Alan Gauld wrote:
> The ref manual ... for extended slicing says:
>
> ----------------------
> The semantics for an extended slicing are as follows. The primary must
> evaluate to a mapping object
d = {}
> It is indexed with a key that is constructed from the slice list, as follows. 
Note that
slice_list 	::= 	slice_item <slicings.html#tok-slice_item> ("," 
slice_item <slicings.html#tok-slice_item>)* [","]

slice_item 	::= 	expression <Booleans.html#tok-expression> | 
proper_slice <slicings.html#tok-proper_slice> | ellipsis 
<slicings.html#tok-ellipsis>

> If the slice list contains at least one comma, the key is a tuple containing the conversion of the slice items
d[1,2] = 3
> otherwise, the conversion of the lone slice item is the key. 
This is the usual application of a key where slice_item is an expression

d[313] = 4
> 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. 
print d
{(1, 2): 3, Ellipsis: 4, 313: 4}

Now consider:

slice_item 	::= 	expression <Booleans.html#tok-expression> | 
proper_slice <slicings.html#tok-proper_slice> | ellipsis 
<slicings.html#tok-ellipsis>



We took care of the cases where slice_item is expression or ellipsis.

How to apply a slice_list consisting of slice_item(s) that are 
proper_slices to a mapping object???

-- 
Bob Gailer
510-978-4454 Oakland, CA
919-636-4239 Chapel Hill, NC