[Python-Dev] Python version of PySlice_GetIndicesEx

[Guido van Rossum]

(I changed the subject)

> When I was going through the sources of sliceobject.c I found the function 
> PySlice_GetIndicesEx.  It performs the magic of trimming a slice into the 
> range of indices of a sequence, including negative indices and intervals 
> with None as start or stop value.  A comment in this function says:
> 
>  /* this is harder to get right than you might think */
> 
> Wouldn't it be a good idea to expose this nontrivial functionality to 
> Python code as a method of slice objects?

I dunno.  It seems that most code that actually uses slices is written
in C anyway.

> The method would take an integer argument (length) and return an
> xrange object.

Why an xrange object?  That's not inspectable.  *If* we were to do
this (which I doubt) it should return a tuple of three ints.

> It should make it much 
> easier to implement user types that support extended slicing:
> 
>     def __getitem__(self, index):
>         if isinstance(index, slice):
>             return [get_item_at(i) for i in index.trim(len(self))]
>         else:
>             return get_item_at(index)
> 
> Suggestions for a better name than trim?

getindices()

> Any reason why this API should stay exposed only to C and not to
> Python?

Have you got a real use case?  I'm a bit weary of hypothetical use
cases (that's what got us xrange repetition in the first place).

--Guido van Rossum (home page: http://www.python.org/~guido/)