[Python-Dev] Range __contains__ and objects with __index__ methods

[Terry Reedy]

On 12/26/2010 7:15 PM, Nick Coghlan wrote:
> Starting in Python 3.2, range() supports fast containment checking for
> integers (i.e. based on an O(1) arithmetic calculation rather than an
> O(N) iteration through the entire sequence).
>
> Currently, this fast path ignores objects that implement __index__ -
> they are relegated to the slow iterative search.
>
> This seems wrong to me - the semantics of __index__ are such that it
> is meant to be used for objects that are alternative representations
> of the corresponding Python integers (e.g. numpy scalars, or integers
> that use a particular number of bits in memory). Under that
> interpretation, if an object provides __index__, we should use the
> fast path instead of calling __eq__ multiple times.

If I understand, you are proposing 'replacing' o with o.__index__() 
(when possible) and proceeding on the fast path rather than iterating 
the range and comparing o for equality each value in the range (the slow 
path).

I suppose this would change semantics if o != o.__index__().
Equality is not specified in the manual:
"object.__index__(self)
Called to implement operator.index(). Also called whenever Python needs 
an integer object (such as in slicing, or in the built-in bin(), hex() 
and oct() functions). Must return an integer."
However
"operator.__index__(a)
Return a converted to an integer. Equivalent to a.__index__()."
comes close to implying equality (if possible).

What are the actual used of .__index__?

-- 
Terry Jan Reedy