[Python-Dev] Range __contains__ and objects with __index__ methods
tjreedy at udel.edu
Mon Dec 27 02:52:23 CET 2010
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
I suppose this would change semantics if o != o.__index__().
Equality is not specified in the manual:
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."
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
More information about the Python-Dev