[Python-ideas] Non-boolean return from __contains__

[Alexander Belopolsky]

On Tue, Jul 27, 2010 at 11:59 AM, Guido van Rossum <guido at python.org> wrote:
..
>> So it doesn't help that 'in' may return something else than a bool
>> because the method is called on the wrong object for your purposes.
>
> Well that pretty much kills the proposal. I can't believe nobody
> (including myself) figured this out earlier in the thread. :-(

It may kill a use case or two, but not the proposal.   In the
libraries like numpy where all python containers get replaced, this is
not an issue.   Also this problem invites __rcontains__ solution, but
the proposal is not very attractive to begin with.   IMO, operators
that are not symbols  such as +, - or &, but words such as 'in', 'not'
or 'and' don't offer much advantage over function calls.