[Python-ideas] Non-boolean return from __contains__

[Michael Foord]

On 27 July 2010 17:42, Alexander Belopolsky
<alexander.belopolsky at gmail.com>wrote:

> 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,



Wasn't the lack of an __rcontains__ a problem for the web-sig guys trying to
work out the bytes / strings issue?

For what it's worth I think that guido is correct that a better solution for
the expression -> query problem is to introduce an expression tree, as is
done for LINQ (which has been enormously popular amongst .NET developers).

All the best,

Michael Foord



> 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.
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