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keying by identity in dict and set


Hi,

Yes, there are several options, but they all involve an extra layer
that detracts between the interface I am building and my user's code.
In this situation, the objects being used as keys are conceptually the
unique entities that the user deals with, even if their data is
non-unique.  And I do not want to subject the user to the un-pythonic
use of some operator other than '==' to determine their equivalence.

As near as I can tell, returning the id() in __hash__() results in a
perfect hash key.  I really want to know if that is true.
Because if it is true, any further layer is simply covering for a
failing in the documentation.

Thanks!

On Sun, Oct 27, 2019 at 4:54 AM Random832 <random832 at fastmail.com> wrote:
>
> On Sat, Oct 19, 2019, at 07:31, Steve White wrote:
> > Hi,
> >
> > I have an application that would benefit from object instances
> > distinguished by identity being used in dict's and set's. To do this,
> > the __hash__ method must be overridden, the obvious return value being
> > the instance's id.
> >
> > This works flawlessly in extensive tests on several platforms, and on
> > a couple of different Python versions and implementations.
> >
> > The documentation seems to preclude a second requirement, however.
> >
> > I also want to use the == operator on these objects to mean a natural
> > comparison of values, different from identity, so that two instances
> > comparing equivalent does not imply that they are identical.
>
> I'd like to jump in to this thread to note that while this is reasonably easily achieved with a custom mapping class that uses a dict along with a wrapper class that stores the identity...
>
> I once tried to make a WeakKeyDictionary that was keyed by identity and had no end of trouble.
> --
> https://mail.python.org/mailman/listinfo/python-list