keying by identity in dict and set

[Steve White]

Hi Chris,

Yes, I am aware of the hash of small integers.  But I am not keying
with small integers here: I am keying with id() values of class
instances.

Precisely what my example shows is that the dict/set algorithms in
fact *never* call __eq__, when the id() of a class instance is
returned by __hash__ (in the implementations of Python I have tested).
Please try the code yourself.  Tell me what I am missing.

What "other problems"?  Please provide an example!

Thanks!

On Sat, Oct 19, 2019 at 9:02 PM Chris Angelico <rosuav at gmail.com> wrote:
>
> On Sun, Oct 20, 2019 at 3:08 AM Steve White <stevan.white at gmail.com> wrote:
> > It would appear that if __hash__ returns the id, then that id is used
> > internally as the key, and since the id is by definition unique, no
> > key collision ever occurs -- at least in every Python implementation
> > I've tried. It also seems that, for a class instance obj,
> >     hash( hash( obj ) ) == hash( obj )
> >     hash( id( obj ) ) == id( obj )
> > These are very strong and useful properties.  Where are they documented?
>
> There are two rules that come into play here. One is that smallish
> integers use their own value as their hash (so hash(1)==1 etc); the
> other is that dictionaries actually look for something that matches on
> identity OR equality. That's why using identity instead of equality
> will appear to work, even though it can cause other problems when you
> mismatch them.
>
> ChrisA
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