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 <ros...@gmail.com> wrote: > > On Sun, Oct 20, 2019 at 3:08 AM Steve White <stevan.wh...@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 > -- > https://mail.python.org/mailman/listinfo/python-list -- https://mail.python.org/mailman/listinfo/python-list
