[Python-Dev] PyObject_RichCompareBool identity shortcut (original) (raw)

Glenn Linderman v+python at g.nevcal.com
Fri Apr 29 02:24:35 CEST 2011

On 4/28/2011 4:40 PM, Nick Coghlan wrote:

Hmm, true. And things like count() and index() would still be thoroughly broken for sequences. OK, so scratch that idea - there's simply no sane way to handle such objects without using an identity-based container that ignores equality definitions altogether.

And the problem with that is that not all values are interned, to share a single identity per value, correct?

On the other hand, proliferation of float objects containing NaN "works", thus so would proliferation of non-float objects of the same value... but "works" would have a different meaning when there could be multiple identities of 6,981,433 in the same set.

But this does bring up an interesting enough point to cause me to rejoin the conversation:

Would it be reasonable to implement 3 types of containers:

  1. using eq (would not use identity comparison optimization)
  2. using is (the case you describe above)
  3. the status quo: is or eq

The first two would require an explicit constructor call because the syntax would be retained for case 3 for backward compatibility.

Heavy users of NaN and other similar values might find case 1 useful, although they would need to be careful with mappings and sets.

Heavy users of NumPy and other similar structures might find case 2 useful.

Offering the choice, and documenting the alternatives may make a lot more programmers choose the proper comparison operations, and less likely to overlook or pooh-pooh the issue with the thought that it won't happen to their program anyway...

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