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

Steven D'Aprano steve at pearwood.info
Thu Apr 28 02:05:59 CEST 2011

Guido van Rossum wrote:

Maybe we should just call off the odd NaN comparison behavior?

This doesn't solve the broader problem that any type might deliberately define non-reflexive equality, and therefore people will still be surprised by

x = SomeObject() x == x False [x] == [x] True

The "problem" (if it is a problem) here is list, not NANs. Please don't break NANs to not-fix a problem with list.

Since we can't (can we?) prohibit non-reflexivity, and even if we can, we shouldn't, reasonable solutions are:

(1) live with the fact that lists and other built-in containers will short-cut equality with identity for speed, ignoring eq;

(2) slow containers down by guaranteeing that they will use eq;

(but how much will it actually hurt performance for real-world cases? and this will have the side-effect that non-reflexivity will propagate to containers)

(3) allow types to register that they are non-reflexive, allowing containers to skip the identity shortcut when necessary.

(but it is not clear to me that the extra complexity will be worth the cost)

My vote is the status quo, (1).

-- Steven

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