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

Stephen J. Turnbull stephen at xemacs.org
Thu Apr 28 05:06:23 CEST 2011

Glenn Linderman writes:

On 4/27/2011 6:11 PM, Ethan Furman wrote:

Totally out of my depth, but what if the a NaN object was allowed to compare equal to itself, but different NaN objects still compared unequal? If NaN was a singleton then the current behavior makes more sense, but since we get a new NaN with each instance creation is there really a good reason why the same NaN can't be equal to itself?

Yes. A NaN is a special object that means "the computation that produced this object is undefined." For example, consider the computation 1/x at x = 0. If you approach from the left, 1/0 "obviously" means minus infinity, while if you approach from the right just as obviously it means plus infinity. So what does the 1/0 that occurs in [1/x for x in range(-5, 6)] mean? In what sense is it "equal to itself"? How can something which is not a number be compared for numerical equality?

n1 = float('NaN') n2 = float('NaN') n3 = n1

n1 nan n2 nan n3 nan

[n1] == [n2] False [n1] == [n3] True

This is the current situation: some NaNs compare equal sometimes, and some don't.

No, Ethan is asking for "n1 == n3" => True. As Mark points out, "[n1] == [n3]" can be interpreted as a containment question, rather than an equality question, with respect to the NaNs themselves. In standard set theory, these are the same question, but that's not necessarily so in other set-like toposes. In particular, getting equality and set membership to behave reasonably with respect to each other one of the problems faced in developing a workable theory of fuzzy sets.

I don't think it matters what behavior you choose for NaNs, somebody is going be unhappy sometimes.

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