[Python-Dev] Why is nan != nan? (original) (raw)

Mark Dickinson dickinsm at gmail.com
Wed Mar 24 23:47:42 CET 2010

On Wed, Mar 24, 2010 at 10:36 PM, Alexander Belopolsky <alexander.belopolsky at gmail.com> wrote:

On Wed, Mar 24, 2010 at 6:31 PM, Mark Dickinson <dickinsm at gmail.com> wrote: ..

Neither is necessary, because Python doesn't actually use == as the equivalence relation for containment testing:  the actual equivalence relation is:  x equivalent to y iff id(x) == id(y) or x == y.  This restores the missing reflexivity (besides being a useful optimization). No, it does not:

float('nan') in [float('nan')] False

Sure, but just think of it as having two different nans there. (You could imagine thinking of the id of the nan as part of the payload.) There's no ideal solution here; IMO, the compromise that currently exists is an acceptable one.

Mark

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