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Subject: Is nan in (nan,) correct?
Date: Thu, 05 Mar 2015 17:26:53 -0500
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Xref: csiph.com comp.lang.python:86958

It's been brought up on Stack Overflow that the "in" operator (on
tuples, and by my testing on dict and list, as well as dict lookup) uses
object identity as a shortcut, and returns true immediately if the
object being tested *is* an element of the container. However, the
contains operation does not specify whether object identity or equality
is to be used. In effect, the built-in container types use a hybrid
test: "a is b or a == b".

My question is, is this a *correct* implementation of the operator, or
are objects "supposed to" use a basis of equality for these tests?