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To: python-list@python.org
From: Ben Finney <ben+python@benfinney.id.au>
Subject: Re: Is nan in (nan,) correct?
Date: Fri, 06 Mar 2015 13:55:32 +1100
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Steven D'Aprano <steve+comp.lang.python@pearwood.info> writes:

> Since reflexivity is *almost* universal, and using object identity
> permits very substantial optimizations, the core developers agreed
> that built-in contain types may assume that `x is y` implies `x == y`.
> Users of NANs and other non-reflexive types can subclass or define
> their own membership function.

On a type (such as a hypothetical SQL NULL type) which does not have
reflexivity – i.e. that ‘(x is x) == (x == x)’ may be False – which
method needs to be implemented so items *containing* values of that type
will have the expected semantics?

I can only think of ‘footype.__contains__’, but that's a method of the
*container* type, and the ‘in’ operator doesn't consult that method of
the items themselves.

So, given the hypothetical NullType::

    class NullType(object):
        """ A type whose value never equals any other.

            This type's values will behave correctly when tested for
            membership in a collection::

                >>> foo = NullType()
                >>> bar = NullType()
                >>> foo is foo
                True
                >>> foo is bar
                False
                >>> foo == foo
                False
                >>> foo == bar
                False
                >>> quux = [foo, "spam"]
                >>> "spam" in quux
                True
                >>> foo in quux
                True
                >>> bar in quux
                False

            """

        def __eq__(self, value):
            return False

        def __method_which_the_in_operator_interrogates__(self, collection):
            """ Method which the ‘is’ operator interrogates for membership. """
            return is_a_member_of(container, self)

What method of NullType replaces the hypothetical
‘__method_which_the_in_operator_interrogates__’, which I've implemented
to as you describe “define their own membership function”, in order to
get the correct behaviour in the doctest above?

-- 
 \     “Why am I an atheist? I ask you: Why is anybody not an atheist? |
  `\      Everyone starts out being an atheist.” —Andy Rooney, _Boston |
_o__)                                                Globe_ 1982-05-30 |
Ben Finney