Is nan in (nan,) correct?
Ben Finney
ben+python at benfinney.id.au
Thu Mar 5 21:55:32 EST 2015
Steven D'Aprano <steve+comp.lang.python at 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
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