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Groups > comp.lang.python > #86975
| From | Ben Finney <ben+python@benfinney.id.au> |
|---|---|
| Subject | Re: Is nan in (nan,) correct? |
| Date | 2015-03-06 13:55 +1100 |
| References | <mailman.67.1425594415.21433.python-list@python.org> <54f90c53$0$12994$c3e8da3$5496439d@news.astraweb.com> |
| Newsgroups | comp.lang.python |
| Message-ID | <mailman.77.1425610573.21433.python-list@python.org> (permalink) |
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
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Is nan in (nan,) correct? random832@fastmail.us - 2015-03-05 17:26 -0500
Re: Is nan in (nan,) correct? sohcahtoa82@gmail.com - 2015-03-05 15:11 -0800
Re: Is nan in (nan,) correct? Ben Finney <ben+python@benfinney.id.au> - 2015-03-06 10:20 +1100
Re: Is nan in (nan,) correct? sohcahtoa82@gmail.com - 2015-03-05 15:27 -0800
Re: Is nan in (nan,) correct? Ben Finney <ben+python@benfinney.id.au> - 2015-03-06 10:39 +1100
Re: Is nan in (nan,) correct? Chris Angelico <rosuav@gmail.com> - 2015-03-06 10:40 +1100
Re: Is nan in (nan,) correct? Chris Angelico <rosuav@gmail.com> - 2015-03-06 10:25 +1100
Re: Is nan in (nan,) correct? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-03-06 13:09 +1100
Re: Is nan in (nan,) correct? Ben Finney <ben+python@benfinney.id.au> - 2015-03-06 13:55 +1100
Re: Is nan in (nan,) correct? Ethan Furman <ethan@stoneleaf.us> - 2015-03-05 19:18 -0800
Re: Is nan in (nan,) correct? Ben Finney <ben+python@benfinney.id.au> - 2015-03-06 14:26 +1100
Re: Is nan in (nan,) correct? Ethan Furman <ethan@stoneleaf.us> - 2015-03-05 19:44 -0800
Re: Is nan in (nan,) correct? Chris Angelico <rosuav@gmail.com> - 2015-03-06 14:49 +1100
Re: Is nan in (nan,) correct? random832@fastmail.us - 2015-03-05 23:37 -0500
Re: Is nan in (nan,) correct? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-03-07 04:07 +1100
Re: Is nan in (nan,) correct? Mark Lawrence <breamoreboy@yahoo.co.uk> - 2015-03-06 04:46 +0000
Re: Is nan in (nan,) correct? Rustom Mody <rustompmody@gmail.com> - 2015-03-06 01:50 -0800
Re: Is nan in (nan,) correct? Chris Angelico <rosuav@gmail.com> - 2015-03-06 21:01 +1100
Re: Is nan in (nan,) correct? Rustom Mody <rustompmody@gmail.com> - 2015-03-06 02:22 -0800
Re: Is nan in (nan,) correct? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-03-07 03:59 +1100
Re: Is nan in (nan,) correct? Rustom Mody <rustompmody@gmail.com> - 2015-03-06 10:04 -0800
Re: Is nan in (nan,) correct? Ethan Furman <ethan@stoneleaf.us> - 2015-03-06 10:16 -0800
Re: Is nan in (nan,) correct? Grant Edwards <invalid@invalid.invalid> - 2015-03-06 15:34 +0000
Re: Is nan in (nan,) correct? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-03-07 03:43 +1100
Re: Is nan in (nan,) correct? Rustom Mody <rustompmody@gmail.com> - 2015-03-06 09:04 -0800
Re: Is nan in (nan,) correct? Chris Angelico <rosuav@gmail.com> - 2015-03-07 04:16 +1100
Re: Is nan in (nan,) correct? Rustom Mody <rustompmody@gmail.com> - 2015-03-06 09:36 -0800
Re: Is nan in (nan,) correct? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-03-07 10:33 +1100
Re: Is nan in (nan,) correct? Rustom Mody <rustompmody@gmail.com> - 2015-03-06 18:37 -0800
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