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To: python-list@python.org
From: Robert Kern <robert.kern@gmail.com>
Subject: Re: Proposal: === and !=== operators
Date: Wed, 09 Jul 2014 16:27:42 +0100
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Xref: csiph.com comp.lang.python:74259

On 2014-07-09 08:00, Steven D'Aprano wrote:
> At the moment, Python has two (in)equality operators, == and != which
> call __eq__ and __ne__ methods. Some problems with those:
>
>
> * Many people expect == to always be reflexive (that is, x == x for
>    every x) but classes which customise __eq__ may not be.
>
> * The == operator requires __eq__ to return True or False
>    (or NotImplemented) and raises TypeError if it doesn't, which
>    makes it impossible to use == with (say) three-valued or fuzzy
>    logic.

No, it doesn't. It can return anything.

[~]
|1> x = np.arange(5)

[~]
|2> x == 3
array([False, False, False,  True, False], dtype=bool)

You can blame Numeric/numpy for that feature getting in. :-)

Now certainly, many uses of __eq__, like containment comparisons, do assume that 
the result is a bool(able).

-- 
Robert Kern

"I have come to believe that the whole world is an enigma, a harmless enigma
  that is made terrible by our own mad attempt to interpret it as though it had
  an underlying truth."
   -- Umberto Eco