Proposal: === and !=== operators
roy at panix.com
Wed Jul 9 14:27:28 CEST 2014
In article <53bce8a3$0$2746$c3e8da3$76491128 at news.astraweb.com>,
Steven D'Aprano <steve at pearwood.info> 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
> I propose:
> * The == operator be redefined to *always* assume reflexivity, that
> is, it first compares the two arguments using `is` before calling
> the __eq__ methods.
> * That's a backwards-incompatible change, so you need to enable it
> using "from __future__ import equals" in Python 3.5, and then to
> become the default behaviour in 3.6.
> * To support non-reflexive types, allow === and !=== operators, which
> are like == and != except they don't call `is` first.
> * The new === and !== operators call __eeq__ and __ene__ (extended
> equal and extended not equal) methods; if they don't exist, they
> fall back on __eq__ and __ne__.
> * To support multi-valued logics, === and !== are not required to
> return True or False, they can return anything you like and it is
> up to the caller to ensure that they are sensible.
> * Returning NotImplemented from __eeq__ and __ene__ has the same
> meaning as for __eq__ and __ne__.
> * For the avoidance of doubt, `if...elif...else` are not expected to
> be aware of multi-valued logics. No other changes to the language
> are expected.
> Thoughts? Comments?
-1. This seems like it will just add additional complexity and
confusion, for very little gain. We would have *three* ways to compare
for equality (==, ===, and is).
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