Proposal: === and !=== operators
steve at pearwood.info
Wed Jul 9 09:00:52 CEST 2014
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
* 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
More information about the Python-list