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. 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? -- Steven -- https://mail.python.org/mailman/listinfo/python-list
