Raymond Hettinger wrote: > > [Nick Coghlan] >> It doesn't quite work the way RDM desribed it - he missed a step. > > Thanks for the clarification. We ought to write-out the process > somewhere in a FAQ.
The closest we currently have to that is the write-up of the coercion rules in 2.x: http://docs.python.org/reference/datamodel.html#id5 Unfortunately, that mixes in a lot of CPython specific special cases along with the old coerce() builtin that obscure the basic behaviour for __op__ and __rop__ pairs. Here's an initial stab at a write-up of the coercion rules for Py3k that is accurate without getting too CPython specific: """ Given "a OP b", the coercion sequence is: 1. Try "a.__op__(b)" 2. If "a.__op__" doesn't exist or the call returns NotImplemented, try "b.__rop__(a)" 3. If "b.__rop__" doesn't exist or the call returns NotImplemented, raise TypeError identifying "type(a)" and "type(b)" as unsupported operands for OP 4. If step 1 or 2 is successful, then the result of the call is the value of the expression Given "a OP= b" the coercion sequence is: 1. Try "a = a.__iop__(b)" 2. If "a.__iop__" doesn't exist or the call returns not implemented, try "a = a OP b" using the normal binary coercion rules above Special cases: - if "type(b)" is a strict subclass of "type(a)", then "b.__rop__" is tried before "a.__op__". This allows subclasses to ensure an instance of the subclass is returned when interacting with instances of the parent class. - rich comparisons are associated into __op__/__rop__ pairs as follows: __eq__/__eq__ (i.e. a == b is considered equivalent to b == a) __ne__/__ne__ (i.e. a != b is considered equivalent to b != a) __lt__/__gt__ (i.e. a < b is considered equivalent to b > a) __le__/__ge__ (i.e. a <= b is considered equivalent to b >= a) - __rpow__ is never invoked for the 3 argument form of pow(), as the coercion rules only apply to binary operations. In this case, a NotImplemented return from the call to __pow__ is converted immediately into a TypeError. """ Cheers, Nick. -- Nick Coghlan | ncogh...@gmail.com | Brisbane, Australia --------------------------------------------------------------- _______________________________________________ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
