Steven D'Aprano <steve+comp.lang.pyt...@pearwood.info>: > On Sun, 08 Jun 2014 18:56:47 +0300, Marko Rauhamaa wrote: >> In fact, what's the point of having the duality? >> x < y <==> x.__lt__(y) > > [...] > > Consider x + y. What happens? > > #1 First, Python checks whether y is an instance of a *subclass* of x. If > so, y gets priority, otherwise x gets priority. > > #2 If y gets priority, y.__radd__(x) is called, if it exists. If it > returns something other than NotImplemented, we are done. > > #3 However if y.__radd__ doesn't exist, or it returns NotImplemented, > then Python continues as if x had priority. > > #3 If x has priority, then x.__add__(y) is called, if it exists. If it > returns something other than NotImplemented, we are done. > > #4 However if it doesn't exist, or it returns NotImplemented, then > y.__radd__(x) is called, provided it wasn't already tried in step #2. > > #5 Finally, if neither object has __add__ or __radd__, or both return > NotImplemented, then Python raises TypeError.
In a word, Python has predefined a handful of *generic functions/methods*, which are general and standard in GOOPS (Guile's object system): (define-method (+ (x <string>) (y <string)) ...) (define-method (+ (x <matrix>) (y <matrix>)) ...) (define-method (+ (f <fish>) (b <bicycle>)) ...) (define-method (+ (a <foo>) (b <bar>) (c <baz>)) ...) <URL: http://www.gnu.org/software/guile/manual/html_node/ Methods-and-Generic-Functions.html> Marko -- https://mail.python.org/mailman/listinfo/python-list
