Re: [VOTE] Userspace operator overloading

On March 28, 2020 5:44:28 PM GMT+01:00, Nikita Popov <nikita.ppv@gmail.com> wrote: >On Sat, Mar 28, 2020 at 5:28 PM Johannes Schlüter ><johannes@schlueters.de> >wrote: > >> On Sat, 2020-03-28 at 17:05 +0100, Christoph M. Becker wrote: >> > On 28.03.2020 at 15:57, Johannes Schlüter wrote: >> > >> > > On March 28, 2020 1:25:11 PM GMT+01:00, "Christoph M. Becker" < >> > > cmbecker69@gmx.de> wrote: >> > > >> > > > This "try left/right" approach is how operator overloading >works >> > > > for >> > > > internal classes[1], and apparently, it works quite well, as >long >> > > > as it >> > > > is not overused. >> > > >> > > The fact that it works in one or two cases as an implementation >> > > detail where the full implementation is controlled by a single >> > > group (internals) is no indication for it to work at large. >> > >> > Fair enough. But maybe Python, where userland operator overloading >> > works similar to the proposal at hand, is? :) >> >> It doesn't: >> >> >> Python 3.6.9 (default, Nov 7 2019, 10:44:02) >> [GCC 8.3.0] on linux >> Type "help", "copyright", "credits" or "license" >> for more >information. >> >>> class A: >> ... def __add__(self, other): >> ... print("add") >> ... >> >>> a = A() >> >>> a + 1 >> add >> >>> 1 + a >> Traceback (most recent call last): >> File "<stdin>", line 1, in <module> >> TypeError: unsupported operand type(s) for +: 'int' and 'A' >> > >It does. > >Because Python uses instance methods for operator overloading, it has >to >use pairs of methods like __add__ and __radd__ to handle commuted >variants. >If you want to implement a commutative add, you need to implement both >__add__ and __radd__. If __add__ is not implemented or returns >NotImplemented, then __radd__ will be called. > >So, as Christoph has said, this is indeed the same system as what is >being >proposed here, with the difference that we do not need two separate >methods >per operator, but can use a static method instead. If we use two methods as well it is a saner design. I won't like it, but lot better than the current one. johannes