[Python-Dev] ++x oddnes

Guido van Rossum guido@python.org
Mon, 13 Aug 2001 10:02:21 -0400

> How?  ++x compiles to
>          LOAD_FAST         x
>          UNARY_POSITIVE      
>          UNARY_POSITIVE      
> I don't see any incrementing going on...

I'm guessing that MAL used a perverse implementation trick, where for
a certain *mutable* counter type '+x' returns an object that contains
a reference to x, and the unary plus operation on *that* object
increments the counter it references.

    class C:
        def __init__(self):
            self.counter = 0
        def inc(self):
            self.counter += 1
        def __str__(self):
            return str(self.counter)
        def __pos__(self):
            return Ref(self)

    class Ref:
        def __init__(self, arg):
            self.referent = arg
        def __pos__(self):
            return self.referent

    a = C()
    print a             # 0
    print ++a   # 1
    print a             # 1

Twisted, sick, call it what you want -- but please don't get used to
this.  The likelihood that I'll ever add a ++ operator is very small,
but not zero. :-)

--Guido van Rossum (home page: http://www.python.org/~guido/)