On Sun, Jan 18, 2009 at 2:56 AM, andrew cooke <and...@acooke.org> wrote: > Context - http://docs.python.org/3.0/reference/datamodel.html?highlight=data > model#object.__iadd__ > > Just a suggestion I thought I'd throw out... There's a restriction in > the language implementation on exactly what can go the left of an > augmented arithmetic expression. > > For example: >>>> a = 3 >>>> a **= 2 > > is ok, but: >>>> class Foo(): > ... def __init__(): > ... self.a = 3 > ... def __ipow__(self, x): > ... self.a **= x > ... >>>> Foo() **= 2 > File "<stdin>", line 1 > SyntaxError: illegal expression for augmented assignment > > Now unless I've done something stupid above (always a possibility :o) > the implementation seems a bit strict (is it really a *syntax* error? > - I am not sure exactly what the restriction is).
IIRC, you can only assign to: - variables (x = ...) - attributes (x.y = ...) - elements (x[y] = ...) Anything else doesn't make sense to assign to because it's not a "storage box" so to speak. There's no way to work out what is meant. In your case, you're assigning to a *value*, specifically a new instance of the Foo class, which is nonsensical; instead of a "box", you're trying to assign to a "value", something that gets stored in boxes. By comparison, '2 = 5' and '[1,2] = 7' would seem to have some sort of meaning under your system, which IMHO seems preposterous. Now true, you are using augmented assignment, which in certain cases is translated to a method call, but in principle the augmented assignment (e.g. x += y) should have roughly the same effect as the non-augmented equivalent (x = x + y), and the fact that a method call is involved is merely an implementation detail of sorts. Therefore, Python requires you to rewrite the code in some other way that makes your intentions more clear. For instance, why not use the << operator instead? Cheers, Chris -- Follow the path of the Iguana... http://rebertia.com -- http://mail.python.org/mailman/listinfo/python-list
