[Zope-dev] Intercepting __getattr__ in a Python 2.3 old-style mixin class?

[Leonardo Rochael Almeida]

Hi Andreas,

As Hanno said, you'll still be in old-style-classes-land even in Python 2.7
unless you inherit from `object` directly.

You said:

My idea is to inject a common mixin class like
>     class Foo(Bar, Mixin):
>         ...
>     class Mixin:
>         def __getattr__(self, k)
>            print repr(self), k
>            return Foo.__getattr__(self, k)


Your recursion error is caused by calling `Foo.__getattr__(self, k)` from
`Mixin`, but `Foo` inherits from `Mixin`. Calling a method of a subclass
from the same-named method in the superclass is a recurssion.

Instead `Mixin` needs to call `Bar.__getattr__` since `Bar` is the
superclass to `Foo`.

Alternatively, if you don't want to name any specific superclass inside the
mixin method, (and since you can't use `super()` since you're using
old-style classes), I suggest a different approach. Instead of a mixin
class, use a method factory, like this:

def my__getattr__factory(SuperClass):
>     def __getattr__(self, k):
>         print repr(self), k
>         return SuperClass.__getattr__(self, k)


Which you can then use as:

class Foo(Bar):  # <- No mixin


>     __getattr__ = my_getattr__generator(Bar)  # <- Bar, not Foo


Regards,

Leo

On 7 November 2016 at 12:29, Hanno Schlichting <hanno at hannosch.eu> wrote:

> On Mon, Nov 7, 2016, at 14:01, Andreas Jung wrote:
> > Python 2.7 is different because all classes are automatically new-style
> > classes.
> > In Python 2.7 you would have to overwrite __getattribute__() but I am on
> > Python 2.3 here.
>
> No :)
>
> In Python 3 all classes are new-style classes. In Python 2 (including
> 2.7) you have to be explicit and either inherit from object (new-style)
> or not (old-style).
>
> In Python 2.7 you get:
>
> >>> class A: pass
> >>> class B(object): pass
> >>> type(A)
> <type 'classobj'>
> >>> type(B)
> <type 'type'>
>
> In Python 3 both are of type 'type'.
>
> And only new-style classes use '__getattribute__'. '__getattr__' is the
> right approach for old-style classes in any Python 2 version, even in
> Python 2.7.
>
> Hanno
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