在Python 2.3旧式mixin类中拦截__getattr__吗？

Question

I have a large Python 2.3 based installation with 200k LOC. As part of a migration project I need to intercept all attribute lookups of all old-style class.

Old legacy code:

class Foo(Bar):
    ...

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)

However I am running always into a recursion because Foo.__getattr__ resolves to Mixin.__getattr__ .

Is there any way to fix the code for Python 2.3 old-style classes?

Answer 1

If you are already injecting mixins, why not add object as parent, to make them new style

class Foo(Mixin, Bar, object):
    ...

And then use super

class Mixin(object):

    def __getattr__(self, k)
        print repr(self), k
        return super(Mixin, self).__getattr__(k)

Answer 2

Assuming that none of the classes in your code base implement __setattr__ or __getattr__ then one approach is to intercept __setattr__ in your Mixin, writing the value to another reserved attribute, then read it back in __getattr__

class Mixin:
    def __setattr__(self, attr, value):
        # write the value into some special reserved space
        namespace = self.__dict__.setdefault("_namespace", {})
        namespace[attr] = value

    def __getattr__(self, attr):
        # reject special methods so e.g. __repr__ can't recurse
        if attr.startswith("__") and attr.endswith("__"):
            raise AttributeError

        # do whatever you wish to do here ...
        print repr(self), attr

        # read the value from the reserved space
        namespace = self.__dict__.get("_namespace", {})
        return namespace[attr]

Example:

class Foo(Mixin):
    def __init__(self):
        self.x = 1

Then

>>> Foo().x
<__main__.Foo instance at 0x10c4dad88> x

Clearly this won't work if any of your Foo classes implement __setattr__ or __getattr__ themselves.