Delegating method call by condition

[Azurin]

Hi,
Imagine we have the following:

Code: Select all

class A(object):
    def m1(self):
        pass
    def m2(self):
        pass

class B(object):
    def __init__(self):
        self.a = A()


What I need is to be able to call either A methods or B methods through B instance.
Smth like

Code: Select all

def __getattribute__(self, name): # B method call
    if condition:
        # try call method from self.a
    else:
        # try call method from self


Hope I described clearly..

[targ]

Code: Select all

def __getattribute__(self, name):
    return object.__getattribute__(self.a if condition else self, name)

Does that work?

[Azurin]

targ,

The problem is, that when you try to access some member via self in __getattribute__ method it is instantly executed, and thus going to "endless" recursion.

[targ]

Yes, that's why I use object.__getattribute__(self, name) instead of self.__getattribute__(name).

The __getattribute__ method (or any method) of a superclass (object in this case) will not be affected by any of its subclasses' definitions of the same method, so it's safe to use.

[Azurin]

Yes.
But.
In your code object.__getattribute__ method will never be called. Before calling it all incoming parameters should be calculated. The step self.a will force recursion.

[targ]

That's true, unless condition also requires, or implies, that name != "a"; or something like:

Code: Select all

instance = object.__getattribute__(self, "a") if condition else self
return object.__getattribute__(instance, name)


But, depending on what condition actually is, there could be a better way to handle the whole thing.

[avisser]

I don't get it.

Code: Select all

class A(object):
    def m1(self):
        pass
    def m2(self):
        pass

class B(object):
    def __init__(self):
        self.a = A()
    def m1(self):
        pass

b = B()
calling_b_method = b.m1()
calling_a_method_through_b = b.a.m1()

or do you mean calling a method of A() as if it were a method of B()?
Sometimes it helps explaining why you want do do something instead of just asking how to do it , this is just theoretical to me.

[Azurin]

avisser,

Yes, the idea is to call A() methods as if they were B() methods.

I put more investigation to my task and find out that I could delegate method calls to A() only if they are not defined in B().
Simple inheritance is not suitable cause I SHOULD WRAP A() methods.
Here __getattr__ does it's job since called right before AttributeError.

I still haven't got time to test it a lot, but looks like working

Code: Select all

class A(object):
    def m1(self):
        pass
    def m2(self):
        pass

class B(object):
    def __init__(self):
        self.a = A()
    def __getattr__(self, name):
        return self._wrapped(name)
    def _wrapped(self, name):
        # do smth
        ret = getattr(self.a, name)
        # do smth
        return ret