Nested objects and setattr & getattr (yes, it's that time)

Question

Overview:

class Inner(object):
    def __init__(self, x):
        self.x = x

class Outer(object):
    def __init__(self, z):
        self.inner = Inner(z)

o = Outer(10)

Now, I want the Outer object to behave transparently -- any attributes set on o should be set on o.inner, same for reading: o.something should return, in fact, the value of o.inner.sommething. Kind of like a proxy or a relay.

__getattr__ for Outer seems simple & works:

def __getattr__(self, item):
    return getattr(self, item)

How would I handle __setattr__? I couldn't come up with anything that didn't cause a recursion error and make me hate myself.

Or is the concept itself flawed?

(Another approach I tried was making Outer a subclass of Inner -- this didn't really play aesthetically the present@classmethods, not to mention the IDE would get lost inside those - not able to resolve some attributes. Let's leave that for now, perhaps?)

Answer 1

The tricky part is setting the inner attribute of the Outer class correctly. What you can do is call the __setattribute__ method of object (base class of Outer):

class Inner(object):
    def __init__(self, x):
        self.x = x

class Outer(object):    
    def __init__(self, z):
        object.__setattr__(self, 'inner', Inner(z))

    def __getattr__(self, name):
        return getattr(self.inner, name)

    def __setattr__(self, name, value):
        return setattr(self.inner, name, value)

Now, it works correctly:

o = Outer(10)
print o.x        # 10
print o.inner.x  # 10
o.g = 3
print o.g        # 3
print o.inner.g  # 3

However, it's not clear to my why you don't want to use inheritance in this case. It seems more natural and pythonic to have Outer inherit from Inner.