Delegating Attribute Access

Contributed by: David Beazley and Brian K. Jones

Difficulty level: Easy

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Problem

You want an instance to delegate attribute access to an internally held instance, possibly as an alternative to inheritance or in order to implement a proxy.

Solution

Simply stated, delegation is a programming pattern where the responsibility for implementing a particular operation is handed off (i.e., delegated) to a different object. In its simplest form, it often looks something like this:

class A(object):
    def spam(self, x):
        pass

    def foo(self):
        pass

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

    def spam(self, x):
        # Delegate to the internal self._a instance
        return self._a.spam(x)

    def foo(self):
        # Delegate to the internal self._a instance
        return self._a.foo()

    def bar(self):
        pass

If there are only a couple of methods to delegate, writing code such as that just given is easy enough. However, if there are many methods to delegate, an alternative approach is to define the __getattr__() method, like this:

class A(object):
    def spam(self, x):
        pass

    def foo(self):
        pass

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

    def bar(self):
        pass

    # Expose all of the methods defined on class A
    def __getattr__(self, name):
        return getattr(self._a, name)

The __getattr__() method is kind of like a catch-all for attribute lookup. It’s a method that gets called if code tries to ...