Delegates

A delegate is a helper object used by another object. The delegator may delegate some operations to the delegate, and provide a default implementation when there is not delegate or the delegate does not implement the operation. This pattern is heavily used in Cocoa framework on Mac OS X

Objects responsibilities:

Delegator:

Keep an optional delegate instance.
Allow setting the delegate when creating the instance or later.
Delegate the methods "thing" and "other" to the delegate.
Provide default implementation if there is no delegate or the delegate does not implement one of the method.

Delegate:

Implement "thing".

Python

class Delegator:
    def __init__(self, delegate=None):
        self.delegate = delegate
    def thing(self):
         if hasattr(self.delegate, 'thing'):
             return self.delegate.thing()
         return 42
    def other(self):
         if hasattr(self.delegate, 'other'):
             return self.delegate.thing()
         return 5

class Delegate:
    def thing(self):
        return 37

if __name__ == '__main__':

    # No delegate
    a = Delegator()
    assert a.thing() == 42
    assert a.other() == 5

    # With delegate
    a.delegate = Delegate()
    assert a.thing() == 37
    assert a.other() == 5