A delegate is a helper object used by another object. The delegator may send the delegate certain messages, and provide a default implementation when there is no delegate or the delegate does not respond to a message. This pattern is heavily used in Cocoa framework on Mac OS X. See also wp:Delegation pattern.

Objects responsibilities:

Delegator:

 Keep an optional delegate instance.
 Implement "operation" method, returning the delegate "thing" if the delegate respond to "thing", or the string "default implementation".
Delegate:

 Implement "thing" and return the string "delegate implementation"
Show how objects are created and used. First, without a delegate, then with a delegate that does not implement "thing", and last with a delegate that implements "thing".

class Delegator:
   def __init__(self):
      self.delegate = None
   def operation(self):
       if hasattr(self.delegate, 'thing') and callable(self.delegate.thing):
          return self.delegate.thing()
       return 'default implementation'
 
class Delegate:
   def thing(self):
      return 'delegate implementation'
 
if __name__ == '__main__':
 
   # No delegate
   a = Delegator()
   assert a.operation() == 'default implementation'
 
   # With a delegate that does not implement "thing"
   a.delegate = 'A delegate may be any object'
   assert a.operation() == 'default implementation'
 
   # With delegate that implements "thing"
   a.delegate = Delegate()
   assert a.operation() == 'delegate implementation'