getattr.py

"""
In the following example, we can pretend that A and B are both classes that are provided by another framework or library
"""
class A:
    cheetahs = "are fast"

    def __getattr__(self, attr):
        print("A is the getter")

        if attr == "donuts":
            return "are delicious"

class B:
    elephants = "are large"

    def __getattr__(self, attr):
        print("B is the getter")
        
        # B.__getattr__ will not be called because A does not delegate to it
        if attr == "frenchtoast":
            return "is filling"


class C(A, B):
    def __init__(self):
        self.data = {
            "apples": "grow on trees",
            "bees": "make honey",
        }

    def __getattr__(self, attr):
        print("C is the getter")

        if attr in self.data.keys():
            return self.data[attr]
        
        # A.__getattr__ will be called only if we check for it and call it explicitly
        elif hasattr(super(), '__getattr__'):
               return getattr(super(), attr)
        else: 
            raise AttributeError


c = C()


print(c.apples)       # will be satisfied by C.__getattr__
print(c.bees)         # will be satisfied by C.__getattr__
print(c.cheetahs)     # will be satisfied by A.cheetah
print(c.donuts)       # will be satisfied by A.__getattr__
print(c.elephants)    # will be satisfied by B.elephants

print(c.frenchtoast)  # will dispatch first to C.__getattr__, then to A.__getattr__, 
                      # and will return None because A does not call getattr on super, 
                      # nor does it raise an attribute error.