We can use multiple inheritance to add this new class as a parent of our existing Friend class. The tricky part is that we now have two parent __init__ methods, both of which need to be initialized. And they need to be initialized with different arguments. How do we do this? Well, we could start with a naive approach:
class Friend(Contact, AddressHolder):
def __init__(
self, name, email, phone, street, city, state, code):
Contact.__init__(self, name, email)
AddressHolder.__init__(self, street, city, state, code)
self.phone = phone
In this example, we directly call the __init__ function on each of the superclasses and explicitly pass the self argument. This example technically works; we can access the different variables directly on the class. But there are a few problems.
First, it is possible for a superclass to go uninitialized if we neglect to explicitly call the initializer. That wouldn't break this example, but it could cause hard-to-debug program crashes in common scenarios. Imagine trying to insert data into a database that has not been connected to, for example.
A more insidious possibility is a superclass being called multiple times because of the organization of the class hierarchy. Look at this inheritance diagram:
The __init__ method from the Friend class first calls __init__ on Contact, which implicitly initializes the object superclass (remember, all classes derive from object). Friend then calls __init__ on AddressHolder, which implicitly initializes the object superclass again. This means the parent class has been set up twice. With the object class, that's relatively harmless, but in some situations, it could spell disaster. Imagine trying to connect to a database twice for every request!
The base class should only be called once. Once, yes, but when? Do we call Friend, then Contact, then Object, and then AddressHolder? Or Friend, then Contact, then AddressHolder, and then Object?
Let's look at a second contrived example, which illustrates this problem more clearly. Here, we have a base class that has a method named call_me. Two subclasses override that method, and then another subclass extends both of these using multiple inheritance. This is called diamond inheritance because of the diamond shape of the class diagram:
Let's convert this diagram to code; this example shows when the methods are called:
class BaseClass:
num_base_calls = 0
def call_me(self):
print("Calling method on Base Class")
self.num_base_calls += 1
class LeftSubclass(BaseClass):
num_left_calls = 0
def call_me(self):
BaseClass.call_me(self)
print("Calling method on Left Subclass")
self.num_left_calls += 1
class RightSubclass(BaseClass):
num_right_calls = 0
def call_me(self):
BaseClass.call_me(self)
print("Calling method on Right Subclass")
self.num_right_calls += 1
class Subclass(LeftSubclass, RightSubclass):
num_sub_calls = 0
def call_me(self):
LeftSubclass.call_me(self)
RightSubclass.call_me(self)
print("Calling method on Subclass")
self.num_sub_calls += 1
This example ensures that each overridden call_me method directly calls the parent method with the same name. It lets us know each time a method is called by printing the information to the screen. It also updates a static variable on the class to show how many times it has been called. If we instantiate one Subclass object and call the method on it once, we get the output:
>>> s = Subclass() >>> s.call_me() Calling method on Base Class Calling method on Left Subclass Calling method on Base Class Calling method on Right Subclass Calling method on Subclass >>> print( ... s.num_sub_calls, ... s.num_left_calls, ... s.num_right_calls, ... s.num_base_calls) 1 1 1 2
Thus, we can clearly see the base class's call_me method being called twice. This could lead to some pernicious bugs if that method is doing actual work, such as depositing into a bank account, twice.
The thing to keep in mind with multiple inheritance is that we only want to call the next method in the class hierarchy, not the parent method. In fact, that next method may not be on a parent or ancestor of the current class. The super keyword comes to our rescue once again. Indeed, super was originally developed to make complicated forms of multiple inheritance possible. Here is the same code written using super:
class BaseClass:
num_base_calls = 0
def call_me(self):
print("Calling method on Base Class")
self.num_base_calls += 1
class LeftSubclass(BaseClass):
num_left_calls = 0
def call_me(self):
super().call_me()
print("Calling method on Left Subclass")
self.num_left_calls += 1
class RightSubclass(BaseClass):
num_right_calls = 0
def call_me(self):
super().call_me()
print("Calling method on Right Subclass")
self.num_right_calls += 1
class Subclass(LeftSubclass, RightSubclass):
num_sub_calls = 0
def call_me(self):
super().call_me()
print("Calling method on Subclass")
self.num_sub_calls += 1
The change is pretty minor; we only replaced the naive direct calls with calls to super(), although the bottom subclass only calls super once rather than having to make the calls for both the left and right. The change is easy enough, but look at the difference when we execute it:
>>> s = Subclass() >>> s.call_me() Calling method on Base Class Calling method on Right Subclass Calling method on Left Subclass Calling method on Subclass >>> print(s.num_sub_calls, s.num_left_calls, s.num_right_calls, s.num_base_calls) 1 1 1 1
Looks good; our base method is only being called once. But what is super() actually doing here? Since the print statements are executed after the super calls, the printed output is in the order each method is actually executed. Let's look at the output from back to front to see who is calling what.
First, call_me of Subclass calls super().call_me(), which happens to refer
to LeftSubclass.call_me(). The LeftSubclass.call_me() method then calls super().call_me(), but in this case, super() is referring to RightSubclass.call_me().
Pay particular attention to this: the super call is not calling the method on the superclass of LeftSubclass (which is BaseClass). Rather, it is calling RightSubclass, even though it is not a direct parent of LeftSubclass! This is the next method, not the parent method. RightSubclass then calls BaseClass and the super calls have ensured each method in the class hierarchy is executed once.