In the last lesson we made the Catalog class for cataloging movies. However, there are other things we can catalog too, such as music. Since cataloging both movies and music should have some common functionalities, something we can do is create classes for them which inherit a common ancestor class. When we do this, it is called multiple inheritance, it means a class can inherit properties from more than one class. This is typically a parent/child relationship. What this often means is the parent is a generalized class and the children are specialized classes. To create these classes, it will look very similar to previously, with some extra work upfront to inherit and initialize the parent class. Lets start with creating a class specifically for cataloging movies. The Catalog class will be the same as the previous lesson.

By writing Movie(Catalog), the Movie class inherits the Catalog class. And the line Catalog.__init__(self, title, release_year) does the actual application of the attributes and methods from the Catalog class to the Movie class.

Why is this useful? Multiple inheritance allows a child class to access methods and properties from all its parent classes. This means we can use and modify instance variables defined in any of the parent classes by referring to them with self. To initialize these variables, we use the __init__ method in the child class, similar to how we would normally invoke a method. Additionally, creating a child class under this system lets us add more specific parameters and functions tailored to the child class's needs. This brings up an important question: how do function calls work when they are defined in both the parent and the child classes?

In the following code editor we initialize a movie object from the Movie class. Note that the code from the previous code editor is present but hidden in the following code block so we can focus on the new content. The way calling functions works in multiple inheritance is to work up the child/parent inheritance tree. So if a function is found in the child class, it will use it. If it's not, the interpreter will search its parent (or parents). If it is there, then it will use that. Additionally, a child class can call a parent class in the same way we called its __init__ function. We have a few examples here. The update_duration function is not found in the parent class, but is in the child class. The update_release_year and __str__ are not found in the child class, but are in the parent class, so those will be used. The update_title function is found in both. In this case, it will call the one found in the child first. Since it's written to call the parent function with a slightly different parameter edit, we can do that as well.

If we have a class that inherits from multiple parents, then it will search the order of inheritance based on which came first. For example if we made a class as: A(B,C), then when an instance of A is created, the interpreter will search A first, followed by B, then C.

Practice Questions

Use the following code for the following questions.

class Character: def __init__(self, name, age): self.name = name self.age = age def __str__(self): return f"{self.name} ({self.age})" class Human(Character): def __init__(self, name, age): Character.__init__(self, name, age) def __str__(self): return f"Human: {self.name} ({self.age})" class Elf(Character): def __init__(self, name, age, affinity): Character.__init__(self, name, age) self.affinity = affinity def __str__(self): return f"Elf: {self.name} ({self.age})" def get_affinity(self): return self.affinity class HalfElf(Human, Elf): def __init__(self, name, age, affinity): Human.__init__(self, name, age)

Q01.a. What will print when we run this code?

lizette = HalfElf("lizette", 19, "hydro") print(lizette, lizette.get_affinity())

Q01.b. If we initialize both Human and Elf such as in the code below, what will print?

class HalfElf(Human, Elf): def __init__(self, name, age, affinity): Human.__init__(self, name, age) Elf.__init__(self, name, age, affinity)

Q01.c. What will print if we change the order of inheritance?

class HalfElf(Elf, Human): def __init__(self, name, age, affinity): Human.__init__(self, name, age) Elf.__init__(self, name, age, affinity) Submit

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