Inheritance and Class Extension

One Instance, Two Initializers

When a child class extends a parent class, the child instance can contain attributes initialized by both classes. The child class adds its own setup, but the parent class's setup does not happen automatically inside the child's __init__ method. The child initializer must use super().__init__() to call the parent initializer.

The Delegation Path

class Account: def __init__(self, owner): self.owner = owner class SavingsAccount(Account): def __init__(self, owner, interest_rate): super().__init__(owner) self.interest_rate = interest_rate

Creation Order

The initialization order is deliberate. First, the child class's __init__ method begins. Next, super().__init__() calls the parent class's initialization method with the correct arguments. The parent initializer sets the parent attributes. Finally, execution returns to the child initializer, which sets the child-specific attributes. Calling super().__init__() at the beginning of the child initializer keeps this order explicit and ensures complete instance setup.

Combined Instance State

A child class instance inherits the parent class's attributes and methods and adds its own. In the generated example, owner comes from the parent initializer and interest_rate comes from the child initializer. These attributes coexist in the same child instance; they are not separate objects created by the two classes.

When the Delegation Is Missing

• Omitting super().__init__() from the child initializer.

  The parent attributes are not initialized by the parent initializer.

  Fix: Call super().__init__() at the beginning of the child initializer.

• Calling super().__init__() without the required parent arguments.

  The parent initializer needs the correct arguments to initialize its attributes.

  Fix: Pass the arguments required by the parent initializer.

• Assuming that child-specific setup replaces parent setup.

  A child instance inherits parent attributes and methods as well as adding its own.

  Fix: Use super().__init__() so both parent and child initialization take place.

Checking Inherited Members

A child class inherits all attributes and methods from its parent class and adds its own. The source pack identifies dir() as one way to verify the members available through the child class. Use it as an inspection tool when you want to examine what the child class provides through inheritance and extension.

class Account:
    def __init__(self, owner):
        self.owner = owner

class SavingsAccount(Account):
    def __init__(self, owner, interest_rate):
        super().__init__(owner)
        self.interest_rate = interest_rate

account = SavingsAccount("Ari", 0.03)
print(account.owner)
print(account.interest_rate)
print(dir(account))

In this example, the parent initializer supplies owner, while the child initializer supplies interest_rate. The call to dir(account) can be used to inspect the attributes and methods available through the resulting child instance.

Practice the Trace

1. Identify the parent class and the child class.
2. Find the child initializer's super().__init__() call.
3. Check that the arguments passed to super().__init__() match the parent initializer's needs.
4. Trace the parent attribute initialization before the child attribute initialization.
5. Confirm that both sets of attributes belong to the one child instance.

Key Takeaways

1. super() calls a method from the parent class.
2. super().__init__() runs the parent initialization method from the child initializer.
3. The parent initializer should run before child-specific attributes are set.
4. A child instance can contain both parent attributes and child attributes.
5. dir() can be used to inspect attributes and methods available through the child instance.