Extending parent behavior in Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does it mean to extend parent behavior in Python classes?

It means adding new features or modifying existing ones in a child class while still using the behavior defined in the parent class.

beginner

How do you call a method from the parent class inside a child class in Python?

You use <code>super().method_name()</code> to call the parent class method from the child class.

beginner

Why would you want to extend parent behavior instead of completely replacing it?

To keep the original functionality and add extra features or changes without losing what the parent class already does.

beginner

Example: What will this code print?
<pre>class Parent:
    def greet(self):
        print('Hello from Parent')

class Child(Parent):
    def greet(self):
        super().greet()
        print('Hello from Child')

c = Child()
c.greet()</pre>

It will print: Hello from Parent Hello from Child Because the child calls the parent's greet method first, then adds its own message.

beginner

What happens if you forget to call super() when overriding a method?

The parent class method won't run, so you lose the original behavior and only the child's code runs.

What keyword is used to call a parent class method in Python?

A base()

B parent()

C self()

D super()

The super() function is used to access methods from the parent class.

If a child class overrides a method but does NOT call super(), what happens?

A Only child method runs

B Error occurs

C Both parent and child methods run automatically

D Parent method runs first

Without super(), only the child method runs, skipping the parent method.

Why use super() inside an overridden method?

A To call the parent class method and extend its behavior

B To call a method from another unrelated class

C To create a new method

D To delete the parent method

super() calls the parent method so you can add to it instead of replacing it.

What is the output of this code?

class A:
    def say(self):
        print('A')

class B(A):
    def say(self):
        print('B')
        super().say()

b = B()
b.say()

A A\nB

B A

C B\nA

D B

The child prints 'B' first, then calls the parent method which prints 'A'.

Extending parent behavior helps to:

A Completely replace parent code

B Reuse code and add new features

C Avoid using inheritance

D Make code slower

Extending behavior allows reuse of existing code while adding new functionality.

Explain how to extend a parent class method in Python and why it is useful.

Think about how to keep old behavior and add new steps.

Describe what happens if you override a method without calling super() in a child class.

Consider what part of the code is skipped.

Practice

1. What does super() do in a child class method?

easy

A. It overrides the child class method completely.

B. It calls the parent class method to reuse its behavior.

C. It deletes the parent class method.

D. It creates a new instance of the child class.

Solution

1. Step 1: Understand the role of super()

   super() is used to call a method from the parent class inside a child class method.

2. Step 2: Recognize code reuse

   By calling the parent method, the child can reuse existing behavior and add new features without rewriting code.

3. Final Answer:

   It calls the parent class method to reuse its behavior. -> Option B

4. Quick Check:

   super() calls parent method = D [OK]

Hint: Remember: super() runs parent method inside child [OK]

Common Mistakes:

• Thinking super() creates new instances
• Believing super() deletes methods
• Assuming super() overrides without calling parent

2. Which of the following is the correct syntax to call a parent class method greet inside a child class method in Python?

easy

A. super().greet()

B. super.greet()

C. parent.greet()

D. self.super.greet()

Solution

1. Step 1: Recall correct super() syntax

   In Python, super() is a function and must be called with parentheses before accessing methods.

2. Step 2: Identify correct method call

   The correct way to call the parent method is super().greet(), not super.greet() or others.

3. Final Answer:

   super().greet() -> Option A

4. Quick Check:

   super() needs parentheses = A [OK]

Hint: Use parentheses with super() to call parent methods [OK]

Common Mistakes:

• Omitting parentheses after super
• Using parent instead of super
• Trying to access super as an attribute

3. What will be the output of this code?

class Parent:
    def greet(self):
        print('Hello from Parent')

class Child(Parent):
    def greet(self):
        super().greet()
        print('Hello from Child')

c = Child()
c.greet()

medium

A. Hello from Child

B. Error: super() not defined

C. Hello from Parent

D. Hello from Parent
Hello from Child

Solution

1. Step 1: Trace the child greet() method

   The child method calls super().greet() first, which prints 'Hello from Parent'.

2. Step 2: Continue child method execution

   After calling the parent method, it prints 'Hello from Child'. So both lines print in order.

3. Final Answer:

   Hello from Parent Hello from Child -> Option D

4. Quick Check:

   super() calls parent then child prints = B [OK]

Hint: super() runs parent code first, then child adds more [OK]

Common Mistakes:

• Ignoring the parent print
• Expecting only child output
• Thinking super() causes error

4. Find the error in this code that tries to extend the parent method:

class Parent:
    def show(self):
        print('Parent show')

class Child(Parent):
    def show(self):
        super.show()
        print('Child show')

medium

A. super.show() should be super().show()

B. Child class must not override show()

C. Parent class method show() is missing self

D. print statements must be inside __init__

Solution

1. Step 1: Check super() usage

   The code uses super.show() which is incorrect syntax; super() must be called as a function.

2. Step 2: Correct the syntax

   It should be super().show() to properly call the parent method.

3. Final Answer:

   super.show() should be super().show() -> Option A

4. Quick Check:

   super() needs parentheses to call methods = A [OK]

Hint: Always use super() with parentheses to call parent methods [OK]

Common Mistakes:

• Forgetting parentheses after super
• Thinking parent method needs no self
• Believing print must be in __init__

5. You want to extend a parent class method calculate so that the child class adds 10 to the parent's result. Which code correctly does this?

class Parent:
    def calculate(self):
        return 5

class Child(Parent):
    def calculate(self):
        # Fill here

hard

A. return Parent.calculate() + 10

B. return calculate() + 10

C. return super().calculate() + 10

D. return self.calculate() + 10

Solution

1. Step 1: Use super() to call parent method

   To get the parent's result, call super().calculate() inside the child method.

2. Step 2: Add 10 to the parent's result

   Return the parent's value plus 10 as super().calculate() + 10.

3. Final Answer:

   return super().calculate() + 10 -> Option C

4. Quick Check:

   super() calls parent, add 10 = C [OK]

Hint: Use return super().method() + extra to extend result [OK]

Common Mistakes:

• Calling calculate() without super causes recursion
• Calling Parent.calculate() without instance
• Using self.calculate() causes infinite loop