Parent and child classes in Python - Time & Space Complexity

Time Complexity: Parent and child classes

O(n)

Understanding Time Complexity

When using parent and child classes, it's important to see how the program runs as the number of objects grows.

We want to know how the time to run changes when we create or use many class instances.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

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

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

children = [Child() for _ in range(n)]
for child in children:
    child.greet()
    

This code creates a list of child objects and calls a method on each one that also calls the parent method.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Looping through the list of child objects and calling the greet method on each.
• How many times: The loop runs once for each child object, so n times.

How Execution Grows With Input

As the number of child objects increases, the total work grows in a straight line with n.

Input Size (n)	Approx. Operations
10	About 10 greet calls
100	About 100 greet calls
1000	About 1000 greet calls

Pattern observation: Doubling the number of objects doubles the total calls and work done.

Final Time Complexity

Time Complexity: O(n)

This means the time to run grows directly with the number of child objects created and used.

Common Mistake

[X] Wrong: "Calling a method on child classes that use parent methods makes the program slower in a way that grows faster than the number of objects."

[OK] Correct: Each method call is simple and happens once per object, so the total time grows just with the number of objects, not faster.

Interview Connect

Understanding how loops over class instances affect time helps you explain object-oriented code performance clearly and confidently.

Self-Check

"What if the greet method called itself recursively inside the child class? How would the time complexity change?"

Practice

1. What is the main purpose of a parent class in Python?

easy

A. To hold common features that child classes can inherit

B. To override methods in child classes

C. To create instances directly without child classes

D. To prevent child classes from adding new features

Solution

1. Step 1: Understand the role of parent class

   A parent class is designed to hold common features like methods and attributes that multiple child classes can share.

2. Step 2: Compare options with this role

   To hold common features that child classes can inherit correctly states this purpose. Other options describe incorrect or unrelated roles.

3. Final Answer:

   To hold common features that child classes can inherit -> Option A

4. Quick Check:

   Parent class = common features [OK]

Hint: Parent class shares features for children to reuse [OK]

Common Mistakes:

• Thinking parent classes prevent changes in children
• Believing parent classes are only for direct instances
• Confusing overriding with inheritance

2. Which of the following is the correct syntax to define a child class Dog that inherits from a parent class Animal?

easy

A. class Dog extends Animal:

B. class Dog inherits Animal:

C. class Dog < Animal:

D. class Dog(Animal):

Solution

1. Step 1: Recall Python inheritance syntax

   In Python, a child class inherits from a parent class by placing the parent class name in parentheses after the child class name.

2. Step 2: Match options with correct syntax

   class Dog(Animal): uses class Dog(Animal): which is correct. Other options use incorrect keywords or symbols not valid in Python.

3. Final Answer:

   class Dog(Animal): -> Option D

4. Quick Check:

   Child class syntax = class Child(Parent): [OK]

Hint: Use parentheses with parent class name after child class [OK]

Common Mistakes:

• Using 'inherits' or 'extends' keywords (not Python)
• Using symbols like '<' instead of parentheses
• Omitting the colon at the end

3. What will be the output of this code?

class Parent:
    def greet(self):
        return "Hello from Parent"

class Child(Parent):
    def greet(self):
        return "Hello from Child"

obj = Child()
print(obj.greet())

medium

A. Hello from ParentChild

B. Hello from Parent

C. Hello from Child

D. Error: greet method not found

Solution

1. Step 1: Understand method overriding in child class

   The child class Child defines its own greet method, which replaces the parent's method when called on a child instance.

2. Step 2: Check which method is called

   Since obj is an instance of Child, calling obj.greet() uses the child's method, returning "Hello from Child".

3. Final Answer:

   Hello from Child -> Option C

4. Quick Check:

   Child method overrides parent method = Hello from Child [OK]

Hint: Child method overrides parent method when called on child [OK]

Common Mistakes:

• Thinking parent method runs instead of child's
• Expecting combined output from both methods
• Assuming method not found error

4. Find the error in this code:

class Parent:
    def __init__(self, name):
        self.name = name

class Child(Parent):
    def __init__(self, name, age):
        self.age = age

c = Child('Anna', 10)
print(c.name, c.age)

medium

A. Child class __init__ does not call Parent __init__, so name is missing

B. Syntax error in class definition

C. Cannot create Child instance with two arguments

D. print statement syntax is wrong

Solution

1. Step 1: Check constructor chaining in child class

   The child class __init__ method sets age but does not call super().__init__(name) to set name from the parent.

2. Step 2: Understand consequence of missing super call

   Because name is not set in Child, accessing c.name will cause an error or missing attribute.

3. Final Answer:

   Child class __init__ does not call Parent __init__, so name is missing -> Option A

4. Quick Check:

   Missing super() call = missing parent attributes [OK]

Hint: Always call super().__init__ in child __init__ to set parent attributes [OK]

Common Mistakes:

• Forgetting to call super().__init__ in child constructor
• Assuming parent attributes set automatically
• Confusing syntax errors with logic errors

5. Given these classes, what will print(c.describe()) output?

class Parent:
    def describe(self):
        return "I am a parent"

class Child(Parent):
    def describe(self):
        parent_desc = super().describe()
        return parent_desc + " and I am a child"

c = Child()

hard

A. I am a parent

B. I am a parent and I am a child

C. I am a child

D. Error: super() used incorrectly

Solution

1. Step 1: Understand super() usage in child describe method

   The child method calls super().describe() which runs the parent method returning "I am a parent".

2. Step 2: Combine parent and child strings

   The child method adds " and I am a child" to the parent's string, so the full return is "I am a parent and I am a child".

3. Final Answer:

   I am a parent and I am a child -> Option B

4. Quick Check:

   super() calls parent method, combined output [OK]

Hint: Use super() to add parent behavior inside child method [OK]

Common Mistakes:

• Expecting only child or only parent output
• Thinking super() causes error without arguments
• Ignoring string concatenation