Parent and child classes in Python - Step-by-Step Execution

Concept Flow - Parent and child classes

Define Parent Class

↓

Define Child Class inherits Parent

↓

Create Child Object

↓

Call Child Method

↓

If method not in Child

Yes↓

Use Parent Method

↓

End

This flow shows how a child class inherits from a parent class and uses its methods if not overridden.

Execution Sample

Python

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

class Child(Parent):
    pass

c = Child()
print(c.greet())

This code defines a parent class with a method and a child class that inherits it, then calls the inherited method.

Execution Table

Step	Action	Evaluation	Result
1	Define class Parent	Parent class created	Parent class ready
2	Define method greet in Parent	Method greet added	Parent.greet() available
3	Define class Child inheriting Parent	Child class created	Child inherits Parent
4	Create object c of Child	c is instance of Child	c created
5	Call c.greet()	Method lookup in Child	Not found in Child
6	Lookup method in Parent	Method greet found	Use Parent.greet()
7	Execute Parent.greet()	Return string	"Hello from Parent"
8	Print output	Output to console	Hello from Parent

💡 Method found in Parent, output printed, program ends

Variable Tracker

Variable	Start	After Step 4	After Step 7	Final
c	undefined	Child instance	unchanged	unchanged

Key Moments - 2 Insights

Why does c.greet() work even though Child class has no greet method?

Because the method is found in the Parent class during lookup (see execution_table step 6). Child inherits Parent's methods.

What happens if Child defines its own greet method?

The Child's greet method will be used instead of Parent's (method lookup stops at Child). This is called overriding.

Visual Quiz - 3 Questions

Look at the execution table, at which step is the method greet found in the Parent class?

A Step 7

B Step 5

C Step 6

D Step 4

💡 Hint

Check the 'Evaluation' column for method lookup results in execution_table.

According to variable_tracker, what is the value of variable c after step 4?

A undefined

B Child instance

C Parent instance

D None

💡 Hint

Look at variable_tracker row for c after step 4.

If we add a greet method to Child class, what changes in the execution_table?

A Method lookup finds greet in Child at step 5

B Method lookup still finds greet in Parent at step 6

C Object c becomes Parent instance

D Output changes to an error

💡 Hint

Think about method overriding and lookup order in execution_table steps 5 and 6.

Concept Snapshot

Parent and child classes:
- Define Parent class with methods.
- Child class inherits Parent using class Child(Parent).
- Child objects can use Parent methods if not overridden.
- Method lookup checks Child first, then Parent.
- Overriding allows Child to replace Parent methods.

Full Transcript

This example shows how a child class inherits from a parent class in Python. First, the Parent class is defined with a greet method. Then, the Child class is defined to inherit from Parent but does not add any methods. When we create an object c of Child and call c.greet(), Python looks for greet in Child. It is not found there, so it looks in Parent and finds it. The greet method from Parent runs and returns the string 'Hello from Parent', which is printed. This shows inheritance where the child class uses the parent's methods if it does not have its own version. If Child had its own greet method, that would be used instead, demonstrating method overriding.