Arithmetic operator overloading in Python - Step-by-Step Execution

Concept Flow - Arithmetic operator overloading

Create object with value

↓

Use operator like +, -, *

↓

Call special method (__add__, __sub__, __mul__)

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Perform custom operation

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Return new object or value

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Use result in expression

When you use operators like + or -, Python calls special methods in your class to decide what to do.

Execution Sample

Python

class Number:
    def __init__(self, value):
        self.value = value
    def __add__(self, other):
        return Number(self.value + other.value)

n1 = Number(3)
n2 = Number(4)
n3 = n1 + n2
print(n3.value)

This code adds two Number objects by overloading the + operator and prints the result.

Execution Table

Step	Action	Evaluation	Result
1	Create n1 with value 3	n1.value = 3	n1 = Number(3)
2	Create n2 with value 4	n2.value = 4	n2 = Number(4)
3	Evaluate n1 + n2	Calls n1.__add__(n2)	Returns Number with value 7
4	Assign result to n3	n3.value = 7	n3 = Number(7)
5	Print n3.value	Output is 7	7

💡 Finished printing result, program ends

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	Final
n1.value	undefined	3	3	3	3	3
n2.value	undefined	undefined	4	4	4	4
n3.value	undefined	undefined	undefined	undefined	7	7

Key Moments - 3 Insights

Why does n1 + n2 call the __add__ method?

Because Python uses the __add__ method to define what + means for objects, as shown in step 3 of the execution_table.

What type is the result of n1 + n2?

The result is a new Number object with value 7, not just a number, as seen in step 3 and 4.

Why do we access n3.value to print the number?

Because n3 is a Number object, and the actual number is stored in its value attribute, shown in step 5.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of n3.value after step 4?

A 3

B 4

C 7

D undefined

💡 Hint

Check the 'After Step 4' column for n3.value in variable_tracker.

At which step does Python call the __add__ method?

A Step 1

B Step 3

C Step 5

D Step 2

💡 Hint

Look at the 'Action' column in execution_table where n1 + n2 is evaluated.

If __add__ returned self.value + other.value instead of a Number, what would change?

A n3 would be an int, not a Number object

B n1 and n2 would change values

C n3 would be a Number object

D The program would crash

💡 Hint

Think about the return type in step 3 of execution_table.

Concept Snapshot

Arithmetic operator overloading lets you define how operators like + work on your objects.
Use special methods like __add__(self, other) to customize behavior.
Return a new object or value from these methods.
This lets you write expressions like obj1 + obj2 naturally.
Python calls these methods automatically when operators are used.

Full Transcript

This lesson shows how Python lets you change what operators like + do when used with your own objects. When you write n1 + n2, Python calls the __add__ method inside your class. This method can do anything you want, like adding values inside the objects and returning a new object. We traced a simple example where two Number objects are added. Step by step, we saw how objects are created, how __add__ is called, and how the result is stored and printed. Key points include understanding that operators call special methods, the result can be a new object, and you access the stored value to see the number. The quizzes check your understanding of these steps and what changes if the method returns a different type.