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__)

↓

Perform custom operation

↓

Return new object or value

↓

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.

Practice

1. What does arithmetic operator overloading allow you to do in Python?

easy

A. Define how operators like +, -, * work for your custom objects

B. Change the behavior of built-in data types like int and str

C. Create new arithmetic operators not available in Python

D. Automatically optimize arithmetic operations for speed

Solution

1. Step 1: Understand operator overloading concept

   Operator overloading lets you tell Python how to use operators like + or * with your own objects.

2. Step 2: Identify what can be customized

   You can define special methods like __add__ to customize + for your class instances.

3. Final Answer:

   Define how operators like +, -, * work for your custom objects -> Option A

4. Quick Check:

   Operator overloading = custom operator behavior [OK]

Hint: Think: 'How do I make + work for my objects?' [OK]

Common Mistakes:

• Confusing operator overloading with creating new operators
• Thinking it changes built-in types behavior
• Assuming it improves performance automatically

2. Which special method should you define to overload the addition operator (+) in a Python class?

easy

A. __append__

B. __plus__

C. __sum__

D. __add__

Solution

1. Step 1: Recall Python special method names for operators

   Python uses __add__ to overload the + operator in classes.

2. Step 2: Check other options

   __plus__, __sum__, and __append__ are not valid special methods for + operator.

3. Final Answer:

   __add__ -> Option D

4. Quick Check:

   + operator method = __add__ [OK]

Hint: Remember: add = __add__, subtract = __sub__ [OK]

Common Mistakes:

• Using __plus__ instead of __add__
• Confusing __sum__ with sum() function
• Using __append__ which is for lists

3. What will be the output of this code?

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

n1 = Number(5)
n2 = Number(10)
print(n1 + n2)

medium

A. TypeError

B. Number object at some memory address

C. 15

D. 5 10

Solution

1. Step 1: Understand __add__ method behavior

   Adding n1 + n2 calls __add__, which returns a new Number with value 5 + 10 = 15.

2. Step 2: Understand __str__ method effect on print

   print calls __str__ on the result, which returns '15' as string.

3. Final Answer:

   15 -> Option C

4. Quick Check:

   Custom + returns Number(15), printed as '15' [OK]

Hint: Check __add__ return and __str__ print output [OK]

Common Mistakes:

• Expecting print to show object memory address
• Forgetting __str__ method for printing
• Thinking it raises TypeError without __add__

4. Find the error in this code that tries to overload the multiplication operator (*) for a class:

class Multiplier:
    def __init__(self, num):
        self.num = num
    def __mul__(self, other):
        return self.num * other.num

m1 = Multiplier(3)
m2 = Multiplier(4)
print(m1 * m2)

medium

A. The __mul__ method should return a Multiplier object, not an int

B. The __init__ method is missing a return statement

C. The print statement should use str(m1 * m2)

D. The __mul__ method should be named __multiply__

Solution

1. Step 1: Check __mul__ return type

   __mul__ returns an int (self.num * other.num), but operator overloading usually returns an object of the class.

2. Step 2: Understand why returning int is a problem

   Returning int means further chained operations on Multiplier objects will fail or behave unexpectedly.

3. Final Answer:

   The __mul__ method should return a Multiplier object, not an int -> Option A

4. Quick Check:

   __mul__ must return class instance for chaining [OK]

Hint: Return class instance in operator methods, not raw values [OK]

Common Mistakes:

• Returning raw int instead of class instance
• Thinking __init__ needs return
• Misnaming __mul__ method

5. You want to create a class Vector that supports adding vectors and multiplying by a number. Which methods should you define to support v1 + v2 and v1 * 3 where v1 and v2 are Vector objects?

hard

A. __add__ for vector + vector, __rmul__ for number * vector

B. __add__ for vector + vector, __mul__ for vector * number

C. __add__ for vector + vector, __mul__ for number * vector

D. __radd__ for vector + vector, __mul__ for vector * number

Solution

1. Step 1: Identify method for vector + vector

   __add__ handles adding two Vector objects like v1 + v2.

2. Step 2: Identify method for vector * number

   __mul__ handles multiplying Vector by a number like v1 * 3.

3. Final Answer:

   __add__ for vector + vector, __mul__ for vector * number -> Option B

4. Quick Check:

   Use __add__ and __mul__ for these operations [OK]

Hint: Use __add__ for +, __mul__ for * with your class [OK]

Common Mistakes:

• Confusing __rmul__ with __mul__
• Using __radd__ instead of __add__ for vector + vector
• Assuming number * vector uses __mul__ (it uses __rmul__)