Arithmetic operator overloading in Python - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of adding two custom objects with overloaded + operator

Consider the following Python class that overloads the + operator. What will be the output of the code below?

Python

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)
result = n1 + n2
print(result)

A 15

B Number object at some memory address

C TypeError

D 510

💡 Hint

Look at how the __add__ method returns a new Number object and how __str__ controls printing.

The __add__ method returns a new Number object with the sum of the two values. The print calls __str__, which returns the string of the summed value, so it prints 15.

❓ Predict Output

intermediate

Output of subtracting custom objects with overloaded - operator

What is the output of this code that overloads the - operator?

Python

class Point:
    def __init__(self, x, y):
        self.x = x
        self.y = y
    def __sub__(self, other):
        return Point(self.x - other.x, self.y - other.y)
    def __repr__(self):
        return f"Point({self.x}, {self.y})"

p1 = Point(10, 5)
p2 = Point(3, 2)
print(p1 - p2)

A Point(7, 3)

B Point(13, 7)

C TypeError

D None

💡 Hint

Check how the __sub__ method calculates the difference of coordinates.

The __sub__ method returns a new Point with each coordinate subtracted. So (10-3, 5-2) = (7, 3).

❓ Predict Output

advanced

Output of multiplying custom objects with overloaded * operator returning int

What will this code print? The * operator is overloaded to return an integer.

Python

class Vector:
    def __init__(self, x, y):
        self.x = x
        self.y = y
    def __mul__(self, other):
        return self.x * other.x + self.y * other.y

v1 = Vector(2, 3)
v2 = Vector(4, 5)
print(v1 * v2)

A None

B Vector(8, 15)

C TypeError

D 23

💡 Hint

The * operator here calculates the dot product of two vectors.

The dot product is (2*4) + (3*5) = 8 + 15 = 23, so the code prints 23.

❓ Predict Output

advanced

Output of in-place addition with overloaded __iadd__ method

What is the output of this code that overloads the += operator?

Python

class Counter:
    def __init__(self, count=0):
        self.count = count
    def __iadd__(self, other):
        self.count += other.count
        return self
    def __str__(self):
        return str(self.count)

c1 = Counter(5)
c2 = Counter(10)
c1 += c2
print(c1)

A Counter object at some memory address

B 15

C TypeError

D 10

💡 Hint

The __iadd__ method modifies the object in place and returns it.

The += operator calls __iadd__, which adds the counts and returns the updated object. Printing calls __str__, so it prints 15.

🧠 Conceptual

expert

Behavior of unsupported arithmetic operator overloading

Given this class that only overloads __add__, what happens if you try to subtract two objects?

class MyNum:
    def __init__(self, val):
        self.val = val
    def __add__(self, other):
        return MyNum(self.val + other.val)

x = MyNum(5)
y = MyNum(3)
z = x - y

A Returns None

B Returns MyNum(2)

C Raises TypeError: unsupported operand type(s) for -: 'MyNum' and 'MyNum'

D Raises AttributeError

💡 Hint

Check what happens if an operator method is not defined for a class.

Since __sub__ is not defined, Python raises a TypeError saying the operator is unsupported for the given types.

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__)