Comparison magic methods in Python - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of custom __eq__ method

What is the output of this code?

Python

class Box:
    def __init__(self, volume):
        self.volume = volume
    def __eq__(self, other):
        return self.volume == other.volume

b1 = Box(10)
b2 = Box(10)
b3 = Box(5)

print(b1 == b2)
print(b1 == b3)

A False\nTrue

B False\nFalse

C True\nTrue

D True\nFalse

💡 Hint

The __eq__ method compares the volume attribute of two Box objects.

The __eq__ method returns True if the volumes are equal. b1 and b2 have volume 10, so b1 == b2 is True. b1 and b3 have different volumes, so b1 == b3 is False.

❓ Predict Output

intermediate

Output of __lt__ and __gt__ methods

What is the output of this code?

Python

class Number:
    def __init__(self, value):
        self.value = value
    def __lt__(self, other):
        return self.value < other.value
    def __gt__(self, other):
        return self.value > other.value

n1 = Number(3)
n2 = Number(5)
print(n1 < n2)
print(n1 > n2)

A True\nFalse

B False\nFalse

C True\nTrue

D False\nTrue

💡 Hint

The __lt__ method checks if one value is less than the other, __gt__ checks if greater.

n1.value is 3 and n2.value is 5. 3 < 5 is True, 3 > 5 is False.

❓ Predict Output

advanced

Output of __le__ and __ge__ with inheritance

What is the output of this code?

Python

class Base:
    def __init__(self, x):
        self.x = x
    def __le__(self, other):
        return self.x <= other.x
    def __ge__(self, other):
        return self.x >= other.x

class Derived(Base):
    pass

b = Base(7)
d = Derived(7)
print(b <= d)
print(b >= d)

A True\nTrue

B False\nTrue

C True\nFalse

D False\nFalse

💡 Hint

Derived inherits comparison methods from Base and both have the same x value.

Both b and d have x = 7. So b <= d and b >= d both return True.

❓ Predict Output

advanced

Output of __eq__ with different types

What is the output of this code?

Python

class Item:
    def __init__(self, id):
        self.id = id
    def __eq__(self, other):
        if not isinstance(other, Item):
            return False
        return self.id == other.id

item = Item(10)
print(item == 10)
print(item == Item(10))

A True\nTrue

B False\nFalse

C False\nTrue

D True\nFalse

💡 Hint

The __eq__ method returns False if the other object is not an Item instance.

item == 10 returns False because 10 is not an Item. item == Item(10) returns True because ids match.

❓ Predict Output

expert

Output of chained comparisons with custom __lt__

What is the output of this code?

Python

class Weird:
    def __init__(self, val):
        self.val = val
    def __lt__(self, other):
        print(f"Comparing {self.val} < {other.val}")
        return self.val < other.val

w1 = Weird(1)
w2 = Weird(2)
w3 = Weird(3)
print(w1 < w2 < w3)

A Comparing 1 < 2\nComparing 2 < 3\nFalse

B Comparing 1 < 2\nComparing 2 < 3\nTrue

C Comparing 1 < 2\nTrue

D Comparing 1 < 2\nComparing 3 < 2\nTrue

💡 Hint

Python evaluates chained comparisons by checking each pair separately.

The expression w1 < w2 < w3 calls __lt__ twice: w1 < w2 and w2 < w3. Both print statements appear, and since 1 < 2 and 2 < 3 are True, the final output is True.

Practice

1. Which magic method in Python is used to define the behavior of the equality operator ==?

easy

A. __eq__

B. __lt__

C. __ne__

D. __gt__

Solution

1. Step 1: Understand the equality operator

   The == operator checks if two objects are equal.

2. Step 2: Identify the corresponding magic method

   In Python, __eq__ is the method that defines equality behavior.

3. Final Answer:

   __eq__ -> Option A

4. Quick Check:

   Equality operator uses __eq__ [OK]

Hint: Remember: eq means equal, so __eq__ handles == [OK]

Common Mistakes:

• Confusing __eq__ with __lt__ or __gt__
• Thinking __ne__ handles equality
• Mixing up method names with comparison operators

2. Which of the following is the correct syntax to define the less than operator < in a Python class?

easy

A. def __lt__(self):

B. def __less_than__(self, other):

C. def __less__(self, other):

D. def __lt__(self, other):

Solution

1. Step 1: Recall the magic method name for <

   The method for < is __lt__ and it takes two parameters: self and other.

2. Step 2: Check method signature correctness

   Correct syntax is def __lt__(self, other):. Other options have wrong names or missing parameters.

3. Final Answer:

   def __lt__(self, other): -> Option D

4. Quick Check:

   Less than operator uses __lt__(self, other) [OK]

Hint: Magic methods for comparisons always take self and other [OK]

Common Mistakes:

• Using wrong method names like __less_than__
• Omitting the other parameter
• Using incorrect method signatures

3. What will be the output of the following code?

class Number:
    def __init__(self, value):
        self.value = value
    def __gt__(self, other):
        return self.value > other.value

n1 = Number(5)
n2 = Number(3)
print(n1 > n2)

medium

A. False

B. TypeError

C. True

D. None

Solution

1. Step 1: Understand the __gt__ method

   The __gt__ method compares self.value and other.value.

2. Step 2: Evaluate the comparison

   n1.value is 5 and n2.value is 3, so 5 > 3 is True.

3. Final Answer:

   True -> Option C

4. Quick Check:

   5 > 3 = True [OK]

Hint: Check the values inside objects when comparing [OK]

Common Mistakes:

• Forgetting to compare attributes inside objects
• Expecting print to show object addresses
• Confusing __gt__ with __lt__

4. Identify the error in the following class that tries to implement the not equal operator !=:

class Item:
    def __init__(self, val):
        self.val = val
    def __ne__(self):
        return self.val != other.val

medium

A. The class should inherit from object explicitly

B. __ne__ method is missing the other parameter

C. The __ne__ method should return True always

D. The __init__ method is missing self

Solution

1. Step 1: Check __ne__ method signature

   The __ne__ method must take two parameters: self and other.

2. Step 2: Identify missing parameter

   Here, other is used but not declared as a parameter, causing an error.

3. Final Answer:

   __ne__ method is missing the other parameter -> Option B

4. Quick Check:

   __ne__ needs (self, other) parameters [OK]

Hint: Comparison methods always take self and other [OK]

Common Mistakes:

• Omitting the other parameter in comparison methods
• Misunderstanding method signatures
• Thinking inheritance from object is required in Python 3

5. You want to create a class Box where two boxes are considered equal if their volumes are equal. Which magic method should you implement and how?

class Box:
    def __init__(self, length, width, height):
        self.length = length
        self.width = width
        self.height = height
    # Your code here

hard

A. Implement __eq__(self, other) to compare volumes: return self.length * self.width * self.height == other.length * other.width * other.height

B. Implement __lt__(self, other) to compare volumes

C. Implement __ne__(self, other) to compare volumes

D. Implement __gt__(self, other) to compare volumes

Solution

1. Step 1: Identify the comparison needed

   Equality means using ==, so implement __eq__.

2. Step 2: Define volume comparison inside __eq__

   Calculate volume for both boxes and compare for equality.

3. Final Answer:

   Implement __eq__(self, other) to compare volumes: return self.length * self.width * self.height == other.length * other.width * other.height -> Option A

4. Quick Check:

   Equality uses __eq__ comparing volumes [OK]

Hint: Use __eq__ to define equality based on volume [OK]

Common Mistakes:

• Using __lt__ or __gt__ for equality
• Not comparing volumes but attributes directly
• Forgetting to implement __eq__ for == operator