Comparison magic methods in Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the __eq__ magic method in Python?

The __eq__ method defines how two objects are compared for equality using the == operator.

beginner

Which magic method is called when using the < operator?

The __lt__ method is called to check if one object is less than another.

intermediate

How does Python use __le__ and __ge__ methods?

__le__ defines behavior for the <= operator (less than or equal), and __ge__ defines behavior for the >= operator (greater than or equal).

intermediate

What happens if you only define __eq__ but not __ne__?

Python will automatically infer __ne__ (not equal) as the opposite of __eq__, so you usually don't need to define __ne__ separately.

beginner

Why would you implement comparison magic methods in your own class?

To allow objects of your class to be compared using standard operators like <code>==</code>, <code>&lt;</code>, <code>&gt;</code>, which helps with sorting, searching, and logical checks.

Which magic method is used for the 'greater than' (>) comparison?

A __gt__

B __ge__

C __lt__

D __le__

The __gt__ method defines behavior for the > operator.

If you want to check if two objects are not equal, which magic method is used?

A __gt__

B __eq__

C __lt__

D __ne__

The __ne__ method defines behavior for the != operator.

What does the __le__ method represent?

A Less than (<)

B Less than or equal (<=)

C Greater than (>)

D Greater than or equal (>=)

__le__ is for the <= operator.

If a class defines __eq__ but not __ne__, what happens when != is used?

A __ne__ is automatically the opposite of __eq__

B Error is raised

C __ne__ returns True always

D Comparison always returns False

Python automatically uses the opposite of __eq__ for __ne__ if not defined.

Which magic method would you implement to allow sorting objects with the < operator?

A __eq__

B __ne__

C __lt__

D __gt__

The __lt__ method defines behavior for the < operator, which is used in sorting.

Explain the role of comparison magic methods in Python and name at least three of them.

Think about how Python knows what to do when you use ==, <, or > with objects.

Describe what happens if you define __eq__ but not __ne__ in a Python class.

Consider how Python handles != when __ne__ is missing.

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