Instance attributes in Python - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of instance attribute modification

What is the output of this Python code involving instance attributes?

Python

class Dog:
    def __init__(self, name):
        self.name = name

d1 = Dog('Buddy')
d2 = Dog('Max')
d1.name = 'Charlie'
print(d2.name)

A Max

B Charlie

C None

D Buddy

💡 Hint

Each instance has its own attributes. Changing one does not affect the other.

Changing d1.name only changes the name for d1. d2.name remains 'Max'.

❓ Predict Output

intermediate

Instance attribute shadowing class attribute

What will be printed by this code?

Python

class Cat:
    species = 'Felis'

    def __init__(self, name):
        self.name = name

c = Cat('Whiskers')
c.species = 'Panthera'
print(Cat.species)
print(c.species)

A Panthera\nPanthera

B Felis\nPanthera

C Felis\nFelis

D Panthera\nFelis

💡 Hint

Instance attribute with the same name hides the class attribute for that instance.

Class attribute Cat.species remains 'Felis'. Instance c.species is set to 'Panthera', shadowing the class attribute.

🔧 Debug

advanced

Identify the error in instance attribute assignment

What error does this code raise when run?

Python

class Person:
    def __init__(self, name):
        name = name

p = Person('Alice')
print(p.name)

A TypeError

B NameError

C None (prints 'Alice')

D AttributeError

💡 Hint

Check if the instance attribute is properly assigned inside __init__.

The code assigns to local variable 'name' instead of instance attribute 'self.name'. So p has no 'name' attribute, causing AttributeError.

❓ Predict Output

advanced

Instance attribute default value behavior

What is the output of this code?

Python

class Counter:
    def __init__(self):
        if not hasattr(self, 'count'):
            self.count = 0
    def increment(self):
        self.count += 1

c1 = Counter()
c2 = Counter()
c1.increment()
c1.increment()
c2.increment()
print(c1.count, c2.count)

A 2 1

B 1 1

C 0 0

D 2 2

💡 Hint

Each instance has its own 'count' attribute initialized to 0.

c1 increments twice, so c1.count is 2. c2 increments once, so c2.count is 1.

🧠 Conceptual

expert

Understanding instance attribute creation timing

When is an instance attribute created in Python classes?

A When the attribute is accessed for the first time

B When the class is defined

C When the instance is created and the attribute is assigned

D When the interpreter starts

💡 Hint

Think about when the attribute actually appears on the object.

Instance attributes are created when assigned to self inside methods or after instance creation, not before.

Practice

1. What is an instance attribute in Python classes?

easy

A. A variable shared by all objects of the class

B. A function that belongs to the class

C. A method to create new objects

D. A variable that stores data unique to each object

Solution

1. Step 1: Understand instance attributes

   Instance attributes are variables that belong to each object separately, not shared.

2. Step 2: Differentiate from class attributes

   Class attributes are shared by all objects, but instance attributes hold unique data per object.

3. Final Answer:

   A variable that stores data unique to each object -> Option D

4. Quick Check:

   Instance attribute = unique data per object [OK]

Hint: Instance attributes belong to objects, not the class itself [OK]

Common Mistakes:

• Confusing instance attributes with class attributes
• Thinking methods are attributes
• Assuming all objects share the same attribute values

2. Which of the following is the correct way to define an instance attribute inside a class?

easy

A. name = "Alice" outside any method

B. def name(self): return "Alice"

C. self.name = "Alice" inside __init__ method

D. class.name = "Alice" inside __init__

Solution

1. Step 1: Identify instance attribute syntax

   Instance attributes are set inside __init__ using self.attribute = value.

2. Step 2: Check each option

   self.name = "Alice" inside __init__ method uses self.name = "Alice" inside __init__, which is correct. Others are class attributes, methods, or invalid.

3. Final Answer:

   self.name = "Alice" inside __init__ method -> Option C

4. Quick Check:

   Instance attribute = self.attribute inside __init__ [OK]

Hint: Use self.attribute = value inside __init__ for instance attributes [OK]

Common Mistakes:

• Defining attributes outside __init__ without self
• Using class.attribute instead of self.attribute
• Confusing methods with attributes

3. What will be the output of this code?

class Dog:
    def __init__(self, name):
        self.name = name

dog1 = Dog("Buddy")
dog2 = Dog("Max")
print(dog1.name)
print(dog2.name)

medium

A. Buddy
Max

B. Max
Buddy

C. Buddy
Buddy

D. Max
Max

Solution

1. Step 1: Understand instance attribute assignment

   dog1.name is set to "Buddy" and dog2.name is set to "Max" separately.

2. Step 2: Print instance attributes

   Printing dog1.name outputs "Buddy" and dog2.name outputs "Max".

3. Final Answer:

   Buddy Max -> Option A

4. Quick Check:

   Each object has its own name attribute [OK]

Hint: Each object keeps its own attribute values [OK]

Common Mistakes:

• Assuming all objects share the same attribute
• Mixing up the order of print statements
• Confusing class and instance attributes

4. Find the error in this code:

class Car:
    def __init__(self, model):
        model = model

car = Car("Tesla")
print(car.model)

medium

A. AttributeError because model is not set as instance attribute

B. SyntaxError due to missing self

C. TypeError because __init__ has wrong parameters

D. No error, prints Tesla

Solution

1. Step 1: Check attribute assignment in __init__

   The code assigns model = model, which only assigns local variable, not instance attribute.

2. Step 2: Accessing car.model causes error

   Since self.model is never set, car.model does not exist, causing AttributeError.

3. Final Answer:

   AttributeError because model is not set as instance attribute -> Option A

4. Quick Check:

   Use self.model = model to set instance attribute [OK]

Hint: Always use self.attribute = value to set instance attributes [OK]

Common Mistakes:

• Forgetting self. when assigning attributes
• Assuming local variable sets instance attribute
• Expecting attribute to exist without self

5. You want to create a class Book where each book has a title and a list of authors. How do you correctly set instance attributes so each book has its own authors list without sharing it between objects?

hard

A. Set self.authors = None and assign list later

B. Set self.authors = [] inside __init__ method

C. Set self.authors = authors where authors is a default empty list in parameters

D. Set authors = [] as a class attribute outside methods

Solution

1. Step 1: Avoid shared mutable class attributes

   Setting authors = [] as class attribute shares the same list across all objects, causing bugs.

2. Step 2: Initialize instance attribute inside __init__

   Setting self.authors = [] inside __init__ creates a new list for each object, avoiding sharing.

3. Final Answer:

   Set self.authors = [] inside __init__ method -> Option B

4. Quick Check:

   Mutable instance attributes must be set inside __init__ [OK]

Hint: Initialize mutable attributes inside __init__ to avoid sharing [OK]

Common Mistakes:

• Using mutable default arguments in method parameters
• Defining mutable attributes as class variables
• Not initializing mutable attributes per instance