__init__ method behavior in Python - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of __init__ with default and custom arguments

What is the output of this Python code when creating an object with and without arguments?

Python

class Car:
    def __init__(self, brand='Toyota', year=2020):
        self.brand = brand
        self.year = year

car1 = Car()
car2 = Car('Honda', 2022)
print(car1.brand, car1.year)
print(car2.brand, car2.year)

A Honda 2022\nHonda 2022

B Toyota 2020\nToyota 2020

C Error: missing required positional arguments

D Toyota 2020\nHonda 2022

💡 Hint

Check how default values work in __init__ parameters.

The __init__ method sets default values for brand and year. car1 uses defaults, car2 uses provided values.

❓ Predict Output

intermediate

Effect of missing self in __init__ method

What happens when the __init__ method is defined without the self parameter?

Python

class Person:
    def __init__(name):
        name.name = name

p = Person('Alice')

A No error, object created successfully

B NameError: name 'name' is not defined

C TypeError: __init__() takes 1 positional argument but 2 were given

D AttributeError: 'str' object has no attribute 'name'

💡 Hint

Remember that the first parameter of __init__ must be self.

The __init__ method is defined with a single parameter `name`. When instantiating `Person('Alice')`, Python passes two arguments to `__init__`: the instance object (implicit `self`) and the string `'Alice'`. This causes `TypeError: __init__() takes 1 positional argument but 2 were given`.

🔧 Debug

advanced

Why does this __init__ method cause unexpected behavior?

Consider this code. What is the output and why?

Python

class Counter:
    count = 0
    def __init__(self):
        self.count += 1

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

A 1 1 0

B AttributeError: 'Counter' object has no attribute 'count'

C 1 2 2

D 0 0 0

💡 Hint

Consider how `self.count += 1` resolves `self.count` on the right-hand side.

`self.count += 1` is equivalent to `self.count = self.count + 1`. Reading `self.count` fetches the class attribute `count` (0) since no instance attribute exists yet. It increments to 1 and sets an instance attribute `self.count = 1`. The class attribute `count` remains 0. Both instances have their own `count=1`.

❓ Predict Output

advanced

Output when __init__ returns a value

What happens if __init__ method returns a value explicitly?

Python

class Sample:
    def __init__(self):
        print('Init called')
        return 5

s = Sample()

A No output, object created silently

B TypeError: __init__ should return None, not 'int'

C Init called

D 5

💡 Hint

Check what Python expects __init__ to return.

__init__ must return None. Returning any other value causes TypeError.

🧠 Conceptual

expert

Why does modifying a mutable default argument in __init__ cause bugs?

Consider this class: class Bag: def __init__(self, items=[]): self.items = items def add(self, item): self.items.append(item) b1 = Bag() b2 = Bag() b1.add('apple') print(b2.items) What is the output and why?

A ['apple'] because both b1 and b2 share the same default list

B [] because each instance has its own list

C Error: cannot append to default argument

D ['apple'] only in b1, b2.items is []

💡 Hint

Default arguments are evaluated once when the function is defined.

The default list is shared across all instances, so adding to b1.items affects b2.items.

Practice

1.

What is the main purpose of the __init__ method in a Python class?

easy

A. To create a new class

B. To delete an object from memory

C. To print information about the class

D. To set up initial values for the object's attributes

Solution

1. Step 1: Understand the role of __init__

   The __init__ method runs automatically when an object is created to set up initial values.

2. Step 2: Compare options with this role

   Only To set up initial values for the object's attributes describes setting initial attribute values, which matches __init__'s purpose.

3. Final Answer:

   To set up initial values for the object's attributes -> Option D

4. Quick Check:

   __init__ sets initial values = A [OK]

Hint: Remember: __init__ sets starting info for objects [OK]

Common Mistakes:

• Thinking __init__ deletes objects
• Confusing __init__ with printing methods
• Believing __init__ creates classes

2.

Which of the following is the correct way to define an __init__ method inside a Python class?

class Car:
    ?

easy

A. def __init__(self, model):

B. def init(self, model):

C. def __start__(self, model):

D. def __init__(model):

Solution

1. Step 1: Check method name and parameters

   The method must be named exactly __init__ and include self as the first parameter.

2. Step 2: Compare options

   Only def __init__(self, model): uses the correct name and includes self properly.

3. Final Answer:

   def __init__(self, model): -> Option A

4. Quick Check:

   Correct __init__ syntax = D [OK]

Hint: Always include self as first parameter in __init__ [OK]

Common Mistakes:

• Omitting self parameter
• Misspelling __init__ method name
• Using wrong method names like init or __start__

3.

What will be the output of this code?

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

my_dog = Dog("Buddy")
print(my_dog.name)

medium

A. Buddy

B. name

C. Dog

D. Error

Solution

1. Step 1: Understand object creation and attribute assignment

   The __init__ method sets self.name to "Buddy" when my_dog is created.

2. Step 2: Check the print statement

   Printing my_dog.name outputs the string "Buddy" stored in the attribute.

3. Final Answer:

   Buddy -> Option A

4. Quick Check:

   Object attribute value = Buddy [OK]

Hint: Print object.attribute to see stored value from __init__ [OK]

Common Mistakes:

• Printing class name instead of attribute
• Expecting attribute name instead of value
• Assuming code causes error

4.

Find the error in this class definition:

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

p = Person("Alice")

medium

A. Wrong class name

B. Missing self parameter in __init__

C. Missing parentheses when creating object

D. No error

Solution

1. Step 1: Check __init__ method parameters

   The __init__ method must have self as the first parameter, but it is missing here.

2. Step 2: Identify impact of missing self

   Without self, self.name causes an error because self is undefined.

3. Final Answer:

   Missing self parameter in __init__ -> Option B

4. Quick Check:

   __init__ needs self first [OK]

Hint: Always put self as first __init__ parameter [OK]

Common Mistakes:

• Forgetting self in method parameters
• Thinking class name must change
• Missing parentheses when creating object

5.

Consider this class:

class Book:
    def __init__(self, title, author="Unknown"):
        self.title = title
        self.author = author

b1 = Book("Python 101")
b2 = Book("Learn AI", "Dr. Smith")
print(b1.author, b2.author)

What will be printed?

hard

A. Dr. Smith Unknown

B. Python 101 Learn AI

C. Unknown Dr. Smith

D. Error due to missing author argument

Solution

1. Step 1: Understand default parameter in __init__

   The author parameter has a default value "Unknown", so it is optional when creating an object.

2. Step 2: Analyze object creations

   b1 is created with only title, so author is "Unknown". b2 provides both title and author "Dr. Smith".

3. Step 3: Check print output

   Printing b1.author and b2.author prints "Unknown Dr. Smith".

4. Final Answer:

   Unknown Dr. Smith -> Option C

5. Quick Check:

   Default parameters fill missing values = A [OK]

Hint: Default values fill missing arguments in __init__ [OK]

Common Mistakes:

• Expecting error when argument is missing
• Mixing up title and author values
• Assuming both arguments are always required