__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.