Object initialization flow in Python - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of __init__ and __post_init__ in dataclasses

What is the output of this Python code using dataclasses?

Python

from dataclasses import dataclass, field

@dataclass
class Person:
    name: str
    age: int = field(default=0)

    def __post_init__(self):
        print(f"Post init: {self.name} is {self.age} years old")

p = Person("Alice", 30)
print(f"Name: {p.name}, Age: {p.age}")

A

Name: Alice, Age: 30
Post init: Alice is 30 years old

B

Name: Alice, Age: 0
Post init: Alice is 30 years old

C

Post init: Alice is 0 years old
Name: Alice, Age: 30

D

Post init: Alice is 30 years old
Name: Alice, Age: 30

💡 Hint

Remember that __post_init__ runs automatically after __init__ in dataclasses.

The __post_init__ method is called right after the dataclass __init__ finishes. It prints the name and age as initialized. Then the final print shows the same values.

❓ Predict Output

intermediate

Order of __new__ and __init__ methods

What will be printed when this code runs?

Python

class MyClass:
    def __new__(cls):
        print("Creating instance")
        return super().__new__(cls)

    def __init__(self):
        print("Initializing instance")

obj = MyClass()

A Creating instance

B

Creating instance
Initializing instance

C

Initializing instance
Creating instance

D Initializing instance

💡 Hint

Think about which method runs first when creating an object.

The __new__ method runs first to create the object, then __init__ initializes it. So the print from __new__ appears before __init__.

❓ Predict Output

advanced

Effect of returning a different object in __new__

What is the output of this code?

Python

class A:
    def __new__(cls):
        print("A.__new__ called")
        return super().__new__(cls)

    def __init__(self):
        print("A.__init__ called")

class B(A):
    def __new__(cls):
        print("B.__new__ called")
        return "Not an instance"

    def __init__(self):
        print("B.__init__ called")

obj = B()
print(type(obj))

A

B.__new__ called
<class 'str'>

B

B.__new__ called
B.__init__ called
<class '__main__.B'>

C

A.__new__ called
A.__init__ called
<class '__main__.B'>

D

B.__new__ called
A.__init__ called
<class '__main__.B'>

💡 Hint

What happens if __new__ returns something that is not an instance of the class?

If __new__ returns an object that is not an instance of the class, __init__ is not called. Here, B.__new__ returns a string, so __init__ is skipped and the object is a string.

🧠 Conceptual

advanced

Understanding attribute initialization order

Consider this class:

class C:
    def __init__(self):
        self.x = 10
        self.setup()

    def setup(self):
        self.x = 20

class D(C):
    def setup(self):
        self.x = 30

obj = D()
print(obj.x)

What is the output?

A 20

B 10

C 30

D AttributeError

💡 Hint

Which setup method runs when obj = D() is created?

The __init__ of C calls self.setup(). Since obj is an instance of D, D's setup runs, setting x to 30.

🔧 Debug

expert

Why does this object initialization fail?

What error does this code raise and why?

class E:
    def __init__(self, value):
        self.value = value

class F(E):
    def __init__(self):
        print("Initializing F")

obj = F(5)

A TypeError: F.__init__() takes 1 positional argument but 2 were given

B AttributeError: 'F' object has no attribute 'value'

C SyntaxError: missing colon in class definition

D No error, prints 'Initializing F'

💡 Hint

Check the parameters expected by F.__init__ versus what is passed.

The class F overrides __init__ but does not accept any arguments except self. When creating F(5), Python tries to pass 5 as an argument, causing a TypeError.