Object initialization flow in Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

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

The __init__ method is called automatically when a new object is created. It initializes the object's attributes with values.

beginner

What happens if you don't define an __init__ method in your Python class?

Python uses a default constructor that does nothing, so the object is created but no attributes are initialized automatically.

beginner

How can you pass values to initialize an object in Python?

You pass values as arguments when creating the object, and the __init__ method receives them to set the object's attributes.

intermediate

What is the order of execution when creating an object in Python?

First, memory is allocated for the new object. Then, the __new__ method runs (usually default). Next, __init__ runs to initialize the object.

intermediate

Can the __init__ method return a value?

No, __init__ must return None. It only initializes the object and does not create or return it.

What is the role of the __init__ method in Python?

A Destroy the object when it is no longer needed

B Initialize the object's attributes when it is created

C Create a new class

D Return a value from a function

The __init__ method sets up the object's initial state by initializing its attributes.

What happens if you create an object without defining __init__?

A The object is created with no initialized attributes

B Python raises an error

C The object is not created

D The object is initialized with default values automatically

Without __init__, Python creates the object but does not set any attributes automatically.

Which method is called first when creating a new object?

A <code>__init__</code>

B <code>__call__</code>

C <code>__del__</code>

D <code>__new__</code>

__new__ creates the object in memory before __init__ initializes it.

Can __init__ return a value other than None?

A No, it must return None

B It depends on the Python version

C Yes, but only integers

D Yes, it can return any value

__init__ must always return None; returning anything else causes an error.

How do you pass values to initialize an object?

A By calling a separate method after creation

B By setting global variables

C By passing arguments to the class name when creating the object

D By modifying the class directly

Arguments passed when creating the object are received by __init__ to set attributes.

Explain the flow of object initialization in Python from creation to attribute setup.

Think about what happens first and what sets the object's data.

Describe how you can customize object creation using __init__ and what happens if it is missing.

Focus on how objects get their starting values.

Practice

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

easy

A. To delete an object when it is no longer needed

B. To define a class-level variable

C. To initialize a new object when it is created

D. To print the object details

Solution

1. Step 1: Understand the role of __init__

   The __init__ method runs automatically when a new object is created from a class.

2. Step 2: Identify what __init__ does

   It sets up the initial state of the object by assigning values to its attributes.

3. Final Answer:

   To initialize a new object when it is created -> Option C

4. Quick Check:

   __init__ initializes objects [OK]

Hint: Remember: __init__ sets up new objects automatically [OK]

Common Mistakes:

• Confusing __init__ with __del__
• Thinking __init__ is for printing
• Believing __init__ defines class variables

2. Which of the following is the correct syntax to define an __init__ method that takes a parameter name in a Python class?

easy

A. def __init__(self, name):

B. def __init__(name):

C. def init(self, name):

D. def __init__(self):

Solution

1. Step 1: Recall __init__ method signature

   The first parameter must be self to refer to the new object.

2. Step 2: Check parameter list

   To accept a name argument, it must be added after self.

3. Final Answer:

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

4. Quick Check:

   First param is self, then others [OK]

Hint: Always put self first in method parameters [OK]

Common Mistakes:

• Omitting self parameter
• Using init instead of __init__
• Missing parameters after self

3. What will be the output of this code?

class Car:
    def __init__(self, brand):
        self.brand = brand

my_car = Car('Toyota')
print(my_car.brand)

medium

A. Car

B. Toyota

C. brand

D. Error

Solution

1. Step 1: Understand object creation

   Creating my_car = Car('Toyota') calls __init__ with 'Toyota' as brand.

2. Step 2: Check attribute assignment and print

   The brand attribute of my_car is set to 'Toyota', so printing my_car.brand outputs 'Toyota'.

3. Final Answer:

   Toyota -> Option B

4. Quick Check:

   Attribute value prints 'Toyota' [OK]

Hint: Print attribute after init to see assigned value [OK]

Common Mistakes:

• Expecting class name instead of attribute value
• Confusing attribute name with value
• Thinking print causes error

4. What is wrong with this class definition?

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

p = Person(30)
print(p.age)

medium

A. The attribute age is not assigned to self

B. The __init__ method is missing self parameter

C. The print statement syntax is incorrect

D. The class name should be lowercase

Solution

1. Step 1: Check attribute assignment inside __init__

   The code assigns age = age, which only reassigns the local variable, not the object's attribute.

2. Step 2: Understand how to assign attributes

   To store the value in the object, it should be self.age = age.

3. Final Answer:

   The attribute age is not assigned to self -> Option A

4. Quick Check:

   Use self.attribute = value to save data [OK]

Hint: Always assign attributes with self.attribute = value [OK]

Common Mistakes:

• Forgetting self in attribute assignment
• Thinking local variable sets object attribute
• Ignoring error messages about missing attributes

5. Given this class:

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

b1 = Book('Python 101')
b2 = Book('Learn AI', 'Alice')

What are the values of b1.author and b2.author?

hard

A. Both b1.author and b2.author are 'Unknown'

B. b1.author is 'Python 101', b2.author is 'Learn AI'

C. b1.author is None, b2.author is 'Alice'

D. b1.author is 'Unknown', b2.author is 'Alice'

Solution

1. Step 1: Understand default parameter usage

   The author parameter has a default value 'Unknown', used if no argument is given.

2. Step 2: Check object creation

   b1 is created with only title, so author defaults to 'Unknown'. b2 provides 'Alice' explicitly.

3. Final Answer:

   b1.author is 'Unknown', b2.author is 'Alice' -> Option D

4. Quick Check:

   Default params fill missing arguments [OK]

Hint: Default values apply when argument is missing [OK]

Common Mistakes:

• Assuming missing argument becomes None
• Mixing title and author values
• Forgetting default parameter behavior