Classes and objects in Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a class in Python?

A class is a blueprint or template for creating objects. It defines properties (attributes) and behaviors (methods) that the objects created from it will have.

beginner

What is an object in Python?

An object is an instance of a class. It is a specific example created using the class blueprint, holding actual data and able to perform actions defined by the class.

beginner

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

The __init__ method is a special function called a constructor. It runs automatically when a new object is created and is used to set up initial values for the object's attributes.

beginner

How do you create an object from a class named Car?

You create an object by calling the class name like a function. For example: <code>my_car = Car()</code> creates an object named my_car from the Car class.

intermediate

What is the difference between a class attribute and an instance attribute?

A class attribute is shared by all objects of the class. An instance attribute is unique to each object and usually set inside the __init__ method.

What keyword is used to define a class in Python?

A object

B class

C def

D func

The keyword class is used to define a new class in Python.

Which method is automatically called when creating a new object?

A __init__

B __str__

C __call__

D __new__

The __init__ method runs automatically to initialize a new object.

How do you access an object's attribute named 'color'?

A color.object

B object->color

C object[color]

D object.color

Use dot notation like object.color to access an attribute.

What is the output of this code? class Dog: def __init__(self, name): self.name = name my_dog = Dog('Buddy') print(my_dog.name)

A Buddy

B name

C Dog

D Error

The object my_dog has the attribute name set to 'Buddy', so printing it shows 'Buddy'.

Which of these is true about instance methods?

A They can only access class attributes.

B They cannot modify object data.

C They must have 'self' as the first parameter.

D They are called without creating an object.

Instance methods always have 'self' as the first parameter to access the object's data.

Explain in your own words what a class and an object are, and how they relate.

Think about how a cookie cutter (class) makes cookies (objects).

Describe the role of the __init__ method and why it is important when creating objects.

It's like setting up your new phone with your name and settings right after buying it.

Practice

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

easy

A. To execute code immediately

B. To store data in variables only

C. To create a blueprint for objects

D. To perform mathematical calculations

Solution

1. Step 1: Understand what a class represents

   A class is like a blueprint or template that defines how objects are created and what they can do.

2. Step 2: Identify the role of a class

   Classes organize code by grouping data and functions that belong together, allowing creation of many objects from the same blueprint.

3. Final Answer:

   To create a blueprint for objects -> Option C

4. Quick Check:

   Class = blueprint for objects [OK]

Hint: Classes define blueprints; objects are instances [OK]

Common Mistakes:

• Thinking classes run code immediately
• Confusing classes with simple variables
• Believing classes only store data

2. Which of the following is the correct way to define a class named Car in Python?

easy

A. class Car():

B. def Car():

C. function Car():

D. Car class:

Solution

1. Step 1: Recall Python class syntax

   In Python, classes are defined using the keyword class followed by the class name and parentheses.

2. Step 2: Check each option

   class Car(): uses class Car(): which is correct syntax. Others use wrong keywords or formats.

3. Final Answer:

   class Car(): -> Option A

4. Quick Check:

   Class definition starts with 'class' keyword [OK]

Hint: Use 'class ClassName():' to define a class [OK]

Common Mistakes:

• Using 'def' instead of 'class'
• Using 'function' keyword (not Python)
• Missing 'class' keyword

3. What will be the output of this code?

class Dog():
    def __init__(self, name):
        self.name = name
    def bark(self):
        return f"{self.name} says Woof!"

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

medium

A. Error: missing self parameter

B. Buddy says Woof!

C. Dog says Woof!

D. Woof!

Solution

1. Step 1: Understand the __init__ method

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

2. Step 2: Analyze the bark method call

   The bark method returns a string using the dog's name, so it returns "Buddy says Woof!".

3. Final Answer:

   Buddy says Woof! -> Option B

4. Quick Check:

   Method uses self.name = Buddy [OK]

Hint: Methods use self to access object data [OK]

Common Mistakes:

• Forgetting to pass 'self' in methods
• Confusing class name with object name
• Expecting method to print instead of return

4. Find the error in this class definition:

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

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

medium

A. Class name should be lowercase

B. Missing self in __init__ parameters

C. print(p.name) should be print(name)

D. Should assign to self.name, not name

Solution

1. Step 1: Check attribute assignment in __init__

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

2. Step 2: Correct assignment to object attribute

   It should be self.name = name to store the value in the object for later access.

3. Final Answer:

   Should assign to self.name, not name -> Option D

4. Quick Check:

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

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

Common Mistakes:

• Assigning to local variable instead of self.attribute
• Forgetting self in method parameters
• Trying to print undefined variables

5. You want to create a class BankAccount that stores an account holder's name and balance. It should have a method deposit(amount) that adds money to the balance only if the amount is positive. Which code correctly implements this?

hard

A. class BankAccount():
def __init__(self, name, balance=0):
self.name = name
self.balance = balance
def deposit(self, amount):
if amount > 0:
self.balance += amount

B. class BankAccount():
def __init__(self, name):
self.name = name
balance = 0
def deposit(self, amount):
self.balance = self.balance + amount

C. class BankAccount():
def __init__(self, name, balance=0):
self.name = name
self.balance = balance
def deposit(self, amount):
self.balance += amount

D. class BankAccount():
def __init__(self, name):
self.name = name
self.balance = 0
def deposit(self, amount):
if amount >= 0:
self.balance = amount

Solution

1. Step 1: Check __init__ method for attributes

   class BankAccount(): def __init__(self, name, balance=0): self.name = name self.balance = balance def deposit(self, amount): if amount > 0: self.balance += amount correctly sets self.name and self.balance with a default balance of 0.

2. Step 2: Verify deposit method logic

   class BankAccount(): def __init__(self, name, balance=0): self.name = name self.balance = balance def deposit(self, amount): if amount > 0: self.balance += amount adds amount to self.balance only if amount > 0, which matches the requirement.

3. Final Answer:

   Correctly implements the class with proper attribute initialization and deposit validation -> Option A

4. Quick Check:

   Check attribute setup and positive amount condition [OK]

Hint: Check attribute setup and validate input in methods [OK]

Common Mistakes:

• Not using self.balance to store balance
• Adding amount without checking if positive
• Overwriting balance instead of adding
• Missing default balance initialization