Instance methods in Python - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of instance method call

What is the output of this Python code?

Python

class Car:
    def __init__(self, brand):
        self.brand = brand
    def show_brand(self):
        return f"This car is a {self.brand}."

my_car = Car("Toyota")
print(my_car.show_brand())

A Error: show_brand() missing 1 required positional argument

B This car is a Toyota.

C Toyota

D This car is a brand.

💡 Hint

Look at how the instance method uses self to access the brand attribute.

The method show_brand uses self.brand to get the brand of the car instance. Since my_car.brand is 'Toyota', the output is 'This car is a Toyota.'.

❓ Predict Output

intermediate

Instance method modifying attribute

What will be the value of x.speed after running this code?

Python

class Bike:
    def __init__(self):
        self.speed = 0
    def accelerate(self, amount):
        self.speed += amount

x = Bike()
x.accelerate(10)

A 10

B 0

C Error: accelerate() missing 1 required positional argument

D None

💡 Hint

The accelerate method adds the amount to the speed attribute.

The accelerate method increases the speed attribute by the given amount. Initially speed is 0, after calling accelerate(10), speed becomes 10.

❓ Predict Output

advanced

Instance method with default argument

What is the output of this code snippet?

Python

class Counter:
    def __init__(self):
        self.count = 0
    def add(self, n=1):
        self.count += n
        return self.count

c = Counter()
print(c.add())
print(c.add(5))

A 0\n5

B 1\n1

C 1\n6

D Error: add() missing 1 required positional argument

💡 Hint

The add method increases count by n, defaulting to 1 if no argument is given.

First call c.add() adds 1 to count (0+1=1). Second call c.add(5) adds 5 (1+5=6). So outputs are 1 and 6.

❓ Predict Output

advanced

Instance method calling another instance method

What will this code print?

Python

class Person:
    def __init__(self, name):
        self.name = name
    def greet(self):
        return f"Hello, {self.name}!"
    def welcome(self):
        return self.greet() + " Welcome to the platform."

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

A Alice Welcome to the platform.

B Hello, welcome to the platform.

C Error: greet() missing 1 required positional argument

D Hello, Alice! Welcome to the platform.

💡 Hint

The welcome method calls greet and adds more text.

The welcome method calls greet(), which returns 'Hello, Alice!'. Then it adds ' Welcome to the platform.' resulting in the full greeting.

🧠 Conceptual

expert

Understanding self in instance methods

Which statement about the use of self in instance methods is correct?

A self refers to the instance on which the method is called and is passed automatically.

B self is a keyword in Python that refers to the current class.

C self must be explicitly passed when calling an instance method.

D self is optional and can be omitted in instance method definitions.

💡 Hint

Think about how Python passes the instance to methods.

self is a conventional name for the first parameter of instance methods. It refers to the instance and is passed automatically by Python when calling the method.

Practice

1. What is the purpose of the self parameter in an instance method?

easy

A. It refers to the specific object calling the method.

B. It is used to define a static method.

C. It is a keyword to create a new object.

D. It stores the return value of the method.

Solution

1. Step 1: Understand what self represents

   self is a reference to the current object that calls the method, allowing access to its attributes and other methods.

2. Step 2: Differentiate from other options

   Static methods, object creation, and return values are unrelated concepts, which are not the role of self.

3. Final Answer:

   It refers to the specific object calling the method. -> Option A

4. Quick Check:

   self = current object [OK]

Hint: Remember: self means 'this object' inside methods [OK]

Common Mistakes:

• Thinking self is a keyword, not a parameter
• Confusing self with class or static methods
• Assuming self is optional in instance methods

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

easy

A. def method_name():

B. def method_name(*args):

C. def method_name(cls):

D. def method_name(self):

Solution

1. Step 1: Recall instance method syntax

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

2. Step 2: Check each option

   def method_name(): misses self, def method_name(cls): uses cls which is for class methods, and def method_name(*args): uses a generic parameter which is not standard for instance methods.

3. Final Answer:

   def method_name(self): -> Option D

4. Quick Check:

   Instance method = first param self [OK]

Hint: Instance methods always start with self parameter [OK]

Common Mistakes:

• Omitting self in method definition
• Using cls instead of self for instance methods
• Using no parameters or *args incorrectly

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. Woof!

B. Buddy says Woof!

C. my_dog says Woof!

D. Error: missing self parameter

Solution

1. Step 1: Understand object creation and method call

   The object my_dog is created with name 'Buddy'. Calling bark() uses self.name which is 'Buddy'.

2. Step 2: Evaluate the return value

   The method returns the string "Buddy says Woof!" which is printed.

3. Final Answer:

   Buddy says Woof! -> Option B

4. Quick Check:

   Method uses self.name = Buddy [OK]

Hint: Instance methods use self to access object data [OK]

Common Mistakes:

• Ignoring self and expecting just 'Woof!'
• Confusing variable name with object name
• Assuming method returns nothing

4. Find the error in this code:

class Car:
    def __init__(self, model):
        self.model = model
    def show_model():
        print(f"Model: {self.model}")

car = Car('Tesla')
car.show_model()

medium

A. Cannot create object without arguments

B. Wrong attribute name used

C. Missing self parameter in show_model method

D. print statement syntax error

Solution

1. Step 1: Check method definition

   The method show_model is missing the self parameter, so it cannot access instance attributes.

2. Step 2: Understand the error cause

   Calling car.show_model() passes the object automatically, but method lacks self to receive it, causing a TypeError.

3. Final Answer:

   Missing self parameter in show_model method -> Option C

4. Quick Check:

   Instance methods need self parameter [OK]

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

Common Mistakes:

• Forgetting self in method definition
• Trying to access self without parameter
• Confusing class and instance methods

5. You want to create a class Counter that counts how many times its method increment is called on each object separately. Which code correctly implements this behavior?

hard

A. class Counter:
def __init__(self):
self.count = 0
def increment(self):
self.count += 1
def get_count(self):
return self.count

B. class Counter:
count = 0
def increment(self):
Counter.count += 1
def get_count(self):
return Counter.count

C. class Counter:
def __init__(self):
self.count = 0
def increment():
self.count += 1
def get_count(self):
return self.count

D. class Counter:
def __init__(self):
self.count = 0
def increment(self):
count += 1
def get_count(self):
return self.count

Solution

1. Step 1: Understand instance vs class variables

   Instance variables (self.count) ensure each object tracks its own count separately. Methods must accept self and update self.count.

2. Step 2: Eliminate incorrect approaches

   Class variables are shared across all instances. Missing self parameter in methods causes TypeError. Updating a local variable doesn't affect the instance attribute.

3. Final Answer:

   class Counter: def __init__(self): self.count = 0 def increment(self): self.count += 1 def get_count(self): return self.count -> Option A

4. Quick Check:

   Instance variables + self = separate counts [OK]

Hint: Use self.variable for per-object data, not class variables [OK]

Common Mistakes:

• Using class variables for per-object data
• Forgetting self in method parameters
• Incrementing local variables instead of instance attributes