Self reference in Python - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to make the method return the instance itself.

Python

class Box:
    def get_self(self):
        return [1]

Drag options to blanks, or click blank then click option'

Aself

BBox

Cthis

Dinstance

Attempts:

3 left

💡 Hint

Common Mistakes

Returning the class name instead of the instance.

Using 'this' which is not a Python keyword.

The keyword self refers to the current instance of the class. Returning self returns the object itself.

2fill in blank

medium

Complete the code to set an attribute on the instance using self reference.

Python

class Person:
    def __init__(self, name):
        [1].name = name

Drag options to blanks, or click blank then click option'

APerson

Bthis

Ccls

Dself

Attempts:

3 left

💡 Hint

Common Mistakes

Using the class name instead of the instance.

Using 'cls' which is for class methods.

Inside instance methods, self is used to access or set attributes on the current object.

3fill in blank

hard

Fix the error in the method to return the instance's attribute using self reference.

Python

class Car:
    def __init__(self, model):
        self.model = model
    def get_model(self):
        return [1].model

Drag options to blanks, or click blank then click option'

Aself

BCar

Ccls

Dmodel

Attempts:

3 left

💡 Hint

Common Mistakes

Using the class name instead of the instance.

Trying to access attribute directly without self.

To access instance attributes inside methods, use self.

4fill in blank

hard

Fill both blanks to create a method that returns a new instance referencing the current instance's attribute.

Python

class Node:
    def __init__(self, value):
        self.value = value
    def copy(self):
        return [1](self.[2])

Drag options to blanks, or click blank then click option'

ANode

Bvalue

Cself

Dcopy

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'copy' as a class name.

Using 'value' instead of 'self' to access attribute.

The method creates a new Node instance using the current instance's value accessed via self.

5fill in blank

hard

Fill all three blanks to define a method that updates the instance's attribute and returns the instance itself.

Python

class Counter:
    def __init__(self, count=0):
        self.count = count
    def increment(self, amount):
        [1].count += [2]
        return [3]

Drag options to blanks, or click blank then click option'

Aself

Bamount

Dcount

Attempts:

3 left

💡 Hint

Common Mistakes

Returning amount instead of the instance.

Using the attribute name instead of self.

The method uses self to update the count attribute by amount and returns self to allow chaining.

Practice

1. What does self represent inside a Python class method?

easy

A. A class method decorator

B. The current instance of the class

C. A global variable

D. A built-in Python keyword

Solution

1. Step 1: Understand the role of self in classes

   self is used to refer to the current object instance inside class methods.

2. Step 2: Differentiate from other options

   It is not a global variable, decorator, or keyword but a conventional name for the instance parameter.

3. Final Answer:

   The current instance of the class -> Option B

4. Quick Check:

   self = current object [OK]

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

Common Mistakes:

• Thinking self is a keyword
• Confusing self with class itself
• Assuming self is optional

2. Which of the following is the correct way to define a method using self in a Python class?

easy

A. def method(self):

B. def method(this):

C. def method(cls):

D. def method():

Solution

1. Step 1: Recall method definition syntax in Python classes

   Instance methods must include self as the first parameter to access instance data.

2. Step 2: Check each option

   Only def method(self): correctly includes self as the first parameter.

3. Final Answer:

   def method(self): -> Option A

4. Quick Check:

   Method needs self parameter [OK]

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

Common Mistakes:

• Omitting self parameter
• Using wrong parameter name like cls or this
• Confusing class and instance methods

3. What is the output of this code?

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

medium

A. 1 2

B. 0 1

C. 1 1

D. 2 3

Solution

1. Step 1: Understand the initial state and method behavior

   When Counter is created, count is 0. Each increment adds 1 and returns the new value.

2. Step 2: Trace the two calls to increment()

   First call: count goes 0 -> 1, returns 1. Second call: count goes 1 -> 2, returns 2.

3. Final Answer:

   1 2 -> Option A

4. Quick Check:

   Increment adds 1 each call [OK]

Hint: Track self.count changes step-by-step [OK]

Common Mistakes:

• Assuming count resets each call
• Confusing return values
• Ignoring self reference updates

4. Find the error in this class definition:

class Person:
    def __init__(name):
        self.name = name
p = Person('Alice')
print(p.name)

medium

A. Using self before assignment

B. Incorrect print statement

C. Missing self parameter in __init__

D. Class name should be lowercase

Solution

1. Step 1: Check method parameters

   The __init__ method must have self as the first parameter to refer to the instance.

2. Step 2: Identify the error

   Here, __init__ only has name, so self is missing, causing a runtime error.

3. Final Answer:

   Missing self parameter in __init__ -> Option C

4. Quick Check:

   __init__ needs self first [OK]

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

Common Mistakes:

• Forgetting self in __init__
• Trying to use self without defining it
• Assuming self is automatic

5. You want to create a class Node for a linked list where each node refers to itself and the next node. Which is the correct way to set the next node using self reference?

class Node:
    def __init__(self, value):
        self.value = value
        self.next = None
    def set_next(self, next_node):
        ???

Choose the correct line to replace ???.

hard

A. next_node = self.next

B. next = self.next_node

C. self.next_node = next_node

D. self.next = next_node

Solution

1. Step 1: Understand attribute assignment with self

   To update the current object's next attribute, use self.next.

2. Step 2: Match the correct assignment

   Assigning self.next = next_node correctly sets the next node reference.

3. Final Answer:

   self.next = next_node -> Option D

4. Quick Check:

   Use self.attribute = value to update instance data [OK]

Hint: Use self.attribute to refer to current object's data [OK]

Common Mistakes:

• Assigning to local variable instead of self attribute
• Mixing attribute names
• Forgetting self in assignment