Self reference in Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does self mean in a Python class?

self is a way for an object to refer to itself inside its methods. It lets the object access its own attributes and other methods.

beginner

Why do we need to use self in method definitions?

Because Python does not automatically pass the object to methods. self is the first parameter that receives the object when a method is called.

beginner

How do you use self to change an object's attribute inside a method?

You write self.attribute_name = new_value inside the method to update the object's attribute.

intermediate

What happens if you forget to include self as the first parameter in a method?

Python will raise an error because it expects the method to receive the object as the first argument, but it won't get it.

intermediate

Can self be named something else?

Yes, but self is the strong convention in Python. Using other names can confuse readers.

In Python, what does self represent inside a class method?

A The instance of the class calling the method

B A global variable

C A class method decorator

D A local variable inside the method

self always refers to the current object instance inside class methods.

What will happen if you define a method without self as the first parameter and try to call it on an object?

A It will work normally

B Python will raise a TypeError

C The method will be treated as a static method

D The method will ignore the object

Python expects the first parameter to be the instance. Without it, calling the method raises a TypeError.

Which of these is the correct way to set an attribute name inside a method using self?

A self->name = 'Alice'

B name = 'Alice'

C self.name = 'Alice'

D set self.name = 'Alice'

Use self.name = value to set or change an attribute of the object.

Is it mandatory to name the first parameter self in Python class methods?

A Yes, it must be <code>self</code>

B Yes, or the code will not run

C No, you can omit it

D No, but it is strongly recommended

You can use any name, but self is the standard and makes code easier to read.

What is the role of self when calling another method inside the same class?

A You use <code>self.method_name()</code> to call it on the same object

B You call methods directly by name without <code>self</code>

C It is not needed to call other methods

D You must import the method first

Use self.method_name() to call another method on the same object.

Explain what self means in Python classes and why it is important.

Think about how an object talks about itself inside its own methods.

Describe how you would use self to change an attribute inside a class method.

Imagine you want to update a property of your own object.

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