Instance methods in Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an instance method in Python?

An instance method is a function defined inside a class that operates on an instance of that class. It can access and modify the object's attributes using the 'self' parameter.

beginner

Why do instance methods use 'self' as their first parameter?

'self' represents the specific object calling the method. It allows the method to access or change the object's own data and other methods.

beginner

How do you call an instance method on an object?

You call it using the dot notation: object.method_name(). Python automatically passes the object as 'self' to the method.

beginner

Example: What will this code print?

class Dog:
    def bark(self):
        print('Woof!')

my_dog = Dog()
my_dog.bark()

It will print: Woof! Because 'bark' is an instance method called on 'my_dog', it prints 'Woof!'.

intermediate

Can instance methods modify the object's attributes? How?

Yes, instance methods can change the object's attributes by using 'self.attribute_name = new_value'. This updates the data stored in that specific object.

What does the 'self' parameter in an instance method represent?

A The instance of the class calling the method

B The class itself

C A global variable

D A static method

'self' always refers to the specific object that calls the instance method.

How do you define an instance method inside a class?

A def method_name(cls):

B def method_name():

C def method_name(self):

D def method_name(static):

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

Which of these calls an instance method correctly?

A Class.method()

B obj.method()

C method(obj)

D method()

You call instance methods on an object using dot notation: obj.method()

Can instance methods access other methods in the same class?

A Yes, using self.other_method()

B No, methods are isolated

C Only if they are static methods

D Only if they are class methods

Instance methods can call other instance methods using self.other_method()

What happens if you forget to include 'self' in an instance method definition?

A The method is ignored

B The method works normally

C The method becomes a static method

D Python raises an error when calling the method

Python expects 'self' as the first parameter; missing it causes errors when calling the method.

Explain what an instance method is and why 'self' is important.

Think about how methods know which object they belong to.

Describe how to call an instance method and what happens behind the scenes.

Remember the dot notation and the role of 'self'.

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