Instance methods in Python - Time & Space Complexity

Time Complexity: Instance methods

O(n)

Understanding Time Complexity

When we use instance methods in Python, we want to know how the time it takes to run changes as the input grows.

We ask: How does calling an instance method affect the program's speed when the input size changes?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

class Counter:
    def __init__(self, numbers):
        self.numbers = numbers

    def total(self):
        total = 0
        for num in self.numbers:
            total += num
        return total

c = Counter([1, 2, 3, 4, 5])
print(c.total())
    

This code defines a class with an instance method that adds up all numbers in a list.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Looping through the list of numbers inside the total method.
• How many times: Once for each number in the list.

How Execution Grows With Input

As the list gets longer, the method has to add more numbers, so it takes more time.

Input Size (n)	Approx. Operations
10	About 10 additions
100	About 100 additions
1000	About 1000 additions

Pattern observation: The time grows directly with the number of items; double the items, double the work.

Final Time Complexity

Time Complexity: O(n)

This means the time to run the method grows in a straight line with the size of the list.

Common Mistake

[X] Wrong: "Calling an instance method always takes the same time no matter the input size."

[OK] Correct: The method's work depends on the data it processes. If it loops over a list, more items mean more work and more time.

Interview Connect

Understanding how instance methods scale with input helps you explain your code's efficiency clearly and confidently in real projects and interviews.

Self-Check

"What if the total method used recursion instead of a loop? How would the time complexity change?"

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