Private attributes in Python - Time & Space Complexity

Time Complexity: Private attributes

O(n)

Understanding Time Complexity

Let's see how using private attributes affects the speed of a program.

We want to know how the time to access or change private attributes grows as the program runs.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

class MyClass:
    def __init__(self, value):
        self.__private = value

    def get_private(self):
        return self.__private

obj = MyClass(10)
for i in range(1000):
    x = obj.get_private()

This code creates an object with a private attribute and accesses it 1000 times in a loop.

Identify Repeating Operations

• Primary operation: Calling the method to get the private attribute.
• How many times: 1000 times in the loop.

How Execution Grows With Input

Each time we increase the number of times we access the private attribute, the total work grows the same way.

Input Size (n)	Approx. Operations
10	10 method calls
100	100 method calls
1000	1000 method calls

Pattern observation: The work grows directly with the number of times we access the private attribute.

Final Time Complexity

Time Complexity: O(n)

This means if you access the private attribute twice as many times, the program takes about twice as long.

Common Mistake

[X] Wrong: "Accessing private attributes is slower because of the double underscore."

[OK] Correct: The double underscore only changes the attribute name internally; accessing it through a method is just a simple operation repeated many times.

Interview Connect

Understanding how attribute access scales helps you write clear and efficient code, which is a useful skill in many programming tasks.

Self-Check

"What if we replaced the method call with direct access to the attribute? How would the time complexity change?"

Practice

1. What is the main purpose of using private attributes (starting with __) in a Python class?

easy

A. To speed up the program execution

B. To hide the attribute from outside the class and protect it

C. To make the attribute accessible everywhere

D. To make the attribute a global variable

Solution

1. Step 1: Understand private attribute naming

   Private attributes start with double underscores to hide them from outside access.

2. Step 2: Purpose of hiding attributes

   This protects the data inside the object from accidental or unauthorized changes.

3. Final Answer:

   To hide the attribute from outside the class and protect it -> Option B

4. Quick Check:

   Private attributes = data protection [OK]

Hint: Private attributes start with __ to hide data inside class [OK]

Common Mistakes:

• Thinking private means accessible everywhere
• Confusing private with global variables
• Believing private speeds up code

2. Which of the following is the correct way to define a private attribute named age in a Python class?

easy

A. self.___age

B. self.age

C. self._age

D. self.__age

Solution

1. Step 1: Identify private attribute syntax

   Private attributes start with exactly two underscores, like __age.

2. Step 2: Check options

   self.__age uses self.__age, which is correct. self.age is public, self._age is protected (single underscore), self.___age has three underscores which is invalid.

3. Final Answer:

   self.__age -> Option D

4. Quick Check:

   Private attribute = double underscore [OK]

Hint: Private attribute = double underscore before name [OK]

Common Mistakes:

• Using single underscore instead of double
• Adding too many underscores
• Forgetting underscores

3. What will be the output of this code?

class Person:
    def __init__(self, name):
        self.__name = name
    def get_name(self):
        return self.__name

p = Person('Anna')
print(p.get_name())
print(p.__name)

medium

A. Anna\nAnna

B. AttributeError\nAnna

C. Anna\nAttributeError

D. AttributeError\nAttributeError

Solution

1. Step 1: Understand private attribute access

   The attribute __name is private and cannot be accessed directly outside the class.

2. Step 2: Check print statements

   Calling p.get_name() returns 'Anna' correctly. But p.__name causes AttributeError because it's private.

3. Final Answer:

   Anna\nAttributeError -> Option C

4. Quick Check:

   Private attribute direct access = AttributeError [OK]

Hint: Private attributes cause error if accessed directly outside class [OK]

Common Mistakes:

• Expecting direct access to private attribute
• Ignoring AttributeError on private access
• Confusing method call with attribute access

4. Find the error in this code and fix it:

class Car:
    def __init__(self, model):
        self.__model = model

c = Car('Tesla')
print(c.__model)

medium

A. Add a method inside class to return __model

B. No error, code runs fine

C. Remove underscores from __model

D. Change __model to _model to fix access

Solution

1. Step 1: Identify the error

   Accessing c.__model outside the class causes AttributeError because __model is private.

2. Step 2: Fix by adding a getter method

   Adding a method inside the class to return self.__model allows safe access.

3. Final Answer:

   Add a method inside class to return __model -> Option A

4. Quick Check:

   Private attribute access needs class method [OK]

Hint: Use class method to access private attributes outside class [OK]

Common Mistakes:

• Trying to access private attribute directly
• Removing underscores breaks privacy
• Ignoring need for getter method

5. You want to store a private attribute __balance in a BankAccount class and allow safe updating only through a method that adds money. Which code snippet correctly implements this?

hard

A. class BankAccount:
def __init__(self):
self.__balance = 0
def add_money(self, amount):
self.__balance += amount
def get_balance(self):
return self.__balance

B. class BankAccount:
def __init__(self):
self._balance = 0
def add_money(self, amount):
self._balance += amount
def get_balance(self):
return self._balance

C. class BankAccount:
def __init__(self):
self.balance = 0
def add_money(self, amount):
self.balance += amount
def get_balance(self):
return self.balance

D. class BankAccount:
def __init__(self):
self.__balance = 0
def add_money(self, amount):
self.balance += amount
def get_balance(self):
return self.__balance

Solution

1. Step 1: Check private attribute usage

   class BankAccount: def __init__(self): self.__balance = 0 def add_money(self, amount): self.__balance += amount def get_balance(self): return self.__balance uses self.__balance consistently and privately.

2. Step 2: Verify method updates and access

   The add_money method safely updates __balance, and get_balance returns it correctly.

3. Final Answer:

   The code using self.__balance consistently in all methods -> Option A

4. Quick Check:

   Private attribute updated only inside class methods [OK]

Hint: Update private attributes only via class methods [OK]

Common Mistakes:

• Updating private attribute outside class
• Mixing private and public attribute names
• Not providing method to access private data