🐍 Python OOP & Advanced Core

1. Classes & Objects

1.1 Basic Class Definition

class Person:
    def __init__(self, name, age):
        self.name = name
        self.age = age

    def greet(self):
        return f"Hello, I'm {self.name}"

p = Person("Vivek", 28)
print(p.greet())

🔹 Important: self

• self represents the instance

• Similar to this in C#

• Must be explicitly written in method definition

2. Instance vs Class Variables

2.1 Instance Variables

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

2.2 Class Variables

class Person:
    species = "Human"  # class variable

3. Dunder (Magic) Methods

3.1 str vs repr

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

    def __str__(self):
        return f"Person({self.name})"

• __str__ → readable

• __repr__ → developer-focused representation

3.2 eq (Equality)

def __eq__(self, other):
    return self.name == other.name

3.3 len

def __len__(self):
    return len(self.items)

4. Inheritance

class Animal:
    def speak(self):
        return "Sound"

class Dog(Animal):
    def speak(self):
        return "Bark"

4.1 super()

class Dog(Animal):
    def __init__(self, name):
        super().__init__()
        self.name = name

5. Multiple Inheritance

class A:
    pass

class B:
    pass

class C(A, B):
    pass

C.__mro__

Python uses C3 linearization for method resolution.

6. Dataclasses (Very Important)

from dataclasses import dataclass

@dataclass
class Person:
    name: str
    age: int

• init

• repr

• eq

7. Exception Handling

7.1 Try / Except

try:
    x = 1 / 0
except ZeroDivisionError:
    print("Cannot divide by zero")

7.2 Multiple Exceptions

except (ValueError, TypeError):
    pass

7.3 Finally

finally:
    print("Always executes")

7.4 Custom Exception

class MyError(Exception):
    pass

8. Modules & Packages

8.1 Importing

import math
from math import sqrt

8.2 name == "main"

if __name__ == "__main__":
    main()

9. Virtual Environments

python -m venv venv

source venv/bin/activate

venv\Scripts\activate

pip install requests

10. File Handling

10.1 Reading File

with open("file.txt", "r") as f:
    content = f.read()

10.2 Writing File

with open("file.txt", "w") as f:
    f.write("Hello")

11. Context Managers

with open("file.txt") as f:
    data = f.read()

11.1 Custom Context Manager

class MyContext:
    def __enter__(self):
        print("Enter")
        return self

    def __exit__(self, exc_type, exc_value, traceback):
        print("Exit")

12. Decorators (Beginner Level)

def logger(func):
    def wrapper():
        print("Before")
        func()
        print("After")
    return wrapper

@logger
def say_hello():
    print("Hello")

13. Closures

def outer():
    x = 10
    def inner():
        return x
    return inner

14. Memory Model & GIL (Interview Critical)

14.1 Reference Counting

14.2 Garbage Collection

14.3 Global Interpreter Lock (GIL)

• Only one thread executes Python bytecode at a time.

• Limits CPU-bound multithreading.

• Does NOT affect I/O-bound tasks.

Python threads are good for I/O-bound tasks but not ideal for CPU-bound tasks due to GIL.

🔥 Quick C# vs Python Mental Comparison