Python (ME-506) - Unit 3 Short Notes

UNIT 3: Python Programming - Intermediate Concepts & Practical Application

3.1. Object-Oriented Programming (OOP) - The Paradigm Shift

3.1.1. Core Concepts & Motivation

• Class: A blueprint/template for creating objects. Defines attributes (data) and methods (functions).

• Object (Instance): A concrete realization of a class. Each object has its own state (attribute values).

• __init__() Method: The constructor. Called automatically when a new object is created (obj = ClassName()). Used to initialize instance attributes.

  
  class Student:
  
      def __init__(self, name, roll_no):  # Constructor
  
          self.name = name    # Instance attribute
  
          self.roll_no = roll_no
  
  

3.1.2. Defining Classes & Creating Objects

• Syntax: class ClassName(ParentClass): (ParentClass is optional for inheritance).

• self Parameter: The first parameter of any instance method. Refers to the specific object instance on which the method is called. Python passes it automatically.

  
  s1 = Student("Alice", 101)  # __init__(self, "Alice", 101) is called
  
  print(s1.name)  # Output: Alice
  
  

3.1.3. Key OOP Principles

[!TIP] Common Pitfall: Forgetting self as the first parameter in instance methods causes TypeError. __init__ must return None.

3.2. Advanced Functions & Scoping

3.2.1. Lambda Functions

• Definition: Small, anonymous, single-expression functions.

• Syntax: lambda arguments: expression

• Use Cases: Short functions for map(), filter(), sorted(key=), tkinter callbacks.

  
  square = lambda x: x**2
  
  nums = [1, 2, 3]
  
  squares = list(map(lambda x: x**2, nums))  # [1, 4, 9]
  
  sorted(students, key=lambda s: s.gpa)      # Sort by GPA
  
  

[!TIP] Rule: Lambda can only contain an expression, not statements (like if-else blocks, for loops). Use a regular def for complex logic.

3.2.2. Comprehensions & Generator Expressions

gen = (x**2 for x in range(5))  # Creates generator object

print(next(gen))  # 0

print(next(gen))  # 1 (computed only when needed)

3.2.3. Variable Scope & Namespaces (LEGB Rule)

Python resolves names by searching scopes in this order:

1. Local: Inside current function.

2. Enclosing: In enclosing (outer) function scopes.

3. Global: At the top level of the module/file.

4. Built-in: In Python's built-in names (e.g., len, print).

• global keyword: Modify a global variable from within a function.

  
  count = 0
  
  def inc():
  
      global count
  
      count += 1  # Now modifies global 'count'
  
  

• nonlocal keyword: Modify a variable from an enclosing (non-global) scope.

  
  def outer():
  
      x = 10
  
      def inner():
  
          nonlocal x
  
          x = 20  # Modifies 'x' in outer()
  
      inner()
  
      print(x)  # 20
  
  

3.3. Error Handling & Exceptions

3.3.1. Understanding Exceptions

• Syntax Errors: Code structure violations (caught by parser). Not exceptions.

• Exceptions: Runtime errors. Occur during execution.

  • Common Built-ins: ValueError, TypeError, IndexError, KeyError, FileNotFoundError, IOError, ZeroDivisionError, Exception (base for all user-defined).

3.3.2. Try-Except Blocks

try:

    # Code that might raise an exception

    result = 10 / 0

except ZeroDivisionError as e:

    # Handle specific exception

    print(f"Cannot divide by zero: {e}")

except (TypeError, ValueError):

    # Handle multiple exceptions

    print("Type or value error occurred.")

except Exception as e:

    # Catch-all (use cautiously)

    print(f"Unexpected error: {e}")

else:

    # Executes ONLY if try block had NO exception

    print("Operation successful.")

finally:

    # ALWAYS executes (cleanup: close files, release resources)

    print("Cleanup complete.")

[!TIP] Order Matters: Catch specific exceptions before general ones (Exception). finally runs even if an exception is caught or uncaught (but not on sys.exit() or keyboard interrupt).

3.3.3. Raising Exceptions

• raise: Signal an error condition.

  
  def set_age(age):
  
      if age < 0:
  
          raise ValueError("Age cannot be negative")
  
      self.age = age
  
  

• Re-raising: Use raise without arguments inside an except block to propagate the caught exception up the call stack.

3.4. File Input/Output (I/O) & Data Persistence

3.4.1. Working with Text Files

• Opening Files: open(filename, mode)

  • Modes: 'r' (read), 'w' (write, truncate), 'a' (append), 'r+' (read/write), 'b' (binary, e.g., 'rb').

• Context Manager (with statement): BEST PRACTICE. Automatically closes file, even on errors.

  
  with open('data.txt', 'r') as f:
  
      content = f.read()  # Read entire file
  
      # f is automatically closed here
  
  

• Reading Methods:

  • f.read(size): Read size bytes/entire file.

  • f.readline(): Read one line (including \n).

  • f.readlines(): Read all lines into a list.

• Writing Methods:

  • f.write(string): Write a string.

  • f.writelines(list_of_strings): Write multiple strings (no newlines added automatically).

3.4.2. Working with Structured Data

3.5. Modules, Packages, and the Standard Library

3.5.1. Modular Programming

• Module: A .py file containing Python definitions and statements.

• Importing:

  
  import module                 # Use: module.func()
  
  from module import func       # Use: func()
  
  from module import *          # Use: all public names (discouraged)
  
  

• if __name__ == "__main__": idiom: Code inside this block runs only when the file is executed directly (not when imported as a module). Used for test code or demo.

3.5.2. Packages

• Package: A directory containing:

  1. Module files (.py).

  2. A special __init__.py file (can be empty). This tells Python the directory is a package.

• Hierarchical Import: from package.subpackage import module

3.5.3. Key Standard Library Modules (Examples)

3.6. Introduction to Virtual Environments & Package Management

3.6.1. Why Virtual Environments?

Isolate project dependencies. Prevents version conflicts between projects (e.g., Project A needs numpy==1.20, Project B needs numpy==1.24).

3.6.2. Using venv

# Create a virtual environment named 'venv' in current directory

python -m venv venv

# Activate (Linux/macOS)

source venv/bin/activate

# Activate (Windows)

venv\Scripts\activate

# Deactivate (any OS)

deactivate

[!TIP] Naming Convention: Common to name the environment directory venv or .venv and add it to .gitignore.

3.6.3. Using pip

[!TIP] Critical: Always activate the correct virtual environment before running pip install. pip installs to the currently active environment.