Lesson 18: Python Error Handling, Exceptions, and Debugging | Coding Class Series

Introduction

Welcome to Lesson 18!
In this lesson, we will learn about handling errors in Python, using exceptions, and debugging techniques.
Error handling ensures that your program does not crash unexpectedly and behaves predictably.

1. Common Errors in Python

• SyntaxError – Mistake in code structure
• NameError – Using a variable before defining it
• TypeError – Using incompatible data types
• ValueError – Wrong type of value for a function
• ZeroDivisionError – Dividing a number by zero

Example:

# SyntaxError example
# print("Hello World"

# NameError example
# print(x)

2. Handling Exceptions with try-except

try:
    num = int(input("Enter a number: "))
    result = 10 / num
    print("Result:", result)
except ZeroDivisionError:
    print("Error: Cannot divide by zero!")
except ValueError:
    print("Error: Invalid input! Enter a number.")

3. Using else and finally

try:
    num = int(input("Enter a number: "))
    result = 10 / num
except Exception as e:
    print("Error occurred:", e)
else:
    print("Division successful! Result is", result)
finally:
    print("This block always executes")

4. Raising Custom Exceptions

def check_age(age):
    if age < 18:
        raise ValueError("Age must be at least 18")
    else:
        print("Access granted")

try:
    check_age(15)
except ValueError as e:
    print("Error:", e)

5. Debugging Techniques

• Use print statements to check variable values
• Use Python debugger (pdb) for step-by-step execution:

import pdb

def add(a, b):
    pdb.set_trace()
    return a + b

result = add(5, "10")  # This will raise TypeError
print(result)

• Use IDE debugging tools (VS Code, PyCharm)

6. Practice Exercises

1. Write a program that divides two numbers and handles all possible exceptions.
2. Create a function to calculate square root, raise a custom exception if the number is negative.
3. Debug a program that adds a string to a number and fix it using try-except.
4. Use finally to close a file after reading its contents, even if an error occurs.

Lesson 17: Python File Handling and Working with CSV/JSON | Coding Class Series

Introduction

Welcome to Lesson 17!
In this lesson, we will learn how to read and write files in Python and work with CSV and JSON data formats.
File handling is essential to store, retrieve, and process data in Python programs.

1. Working with Text Files

Python provides the open() function to work with files.

Modes:

• 'r' – Read (default)
• 'w' – Write (creates or overwrites file)
• 'a' – Append (add to file)
• 'r+' – Read and Write

Example: Writing and reading a text file

# Write to a file
with open("example.txt", "w") as f:
    f.write("Hello, Python File Handling!\n")
    f.write("This is Lesson 17.\n")

# Read from the file
with open("example.txt", "r") as f:
    content = f.read()
    print(content)

2. Reading Files Line by Line

with open("example.txt", "r") as f:
    for line in f:
        print(line.strip())

3. Appending Data to a File

with open("example.txt", "a") as f:
    f.write("Appending a new line.\n")

4. Working with CSV Files

Python provides the csv module to work with CSV data.

Writing CSV:

import csv

data = [["Name", "Age", "City"],
        ["Alice", 25, "New York"],
        ["Bob", 30, "London"]]

with open("people.csv", "w", newline="") as file:
    writer = csv.writer(file)
    writer.writerows(data)

Reading CSV:

import csv

with open("people.csv", "r") as file:
    reader = csv.reader(file)
    for row in reader:
        print(row)

5. Working with JSON Files

Python provides the json module to work with JSON data.

Writing JSON:

import json

person = {
    "name": "Alice",
    "age": 25,
    "city": "New York"
}

with open("person.json", "w") as f:
    json.dump(person, f, indent=4)

Reading JSON:

import json

with open("person.json", "r") as f:
    data = json.load(f)
    print(data)

6. Practice Exercises

1. Create a text file notes.txt, write multiple lines, and read them line by line.
2. Create a CSV file for your favorite movies (Title, Director, Year) and read it back.
3. Create a JSON file containing 3 student records with name, age, and grade. Read and display them in Python.
4. Append new data to both CSV and JSON files.

Lesson 16: Python Modules, Packages, and Pip | Coding Class Series

Introduction

Welcome to Lesson 16!
In this lesson, we will learn about Python modules and packages, how to import them, and how to install external libraries using pip.
Modules and packages help you organize code and reuse functionality efficiently.

1. What is a Module?

A module is a file containing Python code (functions, classes, variables).
You can reuse modules in multiple programs.

Example: math module

import math

print(math.sqrt(16))  # Output: 4.0
print(math.pi)        # Output: 3.141592653589793

2. Importing Modules

You can import a module in different ways:

# Import the whole module
import math
print(math.factorial(5))  # Output: 120

# Import specific functions
from math import factorial, ceil
print(factorial(6))  # Output: 720
print(ceil(4.2))     # Output: 5

# Import with alias
import math as m
print(m.sqrt(25))    # Output: 5.0

3. Creating Your Own Module

You can create a module by saving functions in a .py file.

my_module.py

def greet(name):
    return f"Hello, {name}!"

def add(a, b):
    return a + b

main.py

import my_module

print(my_module.greet("Alice"))
print(my_module.add(10, 5))

4. What is a Package?

A package is a folder containing multiple modules and an __init__.py file.
It allows better organization of related modules.

Example structure:

my_package/
    __init__.py
    module1.py
    module2.py

Usage:

from my_package import module1
module1.function_name()

5. Installing External Libraries with pip

pip is the Python package manager. You can install external libraries easily.

# Install a library
pip install requests

# Check installed libraries
pip list

Example usage:

import requests

response = requests.get("https://api.github.com")
print(response.status_code)  # Output: 200

6. Upgrading and Uninstalling Packages

# Upgrade a package
pip install --upgrade requests

# Uninstall a package
pip uninstall requests

7. Practice Exercises

1. Create a module calculator.py with functions for addition, subtraction, multiplication, and division. Import it in another program.
2. Organize two modules (math_ops.py and string_ops.py) into a package my_utils. Import and use functions.
3. Install the numpy library using pip and create an array.
4. Upgrade numpy and then uninstall it using pip commands.

Lesson 15: Python Exception Handling and Debugging | Coding Class Series

Introduction

Welcome to Lesson 15!
In this lesson, we will learn how to handle errors in Python programs using exceptions and debug our code.
Exception handling is important to prevent program crashes and manage unexpected situations gracefully.

1. What is an Exception?

An exception is an error that occurs during the execution of a program.
Common examples:

• Division by zero
• File not found
• Index out of range

# Division by zero
a = 10
b = 0
print(a / b)  # Raises ZeroDivisionError

2. Try and Except

Use try and except to catch and handle exceptions.

try:
    a = 10
    b = 0
    print(a / b)
except ZeroDivisionError:
    print("Error: Cannot divide by zero!")

3. Handling Multiple Exceptions

You can handle different types of exceptions separately.

try:
    numbers = [1, 2, 3]
    print(numbers[5])
    a = 10 / 0
except IndexError:
    print("Index not found in the list!")
except ZeroDivisionError:
    print("Cannot divide by zero!")

4. Using else and finally

• else: runs if no exception occurs
• finally: always runs, even if an exception occurs

try:
    a = 10
    b = 2
    print(a / b)
except ZeroDivisionError:
    print("Error: Cannot divide by zero!")
else:
    print("Division successful!")
finally:
    print("This runs always!")

5. Raising Exceptions

You can manually raise exceptions using raise.

age = -5

if age < 0:
    raise ValueError("Age cannot be negative!")

6. Debugging Tips

• Print statements: Check the values of variables at different points.
• Python debugger (pdb): Step through code line by line.
• IDE debugging tools: PyCharm, VS Code allow breakpoints and watches.

import pdb

x = 5
y = 0

pdb.set_trace()  # Start debugging here
z = x / y
print(z)

7. Practice Exercises

1. Write a program that asks the user for two numbers and divides them. Handle division by zero errors.
2. Open a file that may not exist and handle FileNotFoundError.
3. Raise an exception if a user's input string length is less than 5.
4. Use pdb to debug a program that calculates the factorial of a number.

Lesson 14: Python File Handling and CSV/JSON | Coding Class Series

Introduction

Welcome to Lesson 14!
In this lesson, we will learn how to work with files in Python, including reading, writing, and updating text files, as well as working with CSV and JSON formats.
File handling is essential for storing data permanently and exchanging it between programs.

1. Opening and Closing Files

Use the open() function to open a file. Always close it after use.

# Open a file in write mode
file = open("example.txt", "w")
file.write("Hello, Python!")
file.close()

# Open the file in read mode
file = open("example.txt", "r")
print(file.read())  # Hello, Python!
file.close()

2. Using with Statement

with automatically closes the file after usage.

with open("example.txt", "w") as f:
    f.write("Using with statement!")

with open("example.txt", "r") as f:
    print(f.read())  # Using with statement!

3. Reading Files

• read(): reads the entire file
• readline(): reads one line at a time
• readlines(): reads all lines into a list

with open("example.txt", "r") as f:
    print(f.readline())
    print(f.readlines())

4. Writing and Appending Files

• "w" mode: write (overwrites existing content)
• "a" mode: append (adds new content)

with open("example.txt", "a") as f:
    f.write("\nAppending a new line!")

with open("example.txt", "r") as f:
    print(f.read())

5. Working with CSV Files

CSV (Comma Separated Values) files store tabular data.

import csv

# Writing to a CSV file
with open("students.csv", "w", newline="") as csvfile:
    writer = csv.writer(csvfile)
    writer.writerow(["Name", "Age", "Grade"])
    writer.writerow(["Alice", 14, "A"])
    writer.writerow(["Bob", 15, "B"])

# Reading from a CSV file
with open("students.csv", "r") as csvfile:
    reader = csv.reader(csvfile)
    for row in reader:
        print(row)

6. Working with JSON Files

JSON is widely used for data exchange between applications.

import json

# Writing JSON to a file
data = {
    "name": "Alice",
    "age": 14,
    "subjects": ["Math", "English", "Science"]
}

with open("data.json", "w") as jsonfile:
    json.dump(data, jsonfile)

# Reading JSON from a file
with open("data.json", "r") as jsonfile:
    loaded_data = json.load(jsonfile)
    print(loaded_data["name"])  # Alice

7. Practice Exercises

1. Create a text file notes.txt and write 5 of your favorite quotes.
2. Create a CSV file inventory.csv for a shop with columns: Item, Quantity, Price. Read it and calculate total value.
3. Create a JSON file with your friends' details (name, age, city) and load it back in Python.
4. Read a text file and count the number of words in it.

Lesson 13: Python Classes and Object-Oriented Programming (OOP) | Coding Class Series

Introduction

Welcome to Lesson 13!
In this lesson, we will learn about Object-Oriented Programming (OOP) in Python, including classes, objects, methods, and inheritance.
OOP helps you structure your code in a way that models real-world objects and makes programming more organized and reusable.

1. What is a Class?

A class is a blueprint for creating objects. It defines attributes (data) and methods (functions).

# Define a class
class Dog:
    def __init__(self, name, age):
        self.name = name
        self.age = age
    
    def bark(self):
        print(f"{self.name} says Woof!")

2. Creating Objects

An object is an instance of a class.

# Create objects
dog1 = Dog("Buddy", 3)
dog2 = Dog("Max", 5)

print(dog1.name)  # Buddy
dog2.bark()       # Max says Woof!

3. The __init__ Method

• __init__ is a constructor used to initialize objects.
• self refers to the instance of the class.

class Cat:
    def __init__(self, name, color):
        self.name = name
        self.color = color
    
    def meow(self):
        print(f"{self.name}, the {self.color} cat, says Meow!")

4. Methods

Methods are functions inside classes that operate on objects.

class Circle:
    def __init__(self, radius):
        self.radius = radius
    
    def area(self):
        return 3.1416 * self.radius ** 2
    
    def perimeter(self):
        return 2 * 3.1416 * self.radius

c = Circle(5)
print(c.area())       # 78.54
print(c.perimeter())  # 31.416

5. Inheritance

Inheritance allows a class to inherit attributes and methods from another class.

class Animal:
    def speak(self):
        print("Some sound")

class Dog(Animal):
    def speak(self):
        print("Woof Woof")

class Cat(Animal):
    def speak(self):
        print("Meow Meow")

d = Dog()
c = Cat()
d.speak()  # Woof Woof
c.speak()  # Meow Meow

6. Encapsulation

Encapsulation hides data inside a class using private attributes.

class BankAccount:
    def __init__(self, balance):
        self.__balance = balance  # private variable
    
    def deposit(self, amount):
        self.__balance += amount
    
    def get_balance(self):
        return self.__balance

account = BankAccount(1000)
account.deposit(500)
print(account.get_balance())  # 1500

7. Polymorphism

Polymorphism allows objects of different classes to be used interchangeably.

class Bird:
    def speak(self):
        print("Tweet")

class Dog:
    def speak(self):
        print("Woof")

def make_speak(animal):
    animal.speak()

b = Bird()
d = Dog()
make_speak(b)  # Tweet
make_speak(d)  # Woof

8. Practice Exercises

1. Create a class Student with attributes name, roll_no, marks. Write a method to calculate percentage.
2. Create a class Vehicle and inherit it with Car and Bike. Add different methods to each.
3. Implement encapsulation for a Library class where books is private.
4. Write a program demonstrating polymorphism with two different classes.

Lesson 12: Python Exception Handling and Debugging | Coding Class Series

Introduction

Welcome to Lesson 12!
In this lesson, we will learn how to handle errors in Python using exceptions and use debugging techniques to make our programs more robust and error-free. Exception handling ensures that your program does not crash unexpectedly.

1. What is an Exception?

An exception is an error that occurs during program execution.
Examples: ZeroDivisionError, FileNotFoundError, ValueError.

# Example: Division by zero
a = 10
b = 0
print(a / b)  # This will cause ZeroDivisionError

2. Using try and except

The try block contains code that might raise an exception.
The except block handles the exception gracefully.

try:
    a = 10
    b = 0
    result = a / b
except ZeroDivisionError:
    print("Error: Cannot divide by zero!")

3. Handling Multiple Exceptions

You can catch multiple types of exceptions:

try:
    num = int(input("Enter a number: "))
    result = 10 / num
except ZeroDivisionError:
    print("Error: Division by zero!")
except ValueError:
    print("Error: Invalid input, not a number!")

4. Using else and finally

• else → Executes if no exception occurs
• finally → Executes always, even if an exception occurs

try:
    file = open("example.txt", "r")
except FileNotFoundError:
    print("File not found!")
else:
    print(file.read())
finally:
    print("Execution finished.")

5. Raising Exceptions

You can raise your own exceptions using raise:

age = int(input("Enter your age: "))
if age < 18:
    raise ValueError("You must be 18 or older to proceed.")

6. Debugging Techniques

1. Print statements – Track variable values and flow.
2. Using IDE debugger – Step through code line by line.
3. Logging module – Log errors for later analysis.

import logging

logging.basicConfig(filename="app.log", level=logging.ERROR)

try:
    x = 10 / 0
except ZeroDivisionError as e:
    logging.error("An error occurred: %s", e)

7. Practice Exercises

1. Write a program that asks for two numbers and divides them. Handle ZeroDivisionError and ValueError.
2. Open a file safely using try-except-else-finally.
3. Raise a custom exception if a user enters a negative number.
4. Implement logging to record all exceptions in your program.