FSD +009

Exception

• Exeptions are throwable objects.
• Checked Exceptions are checked by the compiler and should be handled (thrown or caught).
• Unchecked or runtime Exceptions are not flagged by the compiler. Their occurrence during execution interrupts the program.
• Exceptions can be thrown by developers. Throwing an exception delegates handling the exception to a different class or different level of the program.
• Thrown exception is left not handled, it will cause runtime error.

Error

• Java Errors usually indicate problems with the JVM or system resources.
  • it is not meant to be caught or handled by applications.
  • the base class is Error.
• Python is no separate Error class hierarchy. Errors are represented as exceptions.
  • ValueError, TypeError, MemoryError, SystemError, etc. are subclasses of Exception.

Java vs Pythone Exceiptions

Feature	Java	Python
Syntax	try-catch-finally	try-except-else-finally
try	attempts to execute a block of code	attempts to execute a block of code
catch/except	execute alternative code if exception arises	execute alternative code if exception arises
finally (optional)	executes code regardless of try/catch outcome	executes code regardless of try/except outcome
else (optional)	X	executes code if no exception arises

public class ExceptionExample {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        try {
            System.out.print("Enter first number: ");
            int a = scanner.nextInt();
            System.out.print("Enter second number: ");
            int b = scanner.nextInt();
            int result = a / b; // This will raise ArithmeticException
            System.out.println("Result: " + result);
        } catch (ArithmeticException e) {
            System.out.println("Caught an exception: " + e.getMessage());
        } catch (NumberFormatException e) {
            System.out.println("Caught a number format exception: " + e.getMessage());
        } finally {
            System.out.println("This block always executes.");
        }
    }
}

def div(a, b):
    return a / b

a = input("Enter first number: ")
b = input("Enter second number: ")

try:
    result = div(int(a), int(b))  # This will raise ZeroDivisionError
    print("Result:", result)
except ZeroDivisionError as e:
    print("Caught an exception:", e)
except ValueError as e:
    print("Caught a value error:", e)
else:
    print("No exception occurred.")
finally:
    print("This block always executes.")

Throwing Exceptions

class IncorrectAgeError(Exception):
    def __init__(self, age):
        self.age = age
        self.message = f"Age {age} is not valid. Age must be between 0 and 120."
        super().__init__(self.message)

number = int(input("Enter your age: "))

try:
    if number < 0 or number > 120:
        raise IncorrectAgeError(number)
    print(f"Your age is {number}.")
except IncorrectAgeError as e:
    print(e)

File Handlers

• "r": Opens a file for reading, requires the file to exist
• "w": Opens a file for writing. If the file exists, it is truncated to zero length. If the file does not exist, a new text file is created.
• "a": Opens file for open for writing. The file is created if it does not exist.
• "+": Opens a file in updating mode (reading and writing). It can be used with reading (r+) and writing (w+) modes.
• "t": Opens a file in text mode (default mode).
• "b": Opens a file in binary mode. It can be used with reading (rb) and writing (wb) modes.

File Handler Methods

os.path.exists("file.txt")  # Check if file exists
os.remove("file.txt")  # Delete a file
os.remove("dir")  # Delete an empty directory
os.chdir("path/dir")  # Change current working directory
os.getcwd()  # Get current working directory
os.mkdir("path/dir")  # Create a new directory

file_handler = open("file.txt", "r")  # Open a file in read mode
file_handler.read()  # Read the entire file content
file_handler.readline()  # Read a single line from the file
file_handler.close()  # Close the file

file_handler = open("file.txt", "w")  # Open a file in write mode
file_handler.write("Hello, World!")  # Write to the file
file_handler.write("\n")  # Write a newline character
file_handler.close()  # Close the file


import csv
with open("file.txt", "r") as file_handler:  # Using 'with' to handle file
    csv_object = csv.reader(file_handler)
    for row in csv_object:
        print(row)
    file_handler.close()

with open("file.txt", "a") as file_handler:  # Append mode
    csv_writer = csv.writer(file_handler)
    row = ["1", "gracefullight", "100"]
    csv_writer.writerow(row)
    file_handler.close()

import json
with open("data.json", "r") as file_handler:
    data = json.load(file_handler)  # Load JSON data from file
    print(data)
    file_handler.close()

with open("data.json", "w") as file_handler:
    json.dump(data, file_handler)  # Write JSON data to file
    file_handler.close()

• InputStream: reads byte-based input from various sources like files, memory, or network connections.
• OutputStream: writes byte-based output to various network connections.

Unit Testing

• tests and verifies the smallest unis of an application such as functions, procedures, modules, or objects.
• the conducted during the development phase of the SDLC.
• Developers aim to identify defects and code bugs, which can save time and reduce costs in later testing stages.
• software testing encompasses the fourmain testing phases, and unit testing is at the foundational level of the process.
  • Unit Testing
  • Integration Testing
  • System Testing
  • Acceptance Testing

Python unit testing

• test cases are defined using classes that inherit from unittest.TestCase
• assertions are used to validate results self.assertEquals()
• the unittest.TextTestRunner class can be used to run tests

import unittest

class TestMathOperations(unittest.TestCase):

    def test_addition(self):
        self.assertEqual(2 + 3, 5)

    def test_subtraction(self):
        self.assertEqual(5 - 2, 3)

    def test_multiplication(self):
        self.assertEqual(4 * 3, 12)

    def test_division(self):
        self.assertEqual(10 / 2, 5)