BCA • Chapter 8

Exception Handling

Chapter 8 teaches how to make software robust by intercepting and handling runtime errors using try, except, else, and finally blocks, as well as raising custom exceptions.

16 note blocks5 exam topics

🎯 Exam Focus Areas

Differentiate between SyntaxErrors and runtime Exceptions.Understand the exact execution flow of try -> except -> else -> finally.Identify common built-in exceptions like ValueError, TypeError, ZeroDivisionError, and IndexError.Write code to manually raise exceptions.Create a custom Exception class using inheritance.

No matter how well you write your code, external factors will inevitably cause it to fail. A user might enter a letter when you expected a number, a network connection might drop, or a file might go missing. When Python encounters an error during execution, it raises an Exception. If this exception is not handled, the program crashes instantly. Exception handling allows you to intercept these errors, execute fallback code, and keep your program running smoothly.

8.1 Syntax Errors vs. Exceptions

It is important to distinguish between Syntax Errors and Exceptions (Runtime Errors). Syntax Errors occur before the code even runs because the grammar of the Python language was violated (e.g., missing a colon). Exceptions occur while the code is running, when a syntactically correct statement fails (e.g., dividing by zero).

8.2 The try and except Blocks

The core of exception handling involves wrapping risky code inside a `try` block. If the code inside the `try` block executes successfully, the `except` block is skipped. If an error occurs, the rest of the `try` block is aborted, and Python jumps immediately to the `except` block.

try:
    # Risky code
    num = int(input("Enter a number: "))
    result = 100 / num
    print(f"Result is {result}")
except ZeroDivisionError:
    # Handles division by zero
    print("Error: You cannot divide by zero!")
except ValueError:
    # Handles invalid input (like typing 'abc' instead of a number)
    print("Error: Please enter a valid integer.")
except Exception as e:
    # Catch-all for any other unexpected errors
    # 'as e' captures the error object so we can print the exact reason
    print(f"An unexpected error occurred: {e}")

8.3 The else and finally Blocks

Python's exception handling provides two additional, optional blocks: `else` and `finally`.

1else: Executes ONLY if the try block completes successfully without any exceptions being raised. It is useful for code that should only run if the risky operation succeeded.
2finally: Executes ALWAYS, regardless of whether an exception occurred or not. It is primarily used for cleanup actions, such as closing files or releasing database connections.

try:
    file = open("data.txt", "r")
    data = file.read()
except FileNotFoundError:
    print("The file is missing.")
else:
    print("File read successfully! Length:", len(data))
finally:
    # This will run whether the file was found or not
    print("Execution of try-except block finished.")
    # Note: Checking if 'file' exists before closing is best practice here

8.4 Raising Exceptions Manually

As a developer, you might write a function that requires specific input to work properly. If the user provides bad data, you can actively trigger an error using the `raise` keyword. This forces the calling code to handle the problem.

def set_age(age):
    if age < 0:
        # Manually triggering a ValueError with a custom message
        raise ValueError("Age cannot be negative.")
    print(f"Age set to {age}")

try:
    set_age(-5)
except ValueError as e:
    print(f"Caught an error: {e}")

8.5 Custom User-Defined Exceptions

For large applications, the built-in exceptions (ValueError, TypeError) might not be descriptive enough for your specific business logic. You can create your own custom exception classes by inheriting from Python's base `Exception` class.

# Defining a custom exception
class InsufficientFundsError(Exception):
    pass

def withdraw(balance, amount):
    if amount > balance:
        raise InsufficientFundsError(f"Cannot withdraw {amount}. Balance is {balance}.")
    return balance - amount

try:
    new_balance = withdraw(100, 500)
except InsufficientFundsError as e:
    print(f"Transaction Denied: {e}")

📝 Quick Revision Points

1try: Risky code goes here.
2except SpecificError: Runs if SpecificError is raised.
3else: Runs only if NO exceptions occur.
4finally: Runs ALWAYS (cleanup).
5`raise ValueError('Msg')` throws an error manually.
6Custom errors inherit from `Exception`: `class MyError(Exception): pass`.

← PreviousFile Handling Next →Object-Oriented Programming (OOP)

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🎯 Exam Focus Areas

try: # Risky code num = int(input("Enter a number: ")) result = 100 / num print(f"Result is {result}") except ZeroDivisionError: # Handles division by zero print("Error: You cannot divide by zero!") except ValueError: # Handles invalid input (like typing 'abc' instead of a number) print("Error: Please enter a valid integer.") except Exception as e: # Catch-all for any other unexpected errors # 'as e' captures the error object so we can print the exact reason print(f"An unexpected error occurred: {e}")

try: file = open("data.txt", "r") data = file.read() except FileNotFoundError: print("The file is missing.") else: print("File read successfully! Length:", len(data)) finally: # This will run whether the file was found or not print("Execution of try-except block finished.") # Note: Checking if 'file' exists before closing is best practice here

def set_age(age): if age < 0: # Manually triggering a ValueError with a custom message raise ValueError("Age cannot be negative.") print(f"Age set to {age}") try: set_age(-5) except ValueError as e: print(f"Caught an error: {e}")

# Defining a custom exception class InsufficientFundsError(Exception): pass def withdraw(balance, amount): if amount > balance: raise InsufficientFundsError(f"Cannot withdraw {amount}. Balance is {balance}.") return balance - amount try: new_balance = withdraw(100, 500) except InsufficientFundsError as e: print(f"Transaction Denied: {e}")