Exceptions Handling

• Concept: A mechanism to intercept runtime errors, prevent the program from crashing, and execute alternative logic or cleanup routines.
• Methods & Syntax:
• try: The block of code where you anticipate an error might occur.
• except ExceptionType: The block that executes only if that specific error occurs.
• else: Executes only if the try block succeeds without any exceptions.
• finally: Executes always, regardless of success or failure. Strictly used for resource cleanup (e.g., closing files, dropping database connections).

Program:

def safe_divide(a, b):
    try:
        result = a / b
    except ZeroDivisionError:
        print("Error: Cannot divide by zero.")
    else:
        # Executes only if division succeeded
        print(f"Success! Result is {result}")
    finally:
        # Executes no matter what happened above
        print("Execution of safe_divide completed.\\n")

safe_divide(10, 2)  # Triggers 'else' and 'finally'
safe_divide(10, 0)  # Triggers 'except' and 'finally'

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Catching & Inspecting Exceptions

• Concept: You can target specific errors, group multiple errors together, and extract the system’s exact error message using an alias.
• Methods & Syntax:
• except ValueError as exc: Catches a ValueError and stores the error object in the variable exc.
• except (Exception1, Exception2): Catches multiple specific exceptions in a single block using a tuple.
• except Exception as e: The “Catch-All”. Catches almost any programmatic error. (Best practice: Put this at the very bottom of your except blocks as a last resort).

Program:

def temp_convert(var):
    try:
        return int(var)
    except (TypeError, ValueError) as argument:
        # Extracts the exact string reason the conversion failed
        print(f"Conversion failed. System reason: {argument}")

temp_convert("xyz")

# Output:
# Conversion failed. System reason: invalid literal for int() with base 10: 'xyz'

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Raising & Chaining Exceptions

• Concept: You can manually halt your own program by throwing an error if a specific business logic condition is violated.
• Methods & Syntax:
• raise ExceptionName("Message"): Triggers an exception immediately.
• raise NewException from old_exception: (Crucial Addition) Re-raises a new, clearer exception while preserving the original system traceback so you don’t lose the root cause during debugging.

Program:

def process_level(level):
    if level < 1:
        # Manually triggering a standard exception
        raise ValueError(f"Invalid level! {level} is below minimum.")
    print(f"Processing level {level}")

try:
    # Simulating a database failure
    1 / 0
except ZeroDivisionError as original_error:
    # Exception Chaining: Translating a confusing system error into a clear business error
    raise SystemError("Database connection failed during processing") from original_error

# Output of the chained error will show:
# ZeroDivisionError: division by zero
# The above exception was the direct cause of the following exception:
# SystemError: Database connection failed during processing

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Custom Exceptions

• Concept: Standard Python exceptions (ValueError, TypeError) are often too generic for large applications. You create domain-specific errors by inheriting from Python’s base Exception class.
• Methods & Syntax:
• class CustomError(Exception): Create a class that inherits from Exception (or RuntimeError).
• Add an __init__ method to accept custom error messages or error codes.

Program:

# Defining the Custom Exception
class NetworkError(Exception):
    """Raised when a network operation fails."""
    def __init__(self, message, code):
        self.message = message
        self.code = code
        # Call the base Exception constructor
        super().__init__(self.message)

# Utilizing the Custom Exception
def connect_to_server(hostname):
    if hostname == "bad_host":
        raise NetworkError("Failed to resolve hostname", 404)

try:
    connect_to_server("bad_host")
except NetworkError as e:
    print(f"Network Alert! Code: {e.code}, Reason: {e.message}")

# Output:
# Network Alert! Code: 404, Reason: Failed to resolve hostname