Exception Handling

Python has a number of keywords associated with exception handling:

• try → block of code to test.
• except → block of code that runs if an error occurs.
• else → runs if no exception occurs.
• finally → always runs (cleanup code, e.g., close file).
• raise → manually raise an exception.

Catching Exceptions

Catch specific exceptions:

try:    x = 1 / 0except ZeroDivisionError:    print("Cannot divide by zero!")

Catch multiple:

try:    val = int("abc")except (ValueError, TypeError) as e:    print("Error:", e)

Catch all (not recommended unless logging/debugging):

try:    risky_code()except Exception as e:    print("Unexpected error:", e)

Raising Exceptions

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

Built-in Exceptions

Base Classes:

• BaseException → root of all exceptions.
• Exception → base for most user errors.
• ArithmeticError → base for numeric errors.
• LookupError → base for indexing/key errors.

Common Everyday Exceptions:

• Exception → generic base exception.
• ValueError → invalid value (e.g., int("abc")).
• TypeError → wrong type (e.g., 3 + "hi").
• NameError → variable not defined.
• IndexError → list index out of range.
• KeyError → dict key not found.
• ZeroDivisionError → divide by zero.
• AttributeError → attribute doesn’t exist.
• FileNotFoundError → missing file.
• IOError → I/O error (alias of OSError).
• ImportError → module can’t be imported.
• ModuleNotFoundError → specific import error.
• RuntimeError → generic runtime error.
• StopIteration → end of iterator.
• AssertionError → failed assert.

Arithmetic Errors:

• ArithmeticError (base)
• ZeroDivisionError
• OverflowError (number too large)
• FloatingPointError

Lookup Errors:

• LookupError (base)
• IndexError
• KeyError

OS & System Exceptions:

• OSError → base for system errors
• FileNotFoundError
• PermissionError
• TimeoutError
• EOFError → end of input reached.
• KeyboardInterrupt → Ctrl+C pressed.
• SystemExit → raised by sys.exit().

Other Useful Ones:

• MemoryError → out of memory.
• NotImplementedError → abstract method not implemented.
• IndentationError / TabError → indentation problems.
• UnicodeError (and subclasses: UnicodeEncodeError, UnicodeDecodeError, UnicodeTranslateError).

Demonstrate exception handling

def demo_exceptions():    # Example 1: Simple try/except    try:        result = 10 / 0    except ZeroDivisionError:        print("Error: division by zero")​    # Example 2: Catch multiple errors    try:        num = int("hello")  # ValueError    except (ValueError, TypeError) as e:        print("Conversion error:", e)​    # Example 3: try/except/else    try:        num = int("42")    except ValueError:        print("Invalid number")    else:        print("Converted successfully:", num)​    # Example 4: try/finally    try:        f = open("test.txt", "w")        f.write("Hello")    except IOError as e:        print("File error:", e)    finally:        f.close()        print("File closed safely")​    # Example 5: raise your own exception    try:        age = -5        if age < 0:            raise ValueError("Age cannot be negative")    except ValueError as e:        print("Custom exception caught:", e)​    # Example 6: Generic exception    try:        x = unknown_variable    except Exception as e:        print("Something went wrong:", e)​demo_exceptions()