Chapter 9: Exception Handling

try/except/else/finally

Programs always encounter unexpected situations during runtime (file not found, network disconnected, user input errors...). Exception handling is like an "airbag" — it prevents the program from crashing completely due to a single error.

# Complete exception handling structure
def safe_divide(a, b):
    try:
        result = a / b
    except ZeroDivisionError:
        print("❌ Error: Cannot divide by zero!")
        return None
    except TypeError as e:
        print(f"❌ Type error: {e}")
        return None
    else:
        # Executes when no exception occurs
        print(f"✅ Calculation successful: {a} ÷ {b} = {result:.2f}")
        return result
    finally:
        # Executes no matter what
        print("  (Calculation process ended)")

print("=== Test 1: Normal ===")
safe_divide(10, 3)

print("\n=== Test 2: Division by zero ===")
safe_divide(10, 0)

print("\n=== Test 3: Type error ===")
safe_divide(10, "abc")

=== Test 1: Normal ===
✅ Calculation successful: 10 ÷ 3 = 3.33
  (Calculation process ended)

=== Test 2: Division by zero ===
❌ Error: Cannot divide by zero!
  (Calculation process ended)

=== Test 3: Type error ===
❌ Type error: unsupported operand type(s) for /: 'int' and 'str'
  (Calculation process ended)

Custom Exceptions

# Custom exception class
class AgeError(ValueError):
    def __init__(self, age, message="Age must be between 0-150"):
        self.age = age
        self.message = message
        super().__init__(self.message)

def set_age(age):
    if not isinstance(age, int):
        raise TypeError("Age must be an integer")
    if age < 0 or age > 150:
        raise AgeError(age)
    return f"Age set to: {age}"

# Test
try:
    print(set_age(25))
    print(set_age(-5))
except AgeError as e:
    print(f"❌ Age error: {e.age} - {e.message}")
except TypeError as e:
    print(f"❌ Type error: {e}")

Age set to: 25
❌ Age error: -5 - Age must be between 0-150

⚠️ Note: ⚠️ Best practice: Never use bare except: (without specifying an exception type) to catch all exceptions! This hides real bugs. Always specify the exact exception type.