Python Exceptions

1. What is an Exception

An exception is an error that occurs during program execution and disrupts normal program flow.

print(10 / 0)  # Raises ZeroDivisionError

Without handling, the program terminates immediately.

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2. Basic try...except Block

try:
    result = 10 / 0
except ZeroDivisionError:
    print("Cannot divide by zero")

The try block contains risky code; except handles the error gracefully.

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3. Handling Multiple Exceptions

try:
    value = int("abc")
except ValueError:
    print("Invalid conversion")
except ZeroDivisionError:
    print("Division error")

Different exception types can be handled separately.

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4. Using else with try...except

try:
    num = int("10")
except ValueError:
    print("Conversion failed")
else:
    print("Conversion successful:", num)

The else block executes only if no exception is raised.

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5. Using finally Block

try:
    file = open("data.txt", "r")
except FileNotFoundError:
    print("File not found")
finally:
    print("Execution completed")

finally always executes, whether an exception occurs or not.

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6. Catching All Exceptions (Generic Exception)

try:
    risky_operation()
except Exception as e:
    print("Error occurred:", e)

Catches any exception; useful for logging but should be used cautiously.

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7. Raising Custom Exceptions

def validate_age(age):
    if age < 0:
        raise ValueError("Age cannot be negative")

validate_age(-5)

The raise keyword triggers an exception manually.

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8. Creating Custom Exception Classes

class NegativeNumberError(Exception):
    pass

def check_number(n):
    if n < 0:
        raise NegativeNumberError("Negative numbers are not allowed")

check_number(-3)

Custom exceptions improve clarity in domain-specific logic.

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9. Using Assertions (assert)

age = 15
assert age >= 18, "User must be at least 18 years old"

Assertions are used for debugging and validation during development.

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10. Best Practice: Structured Exception Handling

def divide(a, b):
    try:
        return a / b
    except ZeroDivisionError:
        return "Division by zero error"
    except TypeError:
        return "Invalid input type"

print(divide(10, 0))

Encapsulating exception logic improves reliability and user experience.

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