🌱 Junior Track 12 of 12

12. Error & Exception Handling

Write resilient Python code: `try`, `except`, `else`, `finally`, `Built-in Exceptions`, `raising exceptions`, `custom exceptions`, `exception chaining`, and avoiding overly broad `except` blocks.

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🎓 Junior Python Curriculum — Select Module

1️⃣ Fundamentals 2️⃣ Data Types 3️⃣ Operators 4️⃣ Conditionals 5️⃣ Loops 6️⃣ Collections 7️⃣ Comprehensions 8️⃣ Functions 9️⃣ *args & **kwargs 🔟 Scope & Namespaces 1️⃣1️⃣ OOP 1️⃣2️⃣ Exceptions 1️⃣3️⃣ File Handling 1️⃣4️⃣ Modules & Packages 1️⃣5️⃣ Virtual Environments 1️⃣6️⃣ Standard Library 1️⃣7️⃣ Iterators & Iterables 1️⃣8️⃣ Generators 1️⃣9️⃣ Decorators 2️⃣0️⃣ Context Managers 2️⃣1️⃣ Lambda Functions 2️⃣2️⃣ Functional Programming 2️⃣3️⃣ Type Hints 2️⃣4️⃣ Dataclasses 2️⃣5️⃣ Testing 2️⃣6️⃣ Debugging 2️⃣7️⃣ Logging 2️⃣8️⃣ JSON & Data 2️⃣9️⃣ Regular Expressions 3️⃣0️⃣ Databases

# ==============================================================================
# JUNIOR PYTHON TRACK 12 — ERROR & EXCEPTION HANDLING
# ==============================================================================

# 1. Custom Exception Class
class InvalidAgeError(ValueError):
    """Custom exception raised when user age is out of valid bounds."""
    pass

class DatabaseConnectionError(Exception):
    """Custom exception for database operation failures."""
    pass

# 2. Function with try - except - else - finally
def safe_divide(a: float, b: float) -> float | None:
    """Demonstrates complete exception handling lifecycle."""
    result = None
    try:
        result = a / b  # May raise ZeroDivisionError
    except ZeroDivisionError as err:
        print(f"[ERROR] Division failed: {err}")
    except TypeError as err:
        print(f"[ERROR] Invalid data type provided: {err}")
    else:
        print(f"[SUCCESS] Division completed. Result = {result}")
    finally:
        print("[CLEANUP] Execution of safe_divide completed.")
    return result

# 3. Raising & Exception Chaining
def register_user(name: str, age: int):
    if age < 18:
        raise InvalidAgeError(f"User '{name}' age {age} is below minimum required 18.")
    
    try:
        # Simulating database network failure
        raise ConnectionError("Socket timed out")
    except ConnectionError as err:
        # Exception Chaining using 'from'
        raise DatabaseConnectionError("Failed to persist user in DB") from err

if __name__ == "__main__":
    print("--- Test 1: Successful Division ---")
    safe_divide(10, 2)

    print("\n--- Test 2: Handling ZeroDivisionError ---")
    safe_divide(10, 0)

    print("\n--- Test 3: Custom Exception Handling ---")
    try:
        register_user("Dilshad", 16)
    except InvalidAgeError as e:
        print(f"[CAUGHT] {e}")

$ python3 junior_exceptions.py
--- Test 1: Successful Division ---
[SUCCESS] Division completed. Result = 5.0
[CLEANUP] Execution of safe_divide completed.

--- Test 2: Handling ZeroDivisionError ---
[ERROR] Division failed: division by zero
[CLEANUP] Execution of safe_divide completed.

--- Test 3: Custom Exception Handling ---
[CAUGHT] User 'Dilshad' age 16 is below minimum required 18.
Execution completed successfully.

import unittest

class TestJuniorExceptions(unittest.TestCase):
    def test_safe_divide_zero(self):
        res = safe_divide(10, 0)
        self.assertIsNone(res)

    def test_custom_exception_raised(self):
        with self.assertRaises(InvalidAgeError):
            register_user("Alex", 15)

if __name__ == "__main__":
    unittest.main()

🌱 Junior Track 12 Master Reference

12. Python Error & Exception Handling Reference

1️⃣ Complete Exception Control Structure

                    try:

                        val = int("42")

                    except ValueError as err:

                        print(f"Parsing error: {err}")

                    else:

                        print(f"Parsed integer successfully: {val}")

                    finally:

                        print("Completed parsing attempt.")

2️⃣ Built-in Exceptions Cheat Sheet

Exception	Trigger Cause	Code Example
ZeroDivisionError	Division or modulo by zero.	10 / 0
ValueError	Valid type but inappropriate value.	int("abc")
TypeError	Incompatible operation on data types.	"5" + 5
KeyError	Accessing missing dictionary key.	user_dict["missing"]
IndexError	Sequence index out of range bounds.	items[99]

3️⃣ Custom Exceptions & Chaining (`from`)

Create custom domain-specific exception classes by inheriting from Exception or specialized exceptions like ValueError. Use raise NewError(...) from original_err for clean traceback chaining.

                    class PaymentFailedError(Exception):

                        pass

                    try:

                        process_gateway_card()

                    except NetworkTimeout as err:

                        raise PaymentFailedError("Payment gateway failed") from err

4️⃣ Best Practices: Avoiding Overly Broad `except:`

Never use bare except:: Bare except catches KeyboardInterrupt and SystemExit, preventing user termination.
Catch specific exceptions: Always target exact exception types (`ZeroDivisionError`, `ValueError`).
Write informative error messages: Include contextual information (e.g., variable values, user IDs) in exception messages.

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