Python Custom Exceptions

1) Define & Raise Basic Custom Exception

Define a custom exception class MyError and raise it unconditionally.

---

2) Raise on Negative Input

Ask user for a number; if it’s negative, raise NegativeError(Exception) with message "Negative not allowed".

---

3) Handle Custom Exception

Wrap the input logic from #2 in try/except and catch your NegativeError, printing its message.

---

4) Custom Exception Class With Message

Define a custom exception AgeError that prints "Age must be between 0 and 120" when raised.

---

5) Validate Age Input

Ask user for age. If age < 0 or > 120, raise and catch AgeError.

---

6) Custom Exception in Function

Write:

def safe_divide(a, b)

Raise DivideByZeroError if b == 0; otherwise return result.

---

7) Custom Exception for Even Input

Ask user for a number; if it’s even, raise EvenNotAllowedError, else print odd number.

---

8) Custom Exception in Loop

Loop asking for 3 integers; if any is negative, raise and catch NegativeError but continue the loop.

---

9) Raise on Empty String

Ask for a string; if it’s empty (""), raise EmptyStringError.

---

10) File Name Check

Ask for a filename; if it doesn’t end in .txt, raise InvalidFileError.

---

11) Custom Exception Hierarchy

Define a base class InputError(Exception), then subclass it with TooSmallError and TooLargeError.

Use them for input validation.

---

12) Dictionary Key Check

Ask for a key in {"a":1}; if key not found, raise KeyMissingError.

---

13) Value Range Check

Ask for a number 1–10; if outside range, raise OutOfRangeError.

---

14) Raise in Class Method

Define a class Person with method:

def set_age(self, age)

Raise AgeError if invalid.

---

15) Raise & Pass Message

Raise a custom exception with a dynamic message (include the bad value).

Example:

ValueError: 99 is too big!

---

16) Catch All Custom Errors

Group multiple custom exceptions under one except using a tuple.

---

17) Safe List Access

Ask for an index; if out of bounds, raise and catch IndexTooLargeError.

---

18) Validate Non-Numeric Input

Ask for a number; if input contains letters, raise NotNumericError.

---

19) Custom Exception with Attributes

Define a custom exception with an extra attribute code. Raise it and print both message and code.

---

20) Re-raising Exception

In a function, catch a built-in exception (e.g., ValueError) and re-raise it as your custom exception.

---

🧩 Concepts Practiced

Focus	What You Learn
Defining custom exceptions	class MyError(Exception)
Raising exceptions	raise MyError("msg")
Catching custom errors	except MyError:
Dynamic messages	include values in exception text
Exception inheritance	base and subclass exceptions
Re-raising exceptions	translate built-ins to custom

---

📌 Starter Template

Here’s a minimal custom exception you can reuse:

class MyError(Exception):
    pass

value = int(input("Enter number: "))
if value < 0:
    raise MyError("Negative value not allowed")

Handling it:

try:
    # code that might raise
except MyError as e:
    print("Error:", e)

---

canvil: 333f6c7d-6876-4f7e-b6ad-1bdb2233f5c1