raise and Custom Exceptions

Why raise exceptions?

You can use raiseraise to:

validate function inputs
enforce rules
stop invalid workflows early

raise_example.py

def set_age(age: int) -> int:
    if age < 0:
        raise ValueError("age must be >= 0")
    return age
 
print(set_age(10))
# print(set_age(-1))

raise_example.py

def set_age(age: int) -> int:
    if age < 0:
        raise ValueError("age must be >= 0")
    return age
 
print(set_age(10))
# print(set_age(-1))

Raise with a custom message

raise_message.py

score = -5
if score < 0:
    raise ValueError(f"score must be non-negative, got {score}")

raise_message.py

score = -5
if score < 0:
    raise ValueError(f"score must be non-negative, got {score}")

Creating custom exceptions

Custom exceptions help you represent domain-specific errors.

custom_exception.py

class PaymentError(Exception):
    """Raised when a payment fails."""
 
 
def charge(amount: float) -> None:
    if amount <= 0:
        raise PaymentError("Amount must be positive")
 
 
try:
    charge(0)
except PaymentError as e:
    print("Payment failed:", e)

custom_exception.py

class PaymentError(Exception):
    """Raised when a payment fails."""
 
 
def charge(amount: float) -> None:
    if amount <= 0:
        raise PaymentError("Amount must be positive")
 
 
try:
    charge(0)
except PaymentError as e:
    print("Payment failed:", e)

Re-raising exceptions

Sometimes you want to log something and re-raise.

reraise.py

try:
    int("abc")
except ValueError as e:
    print("Logging error:", e)
    raise

reraise.py

try:
    int("abc")
except ValueError as e:
    print("Logging error:", e)
    raise

Best practices

Don’t leak secrets in error messages.
Use clear messages: what went wrong + what to do.
Prefer built-in exception types when they fit.

🧪 Try It Yourself

Exercise 1 – Raise a Built-in Exception

Exercise 2 – Define a Custom Exception

Exercise 3 – Re-raise an Exception

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