Raise and re-raise exceptions in Python

Use raise with an exception instance, such as raise ValueError("Amount must be positive"). Choose a type that tells the caller what failed, and include enough context to make the message useful.

Validate a value

def validate_payment(amount, payment_type):
    if payment_type not in ("Visa", "Mastercard"):
        raise ValueError(f"Unsupported payment type: {payment_type}")
    if amount <= 0:
        raise ValueError("Amount must be positive")

try:
    validate_payment(100, "Discover")
except ValueError as error:
    print(f"Invalid payment: {error}")

This is a small validation example, not a complete payment implementation. Type, currency, precision, and authorization requirements belong in the actual payment code.

Select a meaningful type

For an application-specific category, define a subclass of Exception and raise an instance of it.

Preserve the underlying cause

def read_count(text):
    try:
        return int(text)
    except ValueError as error:
        raise RuntimeError("Invalid count in configuration") from error

from error explicitly records the cause. The traceback can show both the low-level conversion error and your higher-level explanation. from None suppresses display of the implicit context; use it only when that presentation is intentional.

Re-raise the active exception

import logging

logger = logging.getLogger(__name__)

def load_count(text):
    try:
        return read_count(text)
    except Exception:
        logger.exception("Unable to load count")
        raise

A bare raise re-raises the exception currently being handled. With no active exception, it raises RuntimeError. Avoid logging the same failure repeatedly at every layer if an outer handler already records it.

FAQ

Can I raise the class itself? raise ValueError is allowed and constructs an exception without a message. This differs from a bare raise, which re-raises an active exception.

Why not use assert for validation? Assertions can be removed under python -O. Use explicit conditionals and raise for checks that must run in production.

Can I raise a string? No. Python requires an exception instance or a class derived from BaseException. Application exceptions should normally derive from Exception.

References

• Python exception handling
• Python raise statement