Python Error Handling

Fundamentals Deep Dive

Python Error Handling

Failures happen. Learn how to anticipate them, react with friendly messages, and keep your program from crashing unexpectedly.

Write try/except blocksDesign custom exceptions

try/except anatomy

try:
    payload = json.loads(body)
except json.JSONDecodeError as exc:
    logger.error("Invalid JSON: %s", exc)

• Catch the most specific exception you can.
• Chain exceptions with raise ... from exc to preserve context.

finally + else

try:
    resource = acquire()
except ConnectionError:
    fallback()
else:
    process(resource)
finally:
    release(resource)

• else executes when no exception occurs.
• finally always executes, perfect for cleanup.

Custom exceptions

class PaymentError(Exception):
    """Base exception for payment failures."""

class CardDeclined(PaymentError):
    ...

Benefits:

• Communicate domain-specific issues.
• Catch broad base classes when needed.

Raising exceptions

if not user.active:
    raise PermissionError("User inactive")

Always raise exceptions with helpful messages. Avoid silent failures unless they're intentional.

Logging and observability

• Use logging.exception inside except blocks to capture stack traces.
• Surface fatal errors to monitoring systems (Sentry, Rollbar).

Next up in your learning path

Master try/except/finally patterns

Go deeper with the satellites in this cluster.

Deep dive

Combine with file handling

Protect your I/O operations from corruption.

Companion

Instrument your code

Send structured logs and metrics when exceptions occur.

Workflow

Frequently Asked Questions

Should I use bare `except`?

No. Bare `except` catches `SystemExit` and `KeyboardInterrupt`, making it harder to stop your program. Always specify the exception class.

What about `try/except/pass`?

Swallowing exceptions hides real issues. If you truly want to ignore an error, at least add a comment explaining why.

How do I handle asynchronous errors?

Wrap awaitable calls in try/except inside async functions or use event-loop specific hooks (e.g., `loop.set_exception_handler`).