Python Try...Except

Python Try...Except: Handling Runtime Errors Safely

Python's try ... except blocks catch runtime exceptions so your program recovers gracefully instead of crashing with a traceback. Whenever code reads user input, opens files, or calls external APIs, exception handling keeps failures predictable.

Always catch the narrowest exception class you expect, such as ValueError or FileNotFoundError, rather than catching every error blindly.

Using try, except, else, and finally

The else block runs only if the try block succeeded without errors, and finally always runs for cleanup even when an exception occurred.

def parse_port(raw_value: str) -> int:
    try:
        port = int(raw_value)
    except ValueError as err:
        print(f"Invalid port '{raw_value}': {err}")
        return 8000
    else:
        return port if 1 <= port <= 65535 else 8000
    finally:
        print("Port check finished.")

print(parse_port("8080"))
print(parse_port("abc"))

Raising Your Own Exceptions

Use raise ValueError("...") to enforce preconditions early inside helper functions so callers receive clear feedback immediately.

Common Mistakes to Avoid

• Writing a bare except: clause: Bare except: swallows KeyboardInterrupt and SystemExit, making it impossible to Ctrl+C out of a hung script.
• Wrapping 50 lines inside one try block: Keep try focused on the exact line that can fail so you never hide unrelated bugs.
• Catching an error and ignoring it (except Exception: pass): Silent failures make debugging production logs nearly impossible.

Frequently Asked Questions

What is the difference between except Exception: and bare except:?

except Exception: catches standard program errors while allowing system exit signals like KeyboardInterrupt (Ctrl+C) to pass through normally.

When should I use finally vs a with statement?

Prefer with context managers for files, locks, and database connections. Use finally for custom cleanup steps that lack a context manager.

How do I catch multiple exception types in one handler?

Pass a tuple of exception classes: except (ValueError, TypeError) as err:.