How does exception handling work in Python, and what is wrong with a bare except?

The full structure has four clauses:

try:
    result = risky()
except ValueError as e:
    log.warning("bad input: %s", e)
    raise
except (IOError, OSError):
    ...
else:
    # runs only if no exception was raised
    commit(result)
finally:
    # always runs, exception or not
    cleanup()

Why except: with no exception class is a bug:

• It catches KeyboardInterrupt and SystemExit, so Ctrl-C stops working and the process becomes hard to shut down.
• It catches genuine programming errors — a typo producing a NameError, a TypeError from a bad refactor — and hides them. The code appears to work while doing nothing.
• It gives no information about what you expected to fail.

If you truly need to catch everything, use except Exception:, which excludes the system-exiting ones, log the traceback, and re-raise unless you have a specific reason not to.

Note: Catch the narrowest exception that can actually occur, and keep the try block as short as possible so it cannot accidentally swallow a failure from an unrelated line. raise ... from e preserves the original cause when wrapping.