Python – Does ‘finally’ always execute in Python

exception handlingfinallypythontry-catch-finally

For any possible try-finally block in Python, is it guaranteed that the finally block will always be executed?

For example, let’s say I return while in an except block:

try:
    1/0
except ZeroDivisionError:
    return
finally:
    print("Does this code run?")

Or maybe I re-raise an Exception:

try:
    1/0
except ZeroDivisionError:
    raise
finally:
    print("What about this code?")

Testing shows that finally does get executed for the above examples, but I imagine there are other scenarios I haven't thought of.

Are there any scenarios in which a finally block can fail to execute in Python?

Best Answer

"Guaranteed" is a much stronger word than any implementation of finally deserves. What is guaranteed is that if execution flows out of the whole try-finally construct, it will pass through the finally to do so. What is not guaranteed is that execution will flow out of the try-finally.

  • A finally in a generator or async coroutine might never run, if the object never executes to conclusion. There are a lot of ways that could happen; here's one:

    def gen(text):
    try:
        for line in text:
            try:
                yield int(line)
            except:
                # Ignore blank lines - but catch too much!
                pass
    finally:
        print('Doing important cleanup')

text = ['1', '', '2', '', '3']

if any(n > 1 for n in gen(text)):
    print('Found a number')

print('Oops, no cleanup.')

    Note that this example is a bit tricky: when the generator is garbage collected, Python attempts to run the finally block by throwing in a GeneratorExit exception, but here we catch that exception and then yield again, at which point Python prints a warning ("generator ignored GeneratorExit") and gives up. See PEP 342 (Coroutines via Enhanced Generators) for details.

    Other ways a generator or coroutine might not execute to conclusion include if the object is just never GC'ed (yes, that's possible, even in CPython), or if an async with awaits in __aexit__, or if the object awaits or yields in a finally block. This list is not intended to be exhaustive.

  • A finally in a daemon thread might never execute if all non-daemon threads exit first.

  • os._exit will halt the process immediately without executing finally blocks.

  • os.fork may cause finally blocks to execute twice. As well as just the normal problems you'd expect from things happening twice, this could cause concurrent access conflicts (crashes, stalls, ...) if access to shared resources is not correctly synchronized.

    Since multiprocessing uses fork-without-exec to create worker processes when using the fork start method (the default on Unix), and then calls os._exit in the worker once the worker's job is done, finally and multiprocessing interaction can be problematic (example).

  • A C-level segmentation fault will prevent finally blocks from running.
  • kill -SIGKILL will prevent finally blocks from running. SIGTERM and SIGHUP will also prevent finally blocks from running unless you install a handler to control the shutdown yourself; by default, Python does not handle SIGTERM or SIGHUP.
  • An exception in finally can prevent cleanup from completing. One particularly noteworthy case is if the user hits control-C just as we're starting to execute the finally block. Python will raise a KeyboardInterrupt and skip every line of the finally block's contents. (KeyboardInterrupt-safe code is very hard to write).
  • If the computer loses power, or if it hibernates and doesn't wake up, finally blocks won't run.

The finally block is not a transaction system; it doesn't provide atomicity guarantees or anything of the sort. Some of these examples might seem obvious, but it's easy to forget such things can happen and rely on finally for too much.

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