Does 'finally' always execute in Python?
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
try: 1/0 except ZeroDivisionError: return finally: print("Does this code run?")
Or maybe I re-raise an
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?
"Guaranteed" is a much stronger word than any implementation of
finally deserves. What is guaranteed is that if execution flows out of the whole
finally construct, it will pass through the
finally to do so. What is not guaranteed is that execution will flow out of the
finallyin 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
__aexit__, or if the object
yields in a
finally block. This list is not intended to be exhaustive.
finallyin a daemon thread might never execute if all non-daemon threads exit first.
os._exitwill halt the process immediately without executing
finallyblocks 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.
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,
multiprocessing interaction can be problematic (example).
- A C-level segmentation fault will prevent
finallyblocks from running.
kill -SIGKILLwill prevent
finallyblocks from running.
SIGHUPwill also prevent
finallyblocks from running unless you install a handler to control the shutdown yourself; by default, Python does not handle
- An exception in
finallycan prevent cleanup from completing. One particularly noteworthy case is if the user hits control-C just as we're starting to execute the
finallyblock. Python will raise a
KeyboardInterruptand skip every line of the
finallyblock's contents. (
KeyboardInterrupt-safe code is very hard to write).
- If the computer loses power, or if it hibernates and doesn't wake up,
finallyblocks won't run.
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.
★ Back to homepage or read more recommendations: