Re: test for absence of infinite loop
Robin Becker writes: > A user reported an infinite loop in reportlab. I determined a possible > cause and fix and would like to test for absence of the loop. Is there > any way to check for presence/absence of an infinite loop in python? I > imagine we could do something like call an external process and see if > it takes too long, but that seems a bit flaky. On some systems (among them Linux), you can limit (some) resources used by a process, among them the CPU time. If a process exceeds the limit, it is killed by the operating system. The "bash" makes this available with the "ulimit" command. On Linux, there is also the "timeout" command (with a similar effect). -- https://mail.python.org/mailman/listinfo/python-list
Re: test for absence of infinite loop
On 17Jul2018 12:39, Robin Becker wrote: On 17/07/2018 12:16, Cameron Simpson wrote: On 17Jul2018 10:10, Robin Becker wrote: A user reported an infinite loop in reportlab. I determined a possible cause and fix and would like to test for absence of the loop. Is there any way to check for presence/absence of an infinite loop in python? I imagine we could do something like call an external process and see if it takes too long, but that seems a bit flaky. While others have kindly pointed you at the halting problem (unsolvable in the general case) you can usually verify that the specific problem you fixed is fixed. Can you figure out how long the task should run with your fix in a test case? Not as time, but in loop iterations? Put a counter in the loop and check that its value doesn't exceed that. well I understand the problem about not halting. However as you point out in a fixed case I know that the test should take fractions of a second to complete. I certainly don't want to put instrumentation into the source code. It's relatively easy to imagine polling termination of a separate thread/process, but that's not particularly reliable. I don't know if there is a way to ask a python interpeter how many instructions it has carried out. Hmm. You can set a hard timeout with signal.alarm. Something like: import signal def test_timeout(maxtime): signal.alarm(maxtime) your_looping_function(...) signal.alarm(0) The OS (if UNIX) will send SIGALRM after that number of seconds, which should abort the program by raising an exception. So any test suite will fail the test, or a bare test run will terminate anyway without running forever. Cheers, Cameron Simpson -- https://mail.python.org/mailman/listinfo/python-list
Re: test for absence of infinite loop
On 7/17/2018 7:39 AM, Robin Becker wrote: well I understand the problem about not halting. However as you point out in a fixed case I know that the test should take fractions of a second to complete. If nothing else, you can easily add def test_xyz_completes(self): xyz(args) # Former infinite loop I assume that your test runner has some time limit at some level of granularity. You may be able to add a timeout for a particular test. -- Terry Jan Reedy -- https://mail.python.org/mailman/listinfo/python-list
Re: test for absence of infinite loop
On Tue, 17 Jul 2018 10:10:49 +0100, Robin Becker wrote: > A user reported an infinite loop in reportlab. I determined a possible > cause and fix and would like to test for absence of the loop. Is there > any way to check for presence/absence of an infinite loop in python? I > imagine we could do something like call an external process and see if > it takes too long, but that seems a bit flaky. google halting problem & you will see that this is an impossible request -- I hate quotations. -- Ralph Waldo Emerson -- https://mail.python.org/mailman/listinfo/python-list
Re: test for absence of infinite loop
On Tue, 17 Jul 2018 10:10:49 +0100, Robin Becker wrote: > A user reported an infinite loop in reportlab. I determined a possible > cause and fix and would like to test for absence of the loop. Is there > any way to check for presence/absence of an infinite loop in python? I > imagine we could do something like call an external process and see if > it takes too long, but that seems a bit flaky. In general, no, it is impossible to detect infinite loops. https://en.wikipedia.org/wiki/Halting_problem That's not to say that either human readers or the compiler can't detect *some* infinite loops ahead of time: # obviously an infinite loop while True: pass and then there's this: https://www.usenix.org/legacy/publications/library/proceedings/vhll/ full_papers/koenig.a but Python's compiler isn't capable of anything like that. The way I sometimes deal with that sort of thing is to re-write selected potentially-infinite loops: while condition: # condition may never become False do something to something like this: for counter in range(1000): if not condition: break do something else: raise TooManyIterationsError -- Steven D'Aprano "Ever since I learned about confirmation bias, I've been seeing it everywhere." -- Jon Ronson -- https://mail.python.org/mailman/listinfo/python-list
Re: test for absence of infinite loop
On 17/07/2018 12:16, Cameron Simpson wrote: On 17Jul2018 10:10, Robin Becker wrote: A user reported an infinite loop in reportlab. I determined a possible cause and fix and would like to test for absence of the loop. Is there any way to check for presence/absence of an infinite loop in python? I imagine we could do something like call an external process and see if it takes too long, but that seems a bit flaky. While others have kindly pointed you at the halting problem (unsolvable in the general case) you can usually verify that the specific problem you fixed is fixed. Can you figure out how long the task should run with your fix in a test case? Not as time, but in loop iterations? Put a counter in the loop and check that its value doesn't exceed that. well I understand the problem about not halting. However as you point out in a fixed case I know that the test should take fractions of a second to complete. I certainly don't want to put instrumentation into the source code. It's relatively easy to imagine polling termination of a separate thread/process, but that's not particularly reliable. I don't know if there is a way to ask a python interpeter how many instructions it has carried out. -- Robin Becker -- https://mail.python.org/mailman/listinfo/python-list
Re: test for absence of infinite loop
On 17Jul2018 10:10, Robin Becker wrote: A user reported an infinite loop in reportlab. I determined a possible cause and fix and would like to test for absence of the loop. Is there any way to check for presence/absence of an infinite loop in python? I imagine we could do something like call an external process and see if it takes too long, but that seems a bit flaky. While others have kindly pointed you at the halting problem (unsolvable in the general case) you can usually verify that the specific problem you fixed is fixed. Can you figure out how long the task should run with your fix in a test case? Not as time, but in loop iterations? Put a counter in the loop and check that its value doesn't exceed that. Cheers, Cameron Simpson -- https://mail.python.org/mailman/listinfo/python-list
Re: test for absence of infinite loop
On 17/07/2018 10:32, Chris Angelico wrote: .. All you gotta do is solve the halting problem... https://en.wikipedia.org/wiki/Halting_problem ChrisA ah so it's easy :) -- Robin Becker -- https://mail.python.org/mailman/listinfo/python-list
Re: test for absence of infinite loop
On Tue, Jul 17, 2018 at 7:10 PM, Robin Becker wrote: > A user reported an infinite loop in reportlab. I determined a possible cause > and fix and would like to test for absence of the loop. Is there any way to > check for presence/absence of an infinite loop in python? I imagine we could > do something like call an external process and see if it takes too long, but > that seems a bit flaky. All you gotta do is solve the halting problem... https://en.wikipedia.org/wiki/Halting_problem ChrisA -- https://mail.python.org/mailman/listinfo/python-list
test for absence of infinite loop
A user reported an infinite loop in reportlab. I determined a possible cause and fix and would like to test for absence of the loop. Is there any way to check for presence/absence of an infinite loop in python? I imagine we could do something like call an external process and see if it takes too long, but that seems a bit flaky. -- Robin Becker -- https://mail.python.org/mailman/listinfo/python-list
