test for absence of infinite loop

Steven D'Aprano steve+comp.lang.python at pearwood.info
Tue Jul 17 08:30:41 EDT 2018


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



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