On 22 August 2017 at 15:34, Nick Coghlan <ncogh...@gmail.com> wrote:
> On 21 August 2017 at 11:32, Neil Girdhar <mistersh...@gmail.com> wrote:
>> This question describes an example of the problem:
>> https://stackoverflow.com/questions/8416208/in-python-is-there-a-good-idiom-for-using-context-managers-in-setup-teardown.
>> You want to invoke a context manager in your setup/tearing-down, but the
>> easiest way to do that is to override run, which seems ugly.
>
> Using context managers when you can't use a with statement is one of
> the main use cases for contextlib.ExitStack():
>
> def setUp(self):
> self._resource_stack = stack = contextlib.ExitStack()
> self._resource = stack.enter_context(MyResource())
>
> def tearDown(self):
> self._resource_stack.close()
>
> I posted that as an additional answer to the question:
> https://stackoverflow.com/questions/8416208/in-python-is-there-a-good-idiom-for-using-context-managers-in-setup-teardown/45809502#45809502
Sjoerd pointed out off-list that this doesn't cover the case where
you're acquiring multiple resources and one of the later acquisitions
fails, so I added the ExitStack idiom that covers that case (using
stack.pop_all() as the last operation in a with statement):
def setUp(self):
with contextlib.ExitStack() as stack:
self._resource1 = stack.enter_context(GetResource())
self._resource2 = stack.enter_context(GetOtherResource())
# Failures before here -> immediate cleanup
self.addCleanup(stack.pop_all().close)
# Now cleanup won't happen until the cleanup functions run
I also remember that using addCleanup lets you avoid defining tearDown entirely.
Cheers,
Nick.
--
Nick Coghlan | ncogh...@gmail.com | Brisbane, Australia
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