Stefan Hajnoczi <stefa...@redhat.com> writes:
> On Fri, Aug 25, 2017 at 03:32:29PM +0800, Fam Zheng wrote:
>> On Thu, 08/24 19:04, Stefan Hajnoczi wrote:
>> > On Thu, Aug 24, 2017 at 04:38:43PM +0800, Fam Zheng wrote:
>> > > On Thu, 08/24 08:21, Stefan Hajnoczi wrote:
>> > > > Tests should declare resources upfront in a with statement. Resources
>> > > > are
>> > > > automatically cleaned up whether the test passes or fails:
>> > > >
>> > > > with FilePath('test.img') as img_path,
>> > > > VM() as vm:
>> > > > ...test...
>> > > > # img_path is deleted and vm is shut down automatically
>> > >
>> > > Looks good but still requires test writers to learn and remember to use
>> > > FilePath
>> > > and with.
>> >
>> > You cannot forget to use FilePath() unless you love typing at
>> > os.path.join(iotests.test_dir, 'test.img'). It's much better than open
>> > coding filename generation!
>> >
>> > > These are still boilerplates. Here goes my personal oppinion, so may
>> > > not be plausible:
>> > >
>> > > - For VM() maybe add an atexit in the launch() method also makes sure
>> > > the VM is
>> > > eventually terminated.
>> > >
>> > > This means vm.shutdown() is still needed in tearDown() if there are
>> > > multiple
>> > > test methods and each of them expects a clean state, but that is
>> > > probably
>> > > still less typing (and also indenting) than the with approach, and
>> > > also easy
>> > > to remember (otherwise a test will fail).
>> >
>> > I looked into atexit before going this route. atexit does not have an
>> > unregister() API in Python 2. This makes it ugly to use because some
>> > tests do not want the resource to remain for the duration of the
>> > process.
>> >
>> > A related point is that the Python objects used by atexit handlers live
>> > until the end of the process. They cannot be garbage collected because
>> > the atexit handler still has a reference to them.
>>
>> I think this shortcoming can be solved with a clean up list ("all problems in
>> computer science can be solved by another level of indirection"):
>>
>> _clean_up_list = set()
>> def _clean_up_handler():
>> for i in _clean_up_list:
>> try:
>> i()
>> except:
>> pass
>>
>> atexit.register(_clean_up_handler)
>>
>> class VM(...):
>>
>> def launch():
>> ...
>> _clean_up_list.add(self.launch)
>>
>> def shutdown():
>> _clean_up_list.remove(self.launch)
>> ...
>
> atexit is still less powerful than context managers because its scope is
> fixed. Handler functions are only called when the process terminates.
> Many test cases do not want resources (especially the VMs) around
> forever because they run several iterations or sub-tests.
>
> The with statement can be used both for process-lifetime and for more
> fine-grained scoping. That's why I chose it.
Moreover, context managers are Pythonic.
> If you stick to atexit then sub-tests or iterations require manual
> vm.shutdown() - something that is not necessary using the with
> statement.
>
> Stefan
