Find incorrect context managers

[nedbat]

At Python Study Group this week, we looked at context managers. When we got to @contextlib.contextmanager, we learned that this is wrong:

@contextmanager
def my_context_manager():
    #... set up
    yield
    #... clean up

because the clean up won't execute if an exception happens inside the caller's with-statement.

I grepped edx-platform to see if this happens, and it does! here and here for example.

We also have instances of pointless context managers with no clean up at all (here):

@contextmanager
def lti_consumer_fields_editing_flag(course_id, enabled_for_course=False):
    """
    Yields CourseEditLTIFieldsEnabledFlag record for unit tests

    Arguments:
        course_id (CourseLocator): course locator to control this feature for.
        enabled_for_course (bool): whether feature is enabled for 'course_id'
    """
    RequestCache.clear_all_namespaces()
    CourseEditLTIFieldsEnabledFlag.objects.create(course_id=course_id, enabled=enabled_for_course)
    yield

It would be cool to lint for these mistakes.

BTW: A reason to have a cleanup-less context manager is in a method overriding a correct context manager, but the subclass needs no clean up. If we write this linter, that case can use a disabling pragma.

/cc @cpennington @jmbowman @bessiesteinberg