teardown of a singleton class in pytest

Question

I have a singleton class I'm using in the application (Singleton is implemented in metaclass)

class Singleton(type):
    def __init__(self, *args, **kwargs):
        self.__instance = None
        super(Singleton, self).__init__(*args, **kwargs)

    def __call__(self, *args, **kwargs):
        if self.__instance is None:
            self.__instance = super(Singleton, self).__call__(*args, **kwargs)
            return self.__instance
        else:
            return self.__instance


class ApplicationContext(object):
    __metaclass__ = Singleton

    def __init__(self, db):
        pass

Now I want this object to be in a py.test context - but I want it out of the way in some tests. I tried to create a teardown function but it didn't seem to work...

@pytest.fixture
def context(db, request):
    _context = ApplicationContext(db)

    def teardown():
        print 'teardown'
        del _context   #this is not working. What should be done here?

    request.addfinalizer(teardown)
    return _context

Answer 1

It is a bit hard to say what you mean with:

Now I want this object to be in a py.test context - but I want it out of the way in some tests.

Since we're talking about a singleton, I assume you want the fixture to be available, but not effective in all tests. In that case, Pytest fixtures can have a scope, quoting the relevent part form the docs:

Fixtures requiring network access depend on connectivity and are usually time-expensive to create. Extending the previous example, we can add a scope="module" parameter to the @pytest.fixture invocation to cause a smtp_connection fixture function, responsible to create a connection to a preexisting SMTP server, to only be invoked once per test module (the default is to invoke once per test function). Multiple test functions in a test module will thus each receive the same smtp_connection fixture instance, thus saving time. Possible values for scope are: function, class, module, package or session.

So in your case I'd assume the (default) scope of function should do what you want: the fixture is available in the module (scope), but you have to explicitly use it in each function (out of the way).

Maybe I got your question wrong, but an alternative approach could be to mock the Singleton in your ApplicationContext in your fixture.