Grantee the method of child class return a correct data type with py.test

Java or Python 3.5 is not an option for my organization for now. and I want to make sure the our code has 100% coverage. But I run into the following issues.

in a.py (By developer01)

class API1(object):

    def f(self):
        """return a list of int"""
        raise NotImplementedError()

in b.py (By developer02)

class API1Impl1(a.API1):

    def f(self):
        return 3

in test_b.py(By developer02)

def test_api1_impl1():
    api1impl = API1Impl1()
    assert api1impl.f() == 1

In the above case, if we use py.test cov function, we still can see the 100% coverage. but that's not what I want.

As a developer01, can I did some magic so that any others test their API1ImplX class, the following code will be executed.

ret = api1impl.f()
assert isinstance(ret) and all([isinstance(i, int) for i in ret])