def load(self, requirements_file=None):
        """Load or reload requirements from a requirements.txt file.

        :param requirements_file: if not given, the filename used from
            initialization will be read again.
        """
        if requirements_file is None:
            requirements_file = self.filename
            if requirements_file is None:
                raise ValueError("no filename provided")
        elif isinstance(requirements_file, text_type):
            requirements_file = Path(requirements_file)
        self.clear()
        with requirements_file.open() as f:
            self.loads(f.read())

        if isinstance(requirements_file, (text_type, Path)):
            self.filename = requirements_file
def test_subclass_delayed(self):
        class X(Structure):
            pass
        self.assertEqual(sizeof(X), 0)
        X._fields_ = [("a", c_int)]

        class Y(X):
            pass
        self.assertEqual(sizeof(Y), sizeof(X))
        Y._fields_ = [("b", c_int)]

        class Z(X):
            pass

        self.assertEqual(sizeof(X), sizeof(c_int))
        self.assertEqual(sizeof(Y), sizeof(c_int)*2)
        self.assertEqual(sizeof(Z), sizeof(c_int))
        self.assertEqual(X._fields_, [("a", c_int)])
        self.assertEqual(Y._fields_, [("b", c_int)])
        self.assertEqual(Z._fields_, [("a", c_int)])