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_fields(self):
        # test the offset and size attributes of Structure/Unoin fields.
        class X(Structure):
            _fields_ = [("x", c_int),
                        ("y", c_char)]

        self.assertEqual(X.x.offset, 0)
        self.assertEqual(X.x.size, sizeof(c_int))

        self.assertEqual(X.y.offset, sizeof(c_int))
        self.assertEqual(X.y.size, sizeof(c_char))

        # readonly
        self.assertRaises((TypeError, AttributeError), setattr, X.x, "offset", 92)
        self.assertRaises((TypeError, AttributeError), setattr, X.x, "size", 92)

        class X(Union):
            _fields_ = [("x", c_int),
                        ("y", c_char)]

        self.assertEqual(X.x.offset, 0)
        self.assertEqual(X.x.size, sizeof(c_int))

        self.assertEqual(X.y.offset, 0)
        self.assertEqual(X.y.size, sizeof(c_char))

        # readonly
        self.assertRaises((TypeError, AttributeError), setattr, X.x, "offset", 92)
        self.assertRaises((TypeError, AttributeError), setattr, X.x, "size", 92)

        # XXX Should we check nested data types also?
        # offset is always relative to the class...