def add(self, elem, replace=False):
        """Add a requirement.

        :param elem: a string or :class:`Requirement` instance.
        :param replace: if `True`, packages in the set with the same name will
            be removed first.
        """
        if isinstance(elem, text_type):
            elem = Requirement.coerce(elem)
        if replace and elem in self:
            self.remove(elem)
        super(Requirements, self).add(elem)
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...