'Default value for user.
Returns:
Value of users.get_current_user() if auto_current_user or
auto_current_user_add is set; else None. (But *not* the default
implementation, since we don\'t support the \'default\' keyword
argument.)'

def default_value(self):

    if (self.auto_current_user or self.auto_current_user_add):
        return users.get_current_user()
    return None
