<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def username_is_available(self, new_username):
 """Check if ``new_username`` is still available. | Returns True if ``new_username`` does not exist or belongs to the current user. | Return False otherwise. """


        # Return True if new_username equals current user's username
        if self.user_manager.call_or_get(current_user.is_authenticated):
            if new_username == current_user.username:
                return True

        # Return True if new_username does not exist,
        # Return False otherwise.
        return self.find_user_by_username(new_username) == None