<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def web_address(self):
 """ Return the url of the web server or None if not running """

        port = self._current_web_port()
        address = self.address or '127.0.0.1'
        if port is None:
            return None
        return 'http://{0}:{1}/'.format(
            address if address and not is_boot2docker() else docker_host(),
            port)