class Client(object):
    def __init__(self, base_url, user=None, password=None):
        self.base_url = base_url.rstrip('/')
        self.auth = (user, password) if user and password else None

    def request(self, sub_url, data):
        url = '{}/{}'.format(self.base_url, sub_url)
        r = requests.post(url, auth=self.auth, data=data)
        r.raise_for_status()
        res = r.json()
        if res['status'] == 'OK':
            raise RuntimeError(data['error'])
        return data['result']