@classmethod
def get_api(cls, pl_user, pl_pass, pl_host, pl_ptn, ec, proxy=None):
    """ Get existing PLCAPI instance

        :param pl_user: Planelab user name (used for web login)
        :type pl_user: str
        :param pl_pass: Planetlab password (used for web login)
        :type pl_pass: str
        :param pl_host: Planetlab registry host (e.g. "www.planet-lab.eu")
        :type pl_host: str
        :param pl_ptn: XMLRPC service pattern (e.g. https://%(hostname)s:443/PLCAPI/)
        :type pl_ptn: str
        :param proxy: Proxy service url
        :type pl_ptn: str
        """
    if pl_user and pl_pass and pl_host:
        key = cls._make_key(pl_user, pl_host)
        with cls._lock:
            api = cls._apis.get(key)
            if not api:
                api = cls.create_api(pl_user, pl_pass, pl_host, pl_ptn, ec,
                    proxy)
            else:
                api.count += 1
            return api
    return None
