In [1]: from pymongo import MongoClient

In [2]: MongoClient("mongodb:///tmp/mongodb-27017.sock").database_names()
---------------------------------------------------------------------------
ServerSelectionTimeoutError               Traceback (most recent call last)
<ipython-input-2-857110db2600> in <module>()
----> 1 MongoClient("mongodb:///tmp/mongodb-27017.sock").database_names()

/home/vagrant/.virtualenvs/pymongo/local/lib/python2.7/site-packages/pymongo/mongo_client.pyc in database_names(self)
    903                 self.admin.command(
    904                     "listDatabases",
--> 905                     read_preference=ReadPreference.PRIMARY)["databases"]]
    906
    907     def drop_database(self, name_or_database):

/home/vagrant/.virtualenvs/pymongo/local/lib/python2.7/site-packages/pymongo/database.pyc in command(self, command, value, check, allowable_errors, read_preference, codec_options, **kwargs)
    449         """
    450         client = self.__client
--> 451         with client._socket_for_reads(read_preference) as (sock_info, slave_ok):
    452             return self._command(sock_info, command, slave_ok, value,
    453                                  check, allowable_errors, read_preference,

/usr/lib/python2.7/contextlib.pyc in __enter__(self)
     15     def __enter__(self):
     16         try:
---> 17             return self.gen.next()
     18         except StopIteration:
     19             raise RuntimeError("generator didn't yield")

/home/vagrant/.virtualenvs/pymongo/local/lib/python2.7/site-packages/pymongo/mongo_client.pyc in _socket_for_reads(self, read_preference)
    697         topology = self._get_topology()
    698         single = topology.description.topology_type == TOPOLOGY_TYPE.Single
--> 699         with self._get_socket(read_preference) as sock_info:
    700             slave_ok = (single and not sock_info.is_mongos) or (
    701                 preference != ReadPreference.PRIMARY)

/usr/lib/python2.7/contextlib.pyc in __enter__(self)
     15     def __enter__(self):
     16         try:
---> 17             return self.gen.next()
     18         except StopIteration:
     19             raise RuntimeError("generator didn't yield")

/home/vagrant/.virtualenvs/pymongo/local/lib/python2.7/site-packages/pymongo/mongo_client.pyc in _get_socket(self, selector)
    661     @contextlib.contextmanager
    662     def _get_socket(self, selector):
--> 663         server = self._get_topology().select_server(selector)
    664         try:
    665             with server.get_socket(self.__all_credentials) as sock_info:

/home/vagrant/.virtualenvs/pymongo/local/lib/python2.7/site-packages/pymongo/topology.pyc in select_server(self, selector, server_selection_timeout, address)
    119         return random.choice(self.select_servers(selector,
    120                                                  server_selection_timeout,
--> 121                                                  address))
    122
    123     def select_server_by_address(self, address,

/home/vagrant/.virtualenvs/pymongo/local/lib/python2.7/site-packages/pymongo/topology.pyc in select_servers(self, selector, server_selection_timeout, address)
     95                 if server_timeout == 0 or now > end_time:
     96                     raise ServerSelectionTimeoutError(
---> 97                         self._error_message(selector))
     98
     99                 self._ensure_opened()

ServerSelectionTimeoutError: %d format: a number is required, not NoneType