PYTHON-525 Add method Cluster.select_server().

In addition to plural select_servers().

pymongo/cluster.py

@@ -14,6 +14,7 @@
 
 """Internal classes to monitor clusters of one or more servers."""
 
+import random
 import threading
 import time
 
@@ -83,6 +84,7 @@ class Cluster(object):
                     # ClusterDescription's stringification in exception msg.
                     raise ConnectionFailure("No suitable servers available")
 
+                self._ensure_opened()
                 self._request_check_all()
 
                 # Release the lock and wait for the cluster description to
@@ -96,6 +98,10 @@ class Cluster(object):
             return [self.get_server_by_address(sd.address)
                     for sd in server_descriptions]
 
+    def select_server(self, selector, server_wait_time=None):
+        """Like select_servers, but choose a random server if several match."""
+        return random.choice(self.select_servers(selector, server_wait_time))
+
     def on_change(self, server_description):
         """Process a new ServerDescription after an ismaster call completes."""
         # We do no I/O holding the lock.
@@ -140,6 +146,12 @@ class Cluster(object):
     def description(self):
         return self._description
 
+    def _ensure_opened(self):
+        """Start monitors. Hold the lock when calling this."""
+        if not self._opened:
+            self._opened = True
+            self._update_servers()
+
     def _request_check_all(self):
         """Wake all monitors. Hold the lock when calling this."""
         for server in self._servers.values():

test/test_cluster.py

@@ -172,7 +172,7 @@ class TestSingleServerCluster(unittest.TestCase):
 
             # No matter whether the server is writable,
             # select_servers() returns it.
-            s = c.select_servers(writable_server_selector)[0]
+            s = c.select_server(writable_server_selector)
             self.assertEqual(server_type, s.description.server_type)
 
     def test_reopen(self):
@@ -192,7 +192,7 @@ class TestSingleServerCluster(unittest.TestCase):
                 return IsMaster({'ok': 1}), round_trip_time
 
         c = create_mock_cluster(monitor_class=TestMonitor)
-        s = c.select_servers(writable_server_selector)[0]
+        s = c.select_server(writable_server_selector)
         self.assertEqual(1, s.description.round_trip_time)
 
         round_trip_time = 3
@@ -532,7 +532,7 @@ class TestMultiServerCluster(unittest.TestCase):
         c = create_mock_cluster(seeds=['a', 'b'], set_name='rs')
 
         def write_batch_size():
-            s = c.select_servers(writable_server_selector)[0]
+            s = c.select_server(writable_server_selector)
             return s.description.max_write_batch_size
 
         got_ismaster(c, ('a', 27017), {
@@ -581,7 +581,7 @@ class TestClusterErrors(unittest.TestCase):
 
         c = create_mock_cluster(monitor_class=TestMonitor)
         # Await first ismaster call.
-        s = c.select_servers(writable_server_selector)[0]
+        s = c.select_server(writable_server_selector)
         self.assertEqual(1, ismaster_count[0])
         pool_id = s.pool.pool_id
 
@@ -604,7 +604,7 @@ class TestClusterErrors(unittest.TestCase):
         c = create_mock_cluster(monitor_class=TestMonitor)
 
         # Await first ismaster call.
-        s = c.select_servers(writable_server_selector)[0]
+        s = c.select_server(writable_server_selector)
         self.assertEqual(1, ismaster_count[0])
         self.assertEqual(SERVER_TYPE.Standalone, s.description.server_type)