PYTHON-1915: Prohibit copying ClientSession objects (#480)

Author: ishmum123

doc/contributors.rst

@@ -88,3 +88,4 @@ The following is a list of people who have contributed to
 - Terence Honles (terencehonles)
 - Paul Fisher (thetorpedodog)
 - Julius Park (juliusgeo)
+- Ishmum Jawad Khan (ishmum123)

pymongo/client_session.py

@@ -827,6 +827,9 @@ def _start_retryable_write(self):
         self._check_ended()
         self._server_session.inc_transaction_id()
 
+    def __copy__(self):
+        raise TypeError('A ClientSession cannot be copied, create a new session instead')
+
 
 class _ServerSession(object):
     def __init__(self, generation):

test/test_session.py

@@ -779,6 +779,11 @@ def drop_db():
 
         wait_until(drop_db, 'dropped database after w=0 writes')
 
+    def test_session_not_copyable(self):
+        client = self.client
+        with client.start_session() as s:
+            self.assertRaises(TypeError, lambda: copy.copy(s))
+
 
 class TestCausalConsistency(unittest.TestCase):