PYTHON-1972 Add example usage for withTransaction API

Author: prashantmital

test/test_examples.py

@@ -27,8 +27,7 @@
 from pymongo.write_concern import WriteConcern
 
 from test import client_context, unittest, IntegrationTest
-from test.utils import rs_or_single_client
-
+from test.utils import rs_client, rs_or_single_client
 
 class TestSampleShellCommands(unittest.TestCase):
 
@@ -1037,6 +1036,44 @@ def update_employee_info(session):
         self.assertIsNotNone(employee)
         self.assertEqual(employee['status'], 'Inactive')
 
+        MongoClient = lambda _: rs_client()
+        uriString = None
+
+        # Start Transactions withTxn API Example 1
+
+        # For a replica set, include the replica set name and a seedlist of the members in the URI string; e.g.
+        # uriString = 'mongodb://mongodb0.example.com:27017,mongodb1.example.com:27017/?replicaSet=myRepl'
+        # For a sharded cluster, connect to the mongos instances; e.g.
+        # uriString = 'mongodb://mongos0.example.com:27017,mongos1.example.com:27017/'
+
+        client = MongoClient(uriString)
+        wc_majority = WriteConcern("majority", wtimeout=1000)
+
+        # Prereq: Create collections. CRUD operations in transactions must be on existing collections.
+        client.get_database(
+            "mydb1", write_concern=wc_majority).foo.insert_one({'abc': 0})
+        client.get_database(
+            "mydb2", write_concern=wc_majority).bar.insert_one({'xyz': 0})
+
+        # Step 1: Define the callback that specifies the sequence of operations to perform inside the transactions.
+        def callback(session):
+            collection_one = session.client.mydb1.foo
+            collection_two = session.client.mydb2.bar
+
+            # Important:: You must pass the session to the operations.
+            collection_one.insert_one({'abc': 1}, session=session)
+            collection_two.insert_one({'xyz': 999}, session=session)
+
+        # Step 2: Start a client session.
+        with client.start_session() as session:
+            # Step 3: Use with_transaction to start a transaction, execute the callback, and commit (or abort on error).
+            session.with_transaction(
+                callback, read_concern=ReadConcern('local'),
+                write_concern=wc_majority,
+                read_preference=ReadPreference.PRIMARY)
+
+        # End Transactions withTxn API Example 1
+
     @client_context.require_transactions
     def test_transactions_beta(self):
         # Transaction beta examples