feat: Add put_item_conditional example to Python samples.

Author: Mo Kamioner

examples/SDK/python/data_plane/WorkingWithItems/put_item_conditional.py

@@ -1,2 +1,48 @@
-import boto3, json
-from boto3.dynamodb.conditions import Key
+from __future__ import print_function  # Python 2/3 compatibility
+import boto3
+from boto3.dynamodb.conditions import Attr
+from botocore.exceptions import ClientError
+
+dynamodb = boto3.resource("dynamodb", region_name="us-west-2")
+table = dynamodb.Table("RetailDatabase")
+
+
+def create_item_if_not_exist(created_at):
+    try:
+        table.put_item(
+            Item={
+                "pk": "jim.bob@somewhere.com",
+                "sk": "metadata",
+                "name": "Jim Bob",
+                "first_name": "Jim",
+                "last_name": "Bob",
+                "created_at": created_at,
+            },
+            # If this item exists, then an item exists with this pk and/or sk so we should fail if
+            # we see a matching item with this pk.
+            ConditionExpression=Attr("pk").not_exists(),
+        )
+    except dynamodb.meta.client.exceptions.ConditionalCheckFailedException:
+        print("PutItem failed since the item already exists")
+        return False
+
+    print("PutItem succeeded")
+    return True
+
+
+def get_created_at(email):
+    response = table.get_item(
+        Key={"pk": email, "sk": "metadata"},
+        AttributesToGet=["created_at"],
+    )
+    return response.get("Item", {}).get("created_at")
+
+
+print("The first PutItem should succeed because the item does not exist")
+assert create_item_if_not_exist(100)
+
+print("The same PutItem command should now fail because the item already exists")
+assert not create_item_if_not_exist(200)
+
+assert get_created_at("jim.bob@somewhere.com") == 100
+print("As expected, The second PutItem command failed and the data was not overwritten")