Merge pull request aws-samples#152 from mkamioner/python-conditional-…

…samples

Python conditional samples

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")

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

@@ -1,2 +1,105 @@
-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")
+
+
+class ConditionalCheckFailedError(Exception):
+    """
+    An error indicating that a DynamoDB conditional check failed.
+    Wrapped as a separate error for readability.
+    """
+
+
+def delete_item(email):
+    response = table.delete_item(
+        Key={
+            "pk": email,
+            "sk": "metadata",
+        },
+    )
+
+
+def create_user(email, name):
+    initial_change_version = 0
+
+    try:
+        table.put_item(
+            Item={
+                "pk": email,
+                "sk": "metadata",
+                "name": name,
+                "change_version": initial_change_version,
+            },
+            ConditionExpression=Attr("pk").not_exists(),
+        )
+    except dynamodb.meta.client.exceptions.ConditionalCheckFailedException as e:
+        raise ConditionalCheckFailedError() from e
+
+    return initial_change_version
+
+
+def update_name(email, name, last_change_version):
+    try:
+        response = table.update_item(
+            Key={"pk": email, "sk": "metadata"},
+            UpdateExpression="SET #n = :nm, #cv = #cv + :one",
+            ExpressionAttributeNames={"#n": "name", "#cv": "change_version"},
+            ExpressionAttributeValues={":nm": name, ":one": 1},
+            ReturnValues="UPDATED_NEW",
+            ConditionExpression=Attr("change_version").eq(last_change_version),
+        )
+    except dynamodb.meta.client.exceptions.ConditionalCheckFailedException as e:
+        raise ConditionalCheckFailedError() from e
+
+    return int(response.get("Attributes", {}).get("change_version"))
+
+
+def get_user(email):
+    response = table.get_item(
+        Key={"pk": email, "sk": "metadata"},
+        AttributesToGet=["name", "change_version"],
+    )
+    return {
+        "email": email,
+        "name": response["Item"]["name"],
+        "change_version": int(response["Item"]["change_version"]),
+    }
+
+
+def main():
+    delete_item("jim.bob@somewhere.com")
+
+    print("Creating an item with change_version = 0")
+    last_change_version = create_user("jim.bob@somewhere.com", "Jim Bob")
+    current_user = get_user("jim.bob@somewhere.com")
+    print("current_user = ", current_user)
+
+    print("Change the user's name")
+    last_change_version = update_name(
+        "jim.bob@somewhere.com", "Jim Roberts", last_change_version
+    )
+    current_user = get_user("jim.bob@somewhere.com")
+    print("current_user = ", current_user)
+
+    print(
+        "Try to update the name with an old change_version imitating a race condition"
+    )
+    try:
+        last_change_version = update_name(
+            "jim.bob@somewhere.com", "Jonathan Roberts", last_change_version - 1
+        )
+    except ConditionalCheckFailedError:
+        print("Yup, this failed as expected, the user information did not change")
+    else:
+        raise RuntimeError("This should have failed")
+
+    current_user = get_user("jim.bob@somewhere.com")
+    print("current_user = ", current_user)
+
+
+if __name__ == "__main__":
+    main()