Implement Add to List for dynamodb:UpdateItem (#3585)

This handles the add-to-list case using the legacy `AttributeUpdates` parameter.

* Added test coverage.
* Verified against real AWS backend.

Closes #3561

moto/dynamodb2/models/__init__.py

@@ -156,6 +156,10 @@ class Item(BaseModel):
                     existing = self.attrs.get(attribute_name, DynamoType({"SS": {}}))
                     new_set = set(existing.value).union(set(new_value))
                     self.attrs[attribute_name] = DynamoType({"SS": list(new_set)})
+                elif set(update_action["Value"].keys()) == {"L"}:
+                    existing = self.attrs.get(attribute_name, DynamoType({"L": []}))
+                    new_list = existing.value + new_value
+                    self.attrs[attribute_name] = DynamoType({"L": new_list})
                 else:
                     # TODO: implement other data types
                     raise NotImplementedError(

tests/test_dynamodb2/test_dynamodb.py

@@ -5704,3 +5704,26 @@ def test_dynamodb_update_item_fails_on_string_sets():
         Key={"record_id": {"S": "testrecord"}},
         AttributeUpdates=attribute,
     )
+
+
+@moto.mock_dynamodb2
+def test_update_item_add_to_list_using_legacy_attribute_updates():
+    resource = boto3.resource("dynamodb", region_name="us-west-2")
+    resource.create_table(
+        AttributeDefinitions=[{"AttributeName": "id", "AttributeType": "S"}],
+        TableName="TestTable",
+        KeySchema=[{"AttributeName": "id", "KeyType": "HASH"}],
+        ProvisionedThroughput={"ReadCapacityUnits": 5, "WriteCapacityUnits": 5},
+    )
+    table = resource.Table("TestTable")
+    table.wait_until_exists()
+    table.put_item(Item={"id": "list_add", "attr": ["a", "b", "c"]},)
+
+    table.update_item(
+        TableName="TestTable",
+        Key={"id": "list_add"},
+        AttributeUpdates={"attr": {"Action": "ADD", "Value": ["d", "e"]}},
+    )
+
+    resp = table.get_item(Key={"id": "list_add"})
+    resp["Item"]["attr"].should.equal(["a", "b", "c", "d", "e"])