DynamoDB - Validate empty ExpressionAttributeNames on get_item() (#4393)

moto/dynamodb2/responses.py

@@ -410,8 +410,19 @@ class DynamoHandler(BaseResponse):
             )
         key = self.body["Key"]
         projection_expression = self.body.get("ProjectionExpression")
-        expression_attribute_names = self.body.get("ExpressionAttributeNames", {})
+        expression_attribute_names = self.body.get("ExpressionAttributeNames")
+        if expression_attribute_names == {}:
+            if projection_expression is None:
+                er = "ValidationException"
+                return self.error(
+                    er,
+                    "ExpressionAttributeNames can only be specified when using expressions",
+                )
+            else:
+                er = "ValidationException"
+                return self.error(er, "ExpressionAttributeNames must not be empty")
 
+        expression_attribute_names = expression_attribute_names or {}
         projection_expression = self._adjust_projection_expression(
             projection_expression, expression_attribute_names
         )

tests/test_dynamodb2/test_dynamodb_exceptions.py

@@ -6,9 +6,7 @@ from botocore.exceptions import ClientError
 from moto import mock_dynamodb2
 
 
-@mock_dynamodb2
+table_schema = {
-def test_query_gsi_with_wrong_key_attribute_names_throws_exception():
-    table_schema = {
     "KeySchema": [{"AttributeName": "partitionKey", "KeyType": "HASH"}],
     "GlobalSecondaryIndexes": [
         {
@@ -25,8 +23,11 @@ def test_query_gsi_with_wrong_key_attribute_names_throws_exception():
         {"AttributeName": "gsiK1PartitionKey", "AttributeType": "S"},
         {"AttributeName": "gsiK1SortKey", "AttributeType": "S"},
     ],
-    }
+}
 
+
+@mock_dynamodb2
+def test_query_gsi_with_wrong_key_attribute_names_throws_exception():
     item = {
         "partitionKey": "pk-1",
         "gsiK1PartitionKey": "gsi-pk",
@@ -96,3 +97,53 @@ def test_query_gsi_with_wrong_key_attribute_names_throws_exception():
     err["Message"].should.equal(
         "Query condition missed key schema element: gsiK1SortKey"
     )
+
+
+@mock_dynamodb2
+def test_empty_expressionattributenames():
+    ddb = boto3.resource("dynamodb", region_name="us-east-1")
+    ddb.create_table(
+        TableName="test-table", BillingMode="PAY_PER_REQUEST", **table_schema
+    )
+    table = ddb.Table("test-table")
+    with pytest.raises(ClientError) as exc:
+        table.get_item(
+            Key={"id": "my_id"}, ExpressionAttributeNames={},
+        )
+    err = exc.value.response["Error"]
+    err["Code"].should.equal("ValidationException")
+    err["Message"].should.equal(
+        "ExpressionAttributeNames can only be specified when using expressions"
+    )
+
+
+@mock_dynamodb2
+def test_empty_expressionattributenames_with_empty_projection():
+    ddb = boto3.resource("dynamodb", region_name="us-east-1")
+    ddb.create_table(
+        TableName="test-table", BillingMode="PAY_PER_REQUEST", **table_schema
+    )
+    table = ddb.Table("test-table")
+    with pytest.raises(ClientError) as exc:
+        table.get_item(
+            Key={"id": "my_id"}, ProjectionExpression="", ExpressionAttributeNames={},
+        )
+    err = exc.value.response["Error"]
+    err["Code"].should.equal("ValidationException")
+    err["Message"].should.equal("ExpressionAttributeNames must not be empty")
+
+
+@mock_dynamodb2
+def test_empty_expressionattributenames_with_projection():
+    ddb = boto3.resource("dynamodb", region_name="us-east-1")
+    ddb.create_table(
+        TableName="test-table", BillingMode="PAY_PER_REQUEST", **table_schema
+    )
+    table = ddb.Table("test-table")
+    with pytest.raises(ClientError) as exc:
+        table.get_item(
+            Key={"id": "my_id"}, ProjectionExpression="id", ExpressionAttributeNames={},
+        )
+    err = exc.value.response["Error"]
+    err["Code"].should.equal("ValidationException")
+    err["Message"].should.equal("ExpressionAttributeNames must not be empty")