RDS modify-db-instance: raise DBParameterGroupNotFoundError if the DBParameterGroupName does not exist (#4879)

moto/rds2/models.py

@@ -455,6 +455,12 @@ class Database(CloudFormationModel):
                 )
             ]
         else:
+            if (
+                self.db_parameter_group_name
+                not in rds2_backends[self.region].db_parameter_groups
+            ):
+                raise DBParameterGroupNotFoundError(self.db_parameter_group_name)
+
             return [
                 rds2_backends[self.region].db_parameter_groups[
                     self.db_parameter_group_name

tests/test_rds2/test_rds2.py

@@ -396,6 +396,27 @@ def test_modify_db_instance():
     ].should.equal("sg-123456")
 
 
+@mock_rds2
+def test_modify_db_instance_not_existent_db_parameter_group_name():
+    conn = boto3.client("rds", region_name="us-west-2")
+    conn.create_db_instance(
+        DBInstanceIdentifier="db-master-1",
+        AllocatedStorage=10,
+        DBInstanceClass="postgres",
+        Engine="db.m1.small",
+        MasterUsername="root",
+        MasterUserPassword="hunter2",
+        Port=1234,
+        DBSecurityGroups=["my_sg"],
+    )
+    instances = conn.describe_db_instances(DBInstanceIdentifier="db-master-1")
+    instances["DBInstances"][0]["AllocatedStorage"].should.equal(10)
+    conn.modify_db_instance.when.called_with(
+        DBInstanceIdentifier="db-master-1",
+        DBParameterGroupName="test-sqlserver-se-2017",
+    ).should.throw(ClientError)
+
+
 @mock_rds2
 def test_rename_db_instance():
     conn = boto3.client("rds", region_name="us-west-2")