I'm using xmlrpc, and I'm using a custom Transport, follow the "21.26.8. Example of Client Usage" section of below document: https://docs.python.org/3/library/xmlrpc.client.html I found if I use a custom transport, set use_builtin_types parameter to xmlrpc.client.ServerProxy will have no effect. After read the source code, I found if I use a custom transport, I should set the use_builtin_types parameter in the transport. I think it is better to describe this behavior in the document. best regards.