xmlrpclib.Binary.encode() does base64 encoding.
But there are actually two conflicting specs: RFC 2045 says add newlines every 76 characters, RFC 3548 says do NOT add newlines.

The xmlrpc spec characteristically doesn't mention an RFC, so it's not clear which spec they mean by "base64".
By looking at the dates on the various specs, it can be inferred that they must mean MIME-style with newlines, as per RFC 2045.  That's xmlrpclib does.

But for those of us implementing stuff in the trenches and struggling with interoperability issues, it would be nice if the python docs were more informative so we don't have to go spiraling off into reading multiple specs and figuring out which came first :)

So I'd suggest changing the Binary.encode docstring to something like:

"""Write the XML-RPC base 64 encoding of this binary item to the out stream object.

Note that the encoded data will have newlines every 76 characters as per RFC 2045, which was the de facto standard base64 specification when the xmlrpc spec was written."""

Arguably, the behavior could be changed instead - due to the xmlrpc spec's silence on the subject, not all clients may tolerate newlines (Redstone doesn't)... but that's a separate argument :)