> A feature that may be actually nice to have in the pickle protocol would
be some framing, to help with streaming unpickling (right now unpickling
a stream can read almost one byte at a time, IIRC).
> However, that would also make the protocol and the pickler significantly
more complex.

What if just use io.BufferedReader?

    if not isinstance(file, io.BufferedReader):
        file = io.BufferedReader(file)

(at start of _Unpickler.__init__)