Python3 "pickle" vs. stdin/stdout - unable to get clean byte streams in Python 3

[John Nagle]

  I have working code from Python 2 which uses "pickle"
to talk to a subprocess via stdin/stdio.  I'm trying to
make that work in Python 3.

  First, the subprocess Python is invoked with the "-d' option,
so stdin and stdio are supposed to be unbuffered binary streams.
That was enough in Python 2, but it's not enough in Python 3.

The subprocess and its connections are set up with

  proc = subprocess.Popen(launchargs,stdin=subprocess.PIPE,
    stdout=subprocess.PIPE, env=env)

  ...
  self.reader = pickle.Unpickler(self.proc.stdout)
  self.writer = pickle.Pickler(self.proc.stdin, 2)

after which I get

  result = self.reader.load()
TypeError: 'str' does not support the buffer interface

That's as far as traceback goes, so I assume this is
disappearing into C code.

OK, I know I need a byte stream.  I tried

  self.reader = pickle.Unpickler(self.proc.stdout.buffer)
  self.writer = pickle.Pickler(self.proc.stdin.buffer, 2)

That's not allowed.  The "stdin" and "stdout" that are
fields of "proc" do not have "buffer".  So I can't do that
in the parent process.  In the child, though, where
stdin and stdout come from "sys", "sys.stdin.buffer" is valid.
That fixes the ""str" does not support the buffer interface
error."  But now I get the pickle error "Ran out of input"
on the process child side.  Probably because there's a
str/bytes incompatibility somewhere.

So how do I get clean binary byte streams between parent
and child process?

			John Nagle