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How about if subprocesses have 3 new methods, send(input),
recv(maxlen), and recv_stderr(maxlen).

send(input) would perform like socket.send(), sending as
much as it currently can, returning the number of bytes sent.
recv(maxlen) and recv_stderr(maxlen) would recieve up to the
provided number of bytes from the stdout or stderr pipes
respectively.

I currently have an implementation of the above on Windows
and posix.  I include the context diff against revision 1.20
of subprocess.py in current CVS.