Josiah’s code now seems to handle the four special cases like this:

* Reading a closed pipe raises BrokenPipeError. This is novel to me, and bends the current definition of the exception. But it appears to be the way Windows works at a lower level, and does make a bit of sense to me. However it is inconsistent with how normal reading from files, pipes and sockets works, which return the special value b"" in the equivalent case. Also various communications protocols use an empty packet to signal EOF.

* Reading an empty pipe returns b"". This is consistent with partial reads, which I like. However it could be surprising for someone used to b"" signalling EOF. Alternatives are returning None like RawIOBase.read(), and raising BlockingIOError like os.write() and socket.send().

* Writing to a closed pipe raises BrokenPipeError. This is consistent with how other APIs work, but since it is an asynchronous condition, a naive program may work sometimes without handling it, and then see suprious tracebacks or worse in other situations. Maybe dropping the data and setting a flag could be an alternative.

* Writing to a full pipe returns zero. This is consistent with partial writes, which I like. Alternatives are returning None like RawIOBase.write(), and raising BlockingIOError like os.write(), socket.send(), BufferedIOBase.write().