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It seems the old pyprocessing (http://pyprocessing.berlios.de/) can do 
some things that the new multiprocessing package can not; 
sending/receiving connection objects for one.
This is a quite handy functionality, so it would be nice if it were 
reintroduced.
Also, the error message isn't very helpful.
Failing test below.
$ python2.6 pipetest2.py 
asdf
Traceback (most recent call last):
 File "a.py", line 10, in <module>
 print c1.recv()
TypeError: Required argument 'handle' (pos 1) not found
$ PYTHONPATH=processing-0.52-py2.5-macosx-10.5-i386.egg python2.5 
pipetest2.py 
asdf
Connection(handle=5)
$ PYTHONPATH=multiprocessing-2.6.0.2-py2.5-macosx-10.5-i386.egg 
python2.5 pipetest2.py 
asdf
Traceback (most recent call last):
 File "pipetest2.py", line 10, in <module>
 print c1.recv()
TypeError: function takes at least 1 argument (0 given)
$ uname -a
Darwin midori.local 9.6.0 Darwin Kernel Version 9.6.0: Mon Nov 24 
17:37:00 PST 2008; root:xnu-1228959~1/RELEASE_I386 i386

$ cat pipetest2.py 
try:
	from multiprocessing import Pipe
except ImportError:
	from processing import Pipe
c1, c2 = Pipe(duplex=False)
c2.send('asdf')
print c1.recv()
c2.send(c1)
print c1.recv()

thanks for filing this. I'll need to compare the two code bases and figure 
out why it's either regressed, or Richard removed it prior to the 
integration.

Before I can logically support this, I need a clear use case that supports 
the idea that this should be supported in the current version of 
multiprocessing.

A typical use case would be for a server to receive a connection, and
then send that connection over to another process that does the actual
work. This used to work with pyprocessing, and the support seems to be
available in multiprocessing.c -> multiprocessing_sendfd, but not used.

And to be clear, I have enabled connection pickling by issuing:
multiprocessing.allow_connection_pickling()

When reverting this commit stuff works:
http://svn.python.org/view/python/trunk/Lib/multiprocessing/reduction.py?r1=64257&r2=65016 

Ehm.. completly broken url in prev message.. Revision 65016, "Apply
Amaury's patch to multiprocessing for issue 3125, removes the copy_reg
and replaces it with ForkingPickler.register(), which should resolve the
conflict with the global registry/ctypes" is what I'm refering to.
Without this patch, the reducers/rebuilders are properly registered in
the pickler that connection.h (srsly, code in header files?) later uses.

Wanted to quickly comment here, as I'm dealing with this issue as well, that I did find a workaround for avoiding it as far back as 2.6 (and it's not "don't pass a Pipe through a Pipe")
multiprocessing.reduction can already do this, though I don't entirely know why this isn't automatically done if it's a connection object.
>>> from multiprocessing import Pipe, reduction
>>> i, o = Pipe()
>>> reduced = reduction.reduce_connection(i)
>>> newi = reduced[0](*reduced[1])
>>> newi.send("hi")
>>> o.recv()
'hi'
>>> 
The reduced[0](*reduced[1]) line is actually calling reduction.rebuild_connection, as that function is the first element in the tuple, and the second element is the arguments to be passed to it. I can't seem to find any info on reduction.reduce_connection, so I don't know if this is how this was intended to be handled or not.
P.S. Tested on Win (XP) and Linux (Ubuntu 10.10), so there's no weird windows socket stuff that should go wrong with this.

err, is it possible to edit out those file paths? I didn't intend them to be in the message. I'd appreciate it if someone with the privileges to do so could remove them.

> err, is it possible to edit out those file paths?
I don't know how to do that. If you want I can remove the message altogether. But I don't see anything confidential or exploitable in your message.

Jimbofbx wrote:
> def main():
> from multiprocessing import Pipe, reduction
> i, o = Pipe()
> print(i);
> reduced = reduction.reduce_connection(i)
> print(reduced);
> newi = reduced[0](*reduced[1])
> print(newi);
> newi.send("hi")
> o.recv()
On Windows with a PipeConnection object you should use rebuild_pipe_connection() instead of rebuild_connection(). With that change, on Python 3.3 I get
 <multiprocessing.connection.PipeConnection object at 0x025BBCB0>
 (<function rebuild_pipe_connection at 0x0262F420>, (('\\\\.\\pipe\\pyc-6000年1月30日lq4p', 356, False), True, True))
 <multiprocessing.connection.PipeConnection object at 0x029FF710>
Having said all that I agree multiprocessing.reduction should be fixed. Maybe an enable_pickling_support() function could be added to register the necessary things with copyreg.

> Having said all that I agree multiprocessing.reduction should be
> fixed. Maybe an enable_pickling_support() function could be added to
> register the necessary things with copyreg.
Why not simply use ForkingPickler?

ForkingPickler is only used when creating a child process. The multiprocessing.reduction module is only really intended for sending stuff to *pre-existing* processes.
As things stand, after importing multiprocessing.reduction you can do something like
 buf = io.BytesIO()
 pickler = ForkingPickler(buf)
 pickler.dump(conn)
 data = buf.getvalue()
 writer.send_bytes(data)
But that is rather less simple and obvious than just doing
 writer.send(conn)
which was possible in pyprocessing.
Originally just importing the module magically registered the reduce functions with copyreg. Since this was undesirable, the reduction functions were instead registered with ForkingPickler. But this fix rather missed the point of the module.

> ForkingPickler is only used when creating a child process. The
> multiprocessing.reduction module is only really intended for sending
> stuff to *pre-existing* processes.
But ForkingPickler could be used in multiprocessing.connection, couldn't
it?

@pitrou
You can just delete my original post. I'll repost an edited version here for reference
original post with paths removed:
This is an issue for me (Python 3.2). I have a custom pool that sends arguments for a function call over a pipe. I cannot send another pipe as an argument. 
Tim's workaround also does not work for me (win xp 32bit and 64bit)
From what I can tell, you can only send a connection as a direct argument to a function call. This limits what I can do because I cannot introduce new pipes to a worker process after it is instantiated.
Using this code:
def main():
 from multiprocessing import Pipe, reduction
 i, o = Pipe()
 print(i);
 reduced = reduction.reduce_connection(i)
 print(reduced);
 newi = reduced[0](*reduced[1])
 print(newi);
 newi.send("hi")
 o.recv()
if __name__ == "__main__":
 main();
This is my output:
<read-write PipeConnection, handle 1760>
(<function rebuild_connection at 0x00FD4C00>, (('\\\\.\\pipe\\pyc-3156-1-q5wwnr', 1756, False), True, True))
<read-write Connection, handle 1720>
>>> newi.send("hi")
IOError: [Errno 10038] An operation was attempted on something that is not a socket
As you can see, the handle changes

> But ForkingPickler could be used in multiprocessing.connection,
> couldn't it?
I suppose so.
Note that the way a connection handle is transferred between existing processes is unnecessarily inefficient on Windows. A background server thread (one per process) has to be started and the receiving process must connect back to the sending process to receive its duplicate handle.
There is a simpler way to do this on Windows. The sending process duplicates the handle, and the receiving process duplicates that second handle using DuplicateHandle() and the DUPLICATE_CLOSE_SOURCE flag. That way no server thread is necessary on Windows.
I got this to work recently for pickling references to file handles for mmaps on. (A server thread would still be necessary on Unix.)

> There is a simpler way to do this on Windows. The sending process 
> duplicates the handle, and the receiving process duplicates that second 
> handle using DuplicateHandle() and the DUPLICATE_CLOSE_SOURCE flag. That 
> way no server thread is necessary on Windows.
Note that this should not be done for socket handles since DuplicateHandle() is not supposed to work for them. socket.share() and socket.fromshare() with a server thread can be used for sockets.

Shouldn't reduce_pipe_connection just be an alias for reduce_connection in unix so that using reduce_pipe_connection would work for both win and unix? My understanding after looking at the code is that reduce_pipe_connection isn't defined for non-win32, although I haven't tested it to see if that's true.
Of course, ideally a pipe connection would just pickle and unpickle properly out-of-the-box, which I think was the original intent.
Here's a complete, working example with Python 3.2 tested on Win 7 64-bit:
import sys
from multiprocessing import Process,Pipe, reduction
def main():
 print("starting");
 i, o = Pipe(False)
 parent, child = Pipe();
 reducedchild = reduce_pipe(child);
 p = Process(target=helper, args=(i,));
 p.start();
 parent.send("hi");
 o.send(reducedchild);
 print(parent.recv());
 print("finishing");
 p.join();
 print("done");
def helper(inPipe):
 childPipe = expand_reduced_pipe(inPipe.recv());
 childPipe.send("child got: " + childPipe.recv());
 return;
def reduce_pipe(pipe):
 if sys.platform == "win32":
 return reduction.reduce_pipe_connection(pipe);
 else:
 return reduction.reduce_connection(pipe);
def expand_reduced_pipe(reduced_pipe):
 return reduced_pipe[0](*reduced_pipe[1]);
if __name__ == "__main__":
 main();

There is an undocumented function multiprocessing.allow_connection_pickling() whose docstring claims it allows connection and socket objects to be pickled.
The attached patch fixes the multiprocessing.reduction module so that it works correctly. This means that TestPicklingConnections can be reenabled in the unit tests.
The patch uses the new socket.share() and socket.fromshare() methods on Windows.

Unless there's a technical barrier, I still think it would be better to use ForkingPickler in multiprocessing.connection, rather than modify global state (copyreg). The pickling support is multiprocessing-specific and wouldn't make sense for other pickles (e.g. stored to disk).

I just want to point out that each time socket.share() is called, the resulting data can only be used once by socket.fromshare(). I'm mentioning this because I know there is some caching mechanism in reduction.py and that this data is not cacheable/reuseable.

Updated patch which uses ForkingPickler in Connection.send().
Note that connection sharing still has to be enabled using allow_connection_pickling().
Support could be enabled automatically, but that would introduce more circular imports which confuse me. It might be worthwhile refactoring to eliminate all circular imports.

> Support could be enabled automatically, but that would introduce more
> circular imports which confuse me.
Are you sure? AFAICT:
- connection depends on forking
- reduction depends on forking and connection
But connection doesn't depend on reduction, neither does forking.

> But connection doesn't depend on reduction, neither does forking.
If registration of (Pipe)Connection is done in reduction then you can't make (Pipe)Connection picklable *automatically* unless you make connection depend on reduction (possibly indirectly).
A circular import can be avoided by making reduction not import connection at module level. So not hard to fix.

The last patch did not work on Unix.
Here is a new version where the reduction functions are automatically registered, so allow_connection_pickling() is redundant.

Could you regenerate your patch now that the win32 -> _winapi changes have been applied?

Up to date patch.

A couple of minor changes based on Antoine's earlier review (which I did not notice till now).

New changeset 08d4c2fe51ea by Antoine Pitrou in branch 'default':
Issue #4892: multiprocessing Connections can now be transferred over multiprocessing Connections.
http://hg.python.org/cpython/rev/08d4c2fe51ea 

Thanks, Richard. I have now committed the patch. Hopefully the Windows buildbots will be ok :)