The need to call p1.stdout.close() has now been documented as part of issue7678.  Python 3.2's subprocess also has restore_signals=True as its default behavior so SIGPIPE is restored by default.

I do not think it is appropriate to to add the synchronization Peter suggested to the subprocess module to optimize that close call.  The potential delay due to python having to call p1.stdout.close() is non-fatal and should be assumed to exist anyways as you can't guarantee when an async event like a signal (in this case SIGPIPE) will actually reach the other process.