When I use subprocess.py from a child thread, sometimes it deadlocks. I 
determined that the new process is blocked during an import:

#0  0x90024427 in semaphore_wait_signal_trap ()
#1  0x90028414 in pthread_cond_wait ()
#2  0x004c77bf in PyThread_acquire_lock (lock=0x3189a0, waitflag=1)  
at Python/thread_pthread.h:452
#3  0x004ae2a6 in lock_import () at Python/import.c:266
#4  0x004b24be in PyImport_ImportModuleLevel (name=0xaad74 "errno",  
globals=0xbaed0, locals=0x502aa0, fromlist=0xc1378, level=-1) at  
Python/import.c:2054
#5  0x0048d2e2 in builtin___import__ (self=0x0, args=0x53724c90,  
kwds=0x0) at Python/bltinmodule.c:47
#6  0x0040decb in PyObject_Call (func=0xa94b8, arg=0x53724c90,  
kw=0x0) at Objects/abstract.c:1860

and that the code in question is in os.py:

def _execvpe(file, args, env=None):
     from errno import ENOENT, ENOTDIR

I think the problem is that since exec (the C function) hasn't yet been 
called in the new process, it's inherited from the fork a lock that's already 
held. The main process will eventually release its copy of the lock, but this 
will not unlock it in the new process, so it deadlocks.

If I change os.py so that it imports the constants outside of  
_execvpe, the new process no longer blocks in this way.

This is on Mac OS X 10.4.8.