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Python 3.2 has extra code in _PyImport_ReInitLock() which means that when a fork happens as a side effect of an import, the main thread of the forked process owns the import lock. Therefore other threads in the forked process cannot import anything.
_PyImport_ReInitLock(void)
{
 if (import_lock != NULL)
 import_lock = PyThread_allocate_lock();
 if (import_lock_level > 1) {
 /* Forked as a side effect of import */
 long me = PyThread_get_thread_ident();
 PyThread_acquire_lock(import_lock, 0);
 /* XXX: can the previous line fail? */
 import_lock_thread = me;
 import_lock_level--;
 } else {
 import_lock_thread = -1;
 import_lock_level = 0;
 }
}
I think the reason this code is not triggered in Python 3.3 is the introduction of per-module import locks.