It looks like there's also a separate problem in the multiprocessing
module. The following code hangs after hitting a TypeError trying to
pickle one of the TextIOWrapper objects:

    import multiprocessing

    def read(f): return f.read()

    files = [open(path) for path in 3 * ['/dev/null']]
    pool = multiprocessing.Pool()
    results = pool.map(read, files)
    print(results)