import io
import tempfile
import os

class A:
    def __del__(self):
        print('del A')

class MyFileIO(io.FileIO):
    def __del__(self):
        print('__del__ FileIO')
        super().__del__()


class MyBufferedWriter(io.BufferedWriter):
    def __del__(self):
        print('__del__ BufferedWriter')
        super().__del__()


def test():
    fd, filename = tempfile.mkstemp()
    pid = os.fork()
    data = b'hello'
    if pid == 0:
        a = A()
        a.a = a
        a.raw = MyFileIO(fd, mode='w')
        a.buf = MyBufferedWriter(a.raw, buffer_size=1000)
        a.buf.write(data)
        del a
        import gc
        print('collecting')
        gc.collect()
    else:
        os.waitpid(pid, os.P_WAIT)
        fp = os.fdopen(fd, 'rb+')
        fp.seek(0)
        try:
            data2 = fp.read()
            print('read data', data2)
            if data != data2:
                print('buffer not flushed')
            else:
                print('okay')
        finally:
            fp.close()
            os.unlink(filename)


if __name__ == '__main__':
    test()