The uuid module uses a single global buffer for storing random values 
obtained from the system. This can (and does) cause non-uniqueness of 
generated id's when using the uuid4 function in a multithreaded program.

The following snippet shows the problem - _buffer is the global buffer:

    # When the system provides a version-4 UUID generator, use it.
    if _uuid_generate_random:
        _uuid_generate_random(_buffer)
        return UUID(bytes=_buffer.raw)