I discovered what appears to be a thread-unsafety in uuid.py. This is in the trunk as well as in 3.x; I'm using the trunk here for easy reference. There's some code around like 395: import ctypes, ctypes.util _buffer = ctypes.create_string_buffer(16) This creates a *global* buffer which is used as the output parameter to later calls to _uuid_generate_random() and _uuid_generate_time(). For example, around line 481, in uuid1(): _uuid_generate_time(_buffer) return UUID(bytes=_buffer.raw) Clearly if two threads do this simultaneously they are overwriting _buffer in unpredictable order. There are a few other occurrences of this too. I find it somewhat disturbing that what seems a fairly innocent function that doesn't *appear* to have global state is nevertheless not thread-safe. Would it be wise to fix this, e.g. by allocating a fresh output buffer inside uuid1() and other callers? -- --Guido van Rossum (home page: http://www.python.org/~guido/)