On 2023年08月02日, dn <PythonList at DancesWithMice.info> wrote: > Can you please explain why a multi-part second-argument must be a tuple > and not any other form of collection-type? The following comment may hold a clue: if (PyTuple_Check(cls)) { /* Not a general sequence -- that opens up the road to recursion and stack overflow. */ https://github.com/python/cpython/blob/main/Objects/abstract.c#L2684 Plus an almost total lack of demand for change I should think.