You can call .borrow_value() on the tuple to get a &[PyObjectRef], and then use that instead of the PyIterable

Oh I see, cause it has to be Vec<PyTypeRef>. Maybe just .borrow_value().iter().map(typeref tryfromobject).collect::<PyResult<Vec<PyTypeRef>>()?

I recently added a builtins::tuple::PyTupleTyped type, you could try making the type of bases: PyTupleTyped<PyTypeRef>

Updated. Is there a reason you added the Transmute... trait instead of just TryFromObject?

TryFromObject is implemented for a ton of different types and doesn't have any invariants regarding representation, e.g. if it just required TryFromObject you could have PyTupleTyped<u8>, and that obviously wouldn't be safe to cast PyObjectRef to u8. TransmuteFromObject is an unsafe trait, so if anyone wants to implement it wrong they have to use unsafe to do so 🙃