Normally Python code calls built-in setattr, which calls the C API PyObject_SetAttr. This API interns the attribute name before calling the type's tp_setattro or tp_setattr function. Interning the string is a critical step, since the implementation for updating slots assumes the name is interned. 

The __setattr__ slot wrapper calls wrap_setattr in Objects/typeobject.c. In line with how PyObject_SetAttr works, the attached patch interns the name in wrap_setattr before calling the wrapped setattrofunc. For good measure it also applies the same change to wrap_delattr, though that's not strictly necessary.