As I'm reading the PEP 590 reference implementation, it strikes me how similar it is to https://bugs.python.org/issue29259 The main difference is that bpo-29259 has a per-class pointer tp_fastcall instead of a per-object pointer. But actually, the PEP 590 reference implementation does not make much use of the per-object pointer: for all classes except "type", the vectorcall wrapper is the same for all objects of a given type. One thing that bpo-29259 did not realize is that existing optimizations could be dropped in favor of using tp_fastcall. For example, bpo-29259 has code like if (PyFunction_Check(callable)) { return _PyFunction_FastCallKeywords(...); } if (PyCFunction_Check(callable)) { return _PyCFunction_FastCallKeywords(...); } else if (PyType_HasFeature(..., Py_TPFLAGS_HAVE_FASTCALL) ...) but the first 2 branches are superfluous given the third. Anyway, this is just putting PEP 590 a bit in perspective. It doesn't say anything about the merits of PEP 590. Jeroen.