I think clearing these flags is not needed.

It would be better to use PyCFunction_GET_FLAGS().

In sum this function can be written as:

    return PyFunction_Check(callable) ||
           (PyCFunction_Check(callable) && 
            !(PyCFunction_GET_FLAGS(callable) & METH_VARARGS));

"I think clearing these flags is not needed. It would be better to use PyCFunction_GET_FLAGS()."

Right, done. But I prefer explicit if().

pto->fn can be changed by __setstate__().

Oh, nice catch. Fixed.

Since concatenating with empty tuple is optimized, perhaps these two special cases no longer needed. -10 lines of code. Could you tests this?

I tested: removing the two special case (size==0) has no impact on performance, so I removed it.

This can be used for all branches. -2 lines of code.

done

If there are keyword arguments _PyObject_FastCallDict allocates new array for arguments. I don't know whether it is worth to optimize this case and unpack keyword arguments here. This could complicate the code too much and I don't know how large is a benefit.

There is a corner case: dict.update() really expects a dict.

Your proposed optimizations seem complex for a little gain.

Seems the code for merging two dicts is duplicated. Could it be shared?

done

Does it before Py_SETREF(). The partial object should be consistent at the time of destroying old callable.

done