Sorry if this doesn't fit this issue and needs a separate one.

Since Python switched to 2 byte wordcode, all opcodes which do not imply an argument, technically have it - augmented with 0. So it is convenient to iterate over bytecode like:

op, arg = instruction.

But there is a check in stack_effect that the second argument for this opcodes must be None. 

file::_opcode.c

else if (oparg != Py_None) {
        PyErr_SetString(PyExc_ValueError,
                "stack_effect: opcode does not permit oparg but oparg was specified");
        return -1;
    }


So you need to perform a somewhat _redundant_ check before calling:

arg = arg if op >= opcode.HAVE_ARGUMENT else None.
st = stack_effect(op, arg)

Maybe it's normal to relax this condition - be None or 0 for opcode < opcode.HAVE_ARGUMENT?