Find all globals names read or written to by codeblock co

def extract_code_globals(cls, co):
        """
        Find all globals names read or written to by codeblock co
        """
        out_names = cls._extract_code_globals_cache.get(co)
        if out_names is None:
            try:
                names = co.co_names
            except AttributeError:
                # PyPy "builtin-code" object
                out_names = set()
            else:
                out_names = set(names[oparg]
                                for op, oparg in _walk_global_ops(co))

                # see if nested function have any global refs
                if co.co_consts:
                    for const in co.co_consts:
                        if type(const) is types.CodeType:
                            out_names |= cls.extract_code_globals(const)

            cls._extract_code_globals_cache[co] = out_names

        return out_names