I don't like the idea to copy sys.modules. It's an expensive operation, and this code is executed to evaluate every parameter with a non-trivial default for a function with __text_signature__.

Looking at the documentation of Argument Clinic, I see that this code is supposed to handle simple defaults like mod.attr.

So why don't we rewrite wrap_value to do just that, e.g.:

    def wrap_value(s):
        try:
            value = eval(s, module_dict)
        except NameError:
            match = re.match(r'\s*(?P<mod>\w+)\.(?P<attr>\w+)\s*', s)
            if not match:
                raise RuntimeError

            mod = sys.modules.get(match.group('mod'))
            if mod is None:
                raise RuntimeError
            try:
                value = getattr(mod, match.group('attr'))
            except AttributeError:
                raise RuntimeError

This code is equivalent to the previous code and doesn't use eval() at all for evaluating non-trivial defaults.

sys.modules is a plain dict. Copying a dict will be faster than regular expression.

Depends on how many modules you have in your application. But your solution to copy it once works for me.