def build(self, validator, namespace, config_key, default, help):
        """Build or retrieve a ValueProxy from the attributes. Proxies are
        keyed using a repr because default values can be mutable types.
        """
        proxy_attrs = validator, namespace, config_key, default
        proxy_key = repr(proxy_attrs)
        if proxy_key in self.proxies:
            return self.proxies[proxy_key]

        value_proxy = proxy.ValueProxy(*proxy_attrs)
        register_value_proxy(namespace, value_proxy, help)
        return self.proxies.setdefault(proxy_key, value_proxy)
Build or retrieve a ValueProxy from the attributes. Proxies are
        keyed using a repr because default values can be mutable types.