code: def build(self, validator, namespace, config_key, default, help):
          
          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)
docs: Build or retrieve a ValueProxy from the attributes. Proxies are
          keyed using a repr because default values can be mutable types.