code: def extract_value(proxy):
      
      value = proxy.namespace.get(proxy.config_key, proxy.default)
      if value is UndefToken:
          raise errors.ConfigurationError("%s is missing value for: %s" %
              (proxy.namespace, proxy.config_key))
  
      try:
          return proxy.validator(value)
      except errors.ValidationError as e:
          raise errors.ConfigurationError("%s failed to validate %s: %s" %
              (proxy.namespace, proxy.config_key, e))
docs: Given a value proxy type, Retrieve a value from a namespace, raising
      exception if no value is found, or the value does not validate.