docs: def namespace(self, namespace, key_transform=lambda key: key):
          """
          Return a copy with only the keys from a given namespace.
  
          The common prefix will be removed in the returned dict. Example::
  
              >>> from configloader import ConfigLoader
              >>> config = ConfigLoader(
              ...     MY_APP_SETTING1='a',
              ...     EXTERNAL_LIB_SETTING1='b',
              ...     EXTERNAL_LIB_SETTING2='c',
              ... )
              >>> config.namespace('EXTERNAL_LIB')
              ConfigLoader({'SETTING1': 'b', 'SETTING2': 'c'})
  
          :arg namespace: Common prefix.
          :arg key_transform: Function through which to pass each key when
              creating the new dictionary.
  
          :return: New config dict.
          :rtype: :class:`ConfigLoader`
          """
          namespace = namespace.rstrip('_') + '_'
          return ConfigLoader(
              (key_transform(key[len(namespace):]), value)
              for key, value in self.items()
              if key[:len(namespace)] == namespace
          )