docs: def extract_subkeys(kwargs, prefix, defaults=None):
      """
      Extract mappings of the shape '<base_key>__<sub_key>' to new mappings under '<sub_key>'.
  
      >>> foo = {
      ...     'foo__foo': 1,
      ...     'foo__bar': 2,
      ...     'baz': 3,
      ... }
      >>> assert extract_subkeys(foo, 'foo', defaults={'quux': 4}) == {
      ...     'foo': 1,
      ...     'bar': 2,
      ...     'quux': 4,
      ... }
  
      :type kwargs: dict
      :rtype: dict
      """
  
      prefix += '__'
      result = {k[len(prefix):]: v for k, v in kwargs.items() if k.startswith(prefix)}
      if defaults is not None:
          return setdefaults(result, defaults)
      else:
          return result