docs: def check_partitioners(partitioners, keys):
    """Checks the given partitioners.
  
    This checks that `partitioners` is a dictionary that only contains keys in
    `keys`, and furthermore the entries in `partitioners` are functions or
    further dictionaries (the latter used, for example, in passing partitioners
    to modules inside modules) that must satisfy the same constraints.
  
    Args:
      partitioners: Dictionary of partitioners (allowing nested dictionaries) or
          None.
      keys: Iterable of valid keys for `partitioners`.
  
    Returns:
      Checked dictionary of partitioners. If `partitioners=None`, an empty
      dictionary will be returned.
  
    Raises:
      KeyError: If an partitioner is provided for a key not in `keys`.
      TypeError: If a provided partitioner is not a callable function, or
        `partitioners` is not a Mapping.
    """
    if partitioners is None:
      return {}
    _assert_is_dictlike(partitioners, valid_keys=keys)
  
    keys = set(keys)
  
    if not set(partitioners) <= keys:
      extra_keys = set(partitioners) - keys
      raise KeyError(
          "Invalid partitioner keys {}, partitioners can only "
          "be provided for {}".format(
              ", ".join("'{}'".format(key) for key in extra_keys),
              ", ".join("'{}'".format(key) for key in keys)))
  
    _check_nested_callables(partitioners, "Partitioner")
  
    return partitioners