def pseudonymize(
        self, key_file: KeyFile, identifiers: List["Identifier"]
) -> List["Key"]:
    """Get a pseudonym for each identifier. If identifier is known in PIMS,
    return this. Otherwise, have PIMS generate a new pseudonym and return that.
    Parameters
    ----------
    identifiers: List[Identifier]
        The identifiers to get pseudonyms for
    key_file: KeyFile
        The key_file to use
    Notes
    -----
    Each call this function calls PIMS API twice for each unique source in
    identifiers. This is result of the way the API can be called
    Returns
    -------
    List[Key]
        The PIMS pseudonym for each identifier
    """
    keys = []
    # Each call to process a list of identifiers only allows a single source.
    # Split identifiers by source
    per_source = defaultdict(list)
    for x in identifiers:
        per_source[x.source].append(x)
    for source, items in per_source.items():
        keys = keys + self.deidentify(key_file, [x.value for x in items], source)

    return keys