def METHOD_NAME(cls, source):
    if _isNonStrSequence(source):
        assert len(source) > 0, f"{cls} needs at least format from {source}"
        fmt, remainder = source[0], source[1:]
    elif isinstance(source, collections.abc.Mapping):
        assert "Format" in source, f"{cls} needs at least Format from {source}"
        remainder = source.copy()
        fmt = remainder.pop("Format")
    else:
        raise ValueError(f"Not sure how to populate {cls} from {source}")
    assert isinstance(
        fmt, collections.abc.Hashable
    ), f"{cls} Format is not hashable: {fmt!r}"
    assert fmt in cls.convertersByName, f"{cls} invalid Format: {fmt!r}"
    return fmt, remainder