def _split_what(what):
    """
    Returns a tuple of `frozenset`s of classes and attributes.
    """
    return (
        frozenset(cls for cls in what if isinstance(cls, type)),
        frozenset(cls for cls in what if isinstance(cls, str)),
        frozenset(cls for cls in what if isinstance(cls, Attribute)),
    )

    Returns a tuple of `frozenset`s of classes and attributes.
    
product/gradle-plugin/src/main/python/attr/filters.py