def hash_from_func(cls, func):
        """Return a hashlib-compatible object for the multihash `func`.

        If the `func` is registered but no hashlib-compatible constructor is
        available for it, `None` is returned.  If the `func` is not
        registered, a `KeyError` is raised.

        >>> h = FuncReg.hash_from_func(Func.sha2_256)
        >>> h.name
        'sha256'
        """
        new = cls._func_hash[func].new
        return new() if new else None