@staticmethod
def transform_to_tuple(items):
    """Transform each item to a tuple if it's not one

        :argument items: List of items and tuples
        :type items: iterable

        :returns: List of tuples
        :rtype: iterable of tuple


        >>> list(OptionalPartList.transform_to_tuple([
        ...   '<1>',
        ...   (None, '<2>')
        ... ]))
        [(None, '<1>'), (None, '<2>')]

        """
    for item in items:
        if not isinstance(item, tuple):
            yield None, item
        else:
            yield item
