def distance_rel(tuple1, tuple2):
    """
    Calculates the relative distance of two tuples, returns values between 0.0 and +inf.
    
    >>> distance_rel(('a'), ())
    inf
    
    >>> distance_rel(('a'), ('a'))
    0.0
    
    >>> distance_rel(['a', 'b'], ('b', 'c'))
    2.0

    >>> distance_rel(['a', 'b'], ('c', 'd'))
    inf
    """
    intersectCount = len(set(tuple1).intersection(set(tuple2)))
    if (intersectCount == 0.0):
        return float("+inf")
    else:
        return (0.0 + len(set(tuple1).symmetric_difference(set(tuple2)))) / (float(intersectCount))