Splits a string in the format "a,b:c,d,e:f" into
    [('a', 'b'), ('c', 'd', 'e'), ('f', )]

def lists_from_env(key, default=""):
    """
    Splits a string in the format "a,b:c,d,e:f" into
    [('a', 'b'), ('c', 'd', 'e'), ('f', )]
    """
    try:
        val = os.environ.get(key, default)
        lists = val.split(":")
        return [i.split(',') for i in lists]
    except (KeyError, ValueError):
        return []