     [lion, elephant, zebra]
        1                [1.1, 2.2, 3.3]
        2                [cat, nan, dog]
        3               [cow, 4.5, goat]
        4    [duck, [swan, fish], guppy]
        dtype: object

        Join all lists using a '-'. The lists containing object(s) of types other
        than str will produce a NaN.

        >>> s.str.join('-')
        0    lion-elephant-zebra
        1                    NaN
        2                    NaN
        3                    NaN
        4                    NaN
        dtype: object
        )