def str_join(arr, sep):
return _na_map(sep.join, arr)<EOL>
Join lists contained as elements in the Series/Index with passed delimiter.

If the elements of a Series are lists themselves, join the content of these
lists using the delimiter passed to the function.
This function is an equivalent to :meth:`str.join`.

Parameters
----------
sep : str
    Delimiter to use between list entries.

Returns
-------
Series/Index: object
    The list entries concatenated by intervening occurrences of the
    delimiter.

Raises
-------
AttributeError
    If the supplied Series contains neither strings nor lists.

See Also
--------
str.join : Standard library version of this method.
Series.str.split : Split strings around given separator/delimiter.

Notes
-----
If any of the list items is not a string object, the result of the join
will be `NaN`.

Examples
--------
Example with a list that contains non-string elements.

>>> s = pd.Series([['lion', 'elephant', 'zebra'],
...                [1.1, 2.2, 3.3],
...                ['cat', np.nan, 'dog'],
...                ['cow', 4.5, 'goat'],
...                ['duck', ['swan', 'fish'], 'guppy']])
>>> s
0        [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