At some point the Python 3 documentation of str.join has been copied to Python 2. This includes the sentence "A TypeError will be raised if there are any non-string values in iterable, including bytes objects." The second half of this sentence is wrong for Python 2. Not only is there no "bytes" type in Python 2, but join() in this version will happily join any mixture of unicode and (byte-)str objects, producing unicode output if any of the items (or separator) were unicode. https://docs.python.org/2/library/stdtypes.html#str.join
GH-5333 clarifies that str.join can return a Unicode object if iterable contains a Unicode object, and that it will raise TypeError if there is any non string or non unicode object.
