[2.7] bpo-32640: Clarify the behavior of str.join and unicode object by Mariatta · Pull Request #5333 · python/cpython (original) (raw)

In str.join, if any of the iterable contains a Unicode object,
str.join will return a Unicode object.

A Type error will be raised if iterable contains values other
than a string or Unicode object.

https://bugs.python.org/issue32640