A major, breaking issue[1] was how Python 3 handled missing LC_\* variables. By ...

A major, breaking issue[1] was how Python 3 handled missing LC_* variables. By default, a Python script would simply crash with any non-ASCII input or output. It was fixed[2] in 3.5[3].

[1]: https://bugs.python.org/issue19977

[2]: https://www.python.org/dev/peps/pep-0540/

[3]: https://docs.python.org/release/3.5.0/whatsnew/3.5.html