Thank you for the references, changing the encoding of an open stream sounds interesting, but It's targeted for v. 3.6 so It seems a bit far away.

Anyway, as I said I fixed on my side with monkey-patching on the third-party lib, the broken locale is just the context, just to explain how I got this issue, I opened this because I honestly though that could be better to return the fallback value that raise an exception, but you know better the "big picture" so, if It's better as is, It's ok for me.

BTW, I don't find the opened issue to use utf8 as default instead ascii (except for Windows)