The documentation says that the lib opens files in 'w' mode (which is text). I don't see any mention of the encoding, though. Not only is there no explicit default, there isn't even a documented way to choose an encoding. After a long look I finally found a trail showing that one can, in fact, pass the encoding parameter: the passthrough atomic_write(**cls_kwargs) → AtomicWriter(**open_kwargs) (the latter never mentioned).

The problem with that? Well, most people will probably assume it's UTF-8, but instead it seems to be getdefaultencoding (not even sure, it's hard to find). It seems like that will help to perpetuate the problems described at https://www.python.org/dev/peps/pep-0597/#using-the-default-encoding-is-a-common-mistake
So there are 2 (3?) issues I see:

• The default encoding is implicit.
  • It's not documented that it's implicit.
• A way to choose an encoding is not shown.