I suspect there's a lot of discussion to be had around this topic, so I want to get it started. There are some fairly drastic ideas here and I need help figuring out whether the impact outweighs the value.
Some background: within the Windows API, the preferred encoding is UTF-16. This is a 16-bit format that is typed as wchar_t in the APIs that use it. These APIs are generally referred to as the *W APIs (because they have a W suffix).
There are also (broadly deprecated) APIs that use an 8-bit format (char), where the encoding is assumed to be "the user's active code page". These are *A APIs. AFAIK, there are no cases where a *A API should be preferred over a *W API, and many newer APIs are *W only.
In general, Python passes byte strings into the *A APIs and text strings into the *W APIs.
Right now, sys.getfilesystemencoding() on Windows returns "mbcs", which translates to "the system's active code page". As this encoding generally cannot represent all paths on Windows, it is deprecated and Unicode strings are recommended instead. This, however, means you need to write significantly different code between POSIX (use bytes) and Windows (use text).
ISTM that changing sys.getfilesystemencoding() on Windows to "utf-8" and updating path_converter() (Python/posixmodule.c; likely similar code in other places) to decode incoming byte strings would allow us to undeprecate byte strings and add the requirement that they *must* be encoded with sys.getfilesystemencoding(). I assume that this would allow cross-platform code to handle paths similarly by encoding to whatever the sys module says they should and using bytes consistently (starting this thread is meant to validate/refute my assumption).
(Yes, I know that people on POSIX should just change to using Unicode and surrogateescape. Unfortunately, rather than doing that they complain about Windows and drop support for the platform. If you want to keep hitting them with the stick, go ahead, but I'm inclined to think the carrot is more valuable here.)
Similarly, locale.getpreferredencoding() on Windows returns a legacy value - the user's active code page - which should generally not be used for any reason. The one exception is as a default encoding for opening files when no other information is available (e.g. a Unicode BOM or explicit encoding argument). BOMs are very common on Windows, since the default assumption is nearly always a bad idea.
Making open()'s default encoding detect a BOM before falling back to locale.getpreferredencoding() would resolve many issues, but I'm also inclined towards making the fallback utf-8, leaving locale.getpreferredencoding() solely as a way to get the active system codepage (with suitable warnings about it only being useful for back-compat). This would match the behavior that the .NET Framework has used for many years - effectively, utf_8_sig on read and utf_8 on write.
Finally, the encoding of stdin, stdout and stderr are currently (correctly) inferred from the encoding of the console window that Python is attached to. However, this is typically a codepage that is different from the system codepage (i.e. it's not mbcs) and is almost certainly not Unicode. If users are starting Python from a console, they can use "chcp 65001" first to switch to UTF-8, and then *most* functionality works (input() has some issues, but those can be fixed with a slight rewrite and possibly breaking readline hooks).
It is also possible for Python to change the current console encoding to be UTF-8 on initialize and change it back on finalize. (This would leave the console in an unexpected state if Python segfaults, but console encoding is probably the least of anyone's worries at that point.) So I'm proposing actively changing the current console to be Unicode while Python is running, and hence sys.std[in|out|err] will default to utf-8.
So that's a broad range of changes, and I have little hope of figuring out all the possible issues, back-compat risks, and flow-on effects on my own. Please let me know (either on-list or off-list) how a change like this would affect your projects, either positively or negatively, and whether you have any specific experience with these changes/fixes and think they should be approached differently.
To summarise the proposals (remembering that these would only affect Python 3.6 on Windows):
* change sys.getfilesystemencoding() to return 'utf-8' * automatically decode byte paths assuming they are utf-8 * remove the deprecation warning on byte paths * make the default open() encoding check for a BOM or else use utf-8 * [ALTERNATIVE] make the default open() encoding check for a BOM or else use sys.getpreferredencoding() * force the console encoding to UTF-8 on initialize and revert on finalize
So what are your concerns? Suggestions?