[Python-Dev] Emit a BytesWarning on bytes filenames on Windows

[Victor Stinner]

Hi,

I am not more conviced that raising a UnicodeEncodeError on unencodable 
characters is the right fix for the issue #13247. The problem with this 
solution is that you have to wait until an user get a UnicodeEncodeError.

I have yet another proposition: emit a warning when a bytes filename is used. 
So it doesn't affect the default behaviour, but you can use -Werror to test if 
your program is fully Unicode compliant on Windows (without having to test 
invalid filenames).

I don't know if a BytesWarning or a DeprecationWarning is more apropriate. It 
depends if we plan to drop support of bytes filenames on Windows later (in 
Python 3.5 or later).

List of impacted functions:

os._getfinalpathname(bytes)
os._getfullpathname(bytes)
os._isdir(bytes)
os.access(bytes)
os.chdir(bytes)
os.chmod(bytes)
os.getcwdb()
os.link(bytes, bytes)
os.listdir(bytes)
os.lstat(bytes)
os.mkdir(bytes)
os.readlink(bytes)
os.rename(bytes, bytes)
os.rmdir(bytes)
os.stat(bytes)
os.symlink(bytes, bytes)
os.unlink(bytes)
os.utime(bytes, time)

Note: Unicode filenames are not affected by this change. For example, 
os.listdir(str) will not emit any warning.

Victor