In py3k, subprocess uses _posixsubprocess.fork_exec() function. This function uses surrogateescape error handler for most arguments, but not for the current working directory (cwd).

Attached patch uses PyUnicode_FSConverter() as done for other arguments. I don't know if PyUnicode_FSConverter() result is always a PyBytes, so I added an assertion. It should be fixed.

See also #8391.

New patch supporting bytearray type (remove the buggy assertion).

Commited in py3k (r80135), blocked in 3.1 (r80136).

Python 3.1 has no _posixsubprocess module, it uses os.chdir() which accepts bytes, bytearray and str with surrogates.

It does not work on Windows:

>>> subprocess.Popen("c:/windows/notepad.exe", cwd=b'c:/temp')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "D:\afa\python\py3k-1\lib\subprocess.py", line 681, in __init__
    restore_signals, start_new_session)
  File "D:\afa\python\py3k-1\lib\subprocess.py", line 892, in _execute_child
    startupinfo)
TypeError: must be string or None, not bytes

The function sp_CreateProcess() in PC/_subprocess.c should probably be fixed.
And please add unit tests...

> And please add unit tests...

I'm thinking on this. I plan to write tests for all my last changes about surrogates.

> It does not work on Windows

I always consider Windows as a special case because Windows uses unicode internally. Byte string are converted quickly to unicode using the current locale.

My patch was for UNIX/BSD which uses byte string internally.

sp_CreateProcess() in PC/_subprocess.c uses CreateProcessW. To support byte string, we should use the byte string version of CreateProcess ("CreateProcessA" ?) or convert current directory to unicode (using the current locale). The second solution minimize the changes.

PEP 277 explicitly states that unicode strings should be passed to wide-character functions, whereas byte strings use "standard" functions.
This is done in posixmodule.c, for example.

The "current locale" is a moving thing.

> PEP 277 explicitly states that unicode strings should be passed to
>  wide-character functions, whereas byte strings use "standard"
> functions. This is done in posixmodule.c, for example.

CreateProcessW takes a lot of arguments. Should we CreateProcessA or CreateProcessW if some arguments are byte string and other are unicode string?

> The "current locale" is a moving thing.

Don't CreateProcessA do the same thing? Convert byte string to unicode using the current locale. To use the right words, by "current locale" I mean the "mbcs" Windows codec.

"mbcs" is not a fixed encoding and may change between Windows sessions, see the Rationale in PEP277 http://www.python.org/dev/peps/pep-0277/

The mixed case is interesting. We could use CreateProcessW when at least one string is Unicode, and CreateProcessA when all strings are bytes.
OTOH this is unlikely, because for example the environment will be passed as unicode; so I'm OK to use CreateProcessW in all cases, and use mbcs to convert bytes.

Amaury, I fail to see how the error you get on Windows is related to this issue. AFAICT, neither the issue nor the patch affects Windows at all.

Closing the issue as fixed.

If you think there is also an issue on Windows, please submit a new bug report. However, I don't think the error you get is a bug: what you see is correct behavior.