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Python 3.2 has now its os.environb, the bytes version of os.environ. subprocess should get a new envb argument to be able to use pure bytes environmental variables. Examples:
 subprocess.call([b'env], envb={b'PATH': b'/usr/bin'})
and
 envb = os.environb.copy()
 envb[b'PATH'] = b'/usr/bin'
 subprocess.call([b'env], envb=envb)
Specify both env and envb would raise an exception. envb should only be available on POSIX (as os.environb).
Related issues: #8513 (subprocess: support bytes program name) and #8603 (os.environb).

Why wouldn't you give byte variables in env too?

> Why wouldn't you give byte variables in env too?
The problem with the canonicalization to bytes is to choice of the preferred type. Eg. env={'PATH': 'a', b'PATH': b'b'}: should we use 'a', 'b' or raise an error?
subprocess does already convert environ keys and values to bytes on Unix (in subprocess._execute_child() if _posixsubprocess module is present, and in posix_execve()).
os.get_exec_path() should also support b'PATH' key.

pitou> Why wouldn't you give byte variables in env too?
Ok, attached patch canonicalize env keys and values to bytes. If a variable is defined twice (str name, bytes name), a ValueError is raised.
The patch depends on #8513: it requires that os.get_exec_path() is patched to support b'PATH' key.

Python 3.1 accepts duplicate variables (str name, bytes name). It creates the two variables is a random order:
>>> subprocess.call(['env'], env={'xx': 'str', b'xx': 'bytes'})
xx=str
xx=bytes
0
>>> subprocess.call(['env'], env={'xxx': 'str', b'xxx': 'bytes'})
xxx=bytes
xxx=str
0

os.exeve() and os.exevpe() should also canonicalize env to bytes.
os.exeve() and os.exevpe(), but not os._exevpe() to avoid doing it twice (once in subprocess, once in os._exevpe). Patch os._exevpe() is not enough because subprocess doesn't call it on Unix if _subprocessposix module is present.

My patch to fix #8513 does also fix the examples of this issue and so I think that the canonicalization is no more needed. I will only reopen the issue if I find a good reason to apply the patch :-)