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Bytes program name problem should be splitted in two parts:
 (a) subprocess.call([b'env']) and subprocess.call([b'env'], env={'PATH': '/usr/bin'}): bytes program and unicode environ
 (b) bytes program and bytes environ
Part (a)
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(a) would benefit from the creation of os(.path).fsencode() function (issue #8514).
Part (b)
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If the creation of os.environb is accepted (#8603), I think that subprocess should also be modified to support pure bytes environ. Mix bytes and unicode variables in env would lead to mojibake and headaches.
So I propose the creation of an "envb" (environment bytes) argument to Popen constructor. (b) would be written as:
 subprocess.call([b'env'], envb={b'PATH': b'/usr/bin'})
or
 envb = os.environb.copy()
 envb[b'PATH'] = b'/usr/bin'
 subprocess.call([b'env'], envb=envb)
(and it will not be possible to define env and envb at the same time)
In this case, we will need a pure bytes version of os.get_exec_path(), and os._execvpe() should have a "bytes environment" mode (add an optional argument).
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I have a patch fixing both problems, parts (a) and (b), but the patch depends on os.environb. I prefer to wait until os.environb is accepted to submit my patch.