Following this post instructions:How to make Unicode charset in cmd.exe by default?

Its possible to bypass this encoding problem

import subprocess
output = subprocess.check_output("chcp 65001 | powershell \"Get-ChildItem -LiteralPath 'HKLM:SOFTWARE\\\\Microsoft\\\\Windows\\\\CurrentVersion\\\\Uninstall' -ErrorAction 'Stop' -ErrorVariable '+ErrorUninstallKeyPath'\"", shell=True, stderr=subprocess.STDOUT)

universal_newlines=True enables text mode. Combined with stdout=PIPE, it forces decoding of the child process' output using locale.getpreferredencoding(False) that is not utf-8 on Windows. That is why you see UnicodeDecodeError.

To read the subprocess' output using utf-8 encoding, drop universal_newlines=True:

#!/usr/bin/env python3
from subprocess import Popen, PIPE

with Popen(r'C:\path\to\program.exe "arg 1" "arg 2"',
           stdout=PIPE, stderr=PIPE) as p:
    output, errors = p.communicate()
lines = output.decode('utf-8').splitlines()

str.encode("utf-8") is equivalent to "utf-8".encode(). There is no point to pass it to .communicate() unless you set stdin=PIPE and the child process expects b'utf-8' bytestring as an input.

str.encode(encoding="utf-8", errors="ignore) has the form klass.method(**kwargs). .encode() method expects self (a string object) that is why you see TypeError.

>>> str.encode("abc", encoding="utf-8", errors="ignore") #XXX don't do it
b'abc'
>>> "abc".encode(encoding="utf-8", errors="ignore")
b'abc'

Do not use klass.method(obj) instead of obj.method() without a good reason.

(Answering own question hoping it could be helpfull to others)

I made a short test program. This is what I have found:

1. File system encoding is the key point.
2. Monkey patching does not work. Well, that's OK. It is not acceptable as a solution anyway.
3. LANG=C.UTF-8 requires the locale installed and it was not on my system (checked with locale -a). But on a second system where it was available, it worked.

4. I can make the encoding explicitly and pass bytes as one of the args:

   cmdresult = sub.run(
       [SCRIPT, tid, days, name.encode('utf-8')],
       ...
   

This works, but one question remianed:

Does it comply with the docs?

All I could find is:

args should be a sequence of program arguments or else a single string

And I did understand it as one string or a list of strings, but actually it does not specify a list of what types. I passed also and int to see what will happen. I got this error:

expected str, bytes or os.PathLike object

So my solution seems to be fine.

In Python 3.3+ you can separately indicate that you expect text in a particular encoding. The keyword argument universal_newlines=True was renamed in 3.7 to the more accurate and transparent text=True.

This keyword basically says "just use whatever encoding is default on my system" (so basically UTF-8 on anything reasonably modern except on Windows, where you get some Cthulhu atrocity from the abyss the system's default code page).

In the absence of this keyword, subprocesses receive and return bytes in Python 3.

Of course, if you know the encoding, you can also separately .decode() the bytes you get back.

If you know the encoding it's probably useful to use the encoding= keyword argument (even if you assume it is also the system encoding; this was added in Python 3.6).

response = subprocess.check_output([...], text=True)
response = subprocess.check_output([...], encoding='utf-8')
response = subprocess.check_output([...]).decode('utf-8')