When running Python via subprocess with captured output an encoding error occurs attempting to output a Unicode filename.  The same does not happen when just using spawnl().

Python 3.6.5 (v3.6.5:f59c0932b4, Mar 28 2018, 17:00:18) [MSC v.1900 64 bit (AMD64)] on win32
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>>> import os, sys, subprocess
>>> sys.stdout.encoding, sys.stdout.errors
('utf-8', 'surrogateescape')
>>> args = ['-u', '-c', "print('taqdir\\\u0634\u0645\u0627\u0631.py')"]
>>> os.spawnl(os.P_WAIT, sys.executable, '"%s"' % sys.executable, *args)
taqdir\شمار.py
0
>>> subprocess.run([sys.executable, *args], stdout=subprocess.PIPE, stderr=subprocess.STDOUT)
CompletedProcess(args=['C:\\Program Files (x86)\\Microsoft Visual Studio\\Shared\\Python36_64\\python.exe', '-u', '-c', "print('taqdir\\شمار.py')"], returncode=1, stdout=b'Traceback (most recent call last):\r\n  File "<string>", line 1, in <module>\r\n  File "C:\\Program Files (x86)\\Microsoft Visual Studio\\Shared\\Python36_64\\lib\\encodings\\cp1252.py", line 19, in encode\r\n    return codecs.charmap_encode(input,self.errors,encoding_table)[0]\r\nUnicodeEncodeError: \'charmap\' codec can\'t encode characters in position 7-10: character maps to <undefined>\r\n')

Related to issue34421

The interpreter uses the system ANSI codepage for non-console files. In your case this is codepage 1252. In my current setup I can't reproduce this issue since I'm using the new (beta) support in Windows 10 to configure the ANSI codepage as UTF-8 (65001).

You can force standard I/O to use UTF-8 by setting the environment variable PYTHONIOENCODING. Also, if you want the CompletedProcess stdout decoded as text, in 3.6+ you can pass the parameter `encoding='utf-8'`. For example:

    environ = os.environ.copy()
    environ['PYTHONIOENCODING'] = 'utf-8'
    p = subprocess.run([sys.executable, *args], stdout=subprocess.PIPE, stderr=subprocess.STDOUT, env=environ, encoding='utf-8')
    print(p.stdout)