> For platform semantics, I'd prefer a link to the CreateProcessW docs, 
> with advice to read about the lpApplicationName parameter with 
> respect to `executable` and lpCommandLine with respect to `args` 
> and the platform search semantics. 

For example, let's help someone figure out that because "python3.8" has a ".8" filename 'extension', ".exe" may or may not be appended in a search.

    >>> print(shutil.which('python3.8'))
    C:\Users\someone\AppData\Local\Microsoft\WindowsApps\python3.8.EXE

    >>> subprocess.call(['python3.8.exe', '-V'])
    Python 3.8.6
    0

SearchPathW (called internally by CreateProcessW) won't append the ".exe" default extension to a name that already has a ".8" extension:

    >>> try: subprocess.call(['python3.8', '-V'])
    ... except OSError as e: print(e)
    ...
    [WinError 2] The system cannot find the file specified

But with shell=True it works because CMD always appends the PATHEXT extensions (thankfully there isn't a "python3.8.com" file to get in the way, since .COM is usually listed before .EXE in PATHEXT):

    >>> subprocess.call('python3.8 -V', shell=True)
    Python 3.8.6
    0

SearchPathW does append the default ".exe" extension for a qualified path:

    >>> subprocess.call([r'C:\Users\someone\AppData\Local\Microsoft\WindowsApps\python3.8', '-V'])
    Python 3.8.6
    0