> Does subprocess work from inside Windows GUI applications?

You mean: Python embedded in another program? I don't know. How can I test that?

I never see any warning in subprocess documentation saying that subprocess doesn't work on Windows in some cases. subprocess uses CreateProcessW() (by _subprocess.CreateProcess).

I suppose that CreateProcessW() can always be used.

Note: subprocess does still support Windows 9x. But there is an issue to drop this support: #2405 (Drop w9xpopen and all dependencies).