AFAIU, the change for https://bugs.python.org/issue19764 broke the following usage of subprocess on Windows (re-using a subprocess.STARTUPINFO instance to hide the command window):

    import os, subprocess

    STARTUPINFO = subprocess.STARTUPINFO()
    STARTUPINFO.dwFlags |= subprocess.STARTF_USESHOWWINDOW
    STARTUPINFO.wShowWindow = subprocess.SW_HIDE

    # raises OSError: [WinError 87]
    # in the second loop iteration starting with Python 3.7
    for i in range(2):
        print(i)
        with open(os.devnull, 'w') as stderr:
            subprocess.check_call(['attrib'], stderr=stderr,
                                  startupinfo=STARTUPINFO)

AFAICT, this works on Python 2.7, 3.4, 3.5, and 3.6

I think the documentation in

https://docs.python.org/3/library/subprocess.html#windows-popen-helpers

does not mention that every Popen call should be done with a fresh instance, so either the documentation needs to be changed, or the implementation (e.g. by deep-copying the instance).

See also https://bugs.python.org/issue19764#msg320784

New changeset 483422f57e5d8c8bf8820fec29fc9b96bb15d4ef by Victor Stinner in branch 'master':
bpo-34044: subprocess.Popen copies startupinfo (GH-8090)
https://github.com/python/cpython/commit/483422f57e5d8c8bf8820fec29fc9b96bb15d4ef

New changeset 29be3bd3c9aed0190e60096a603120cacda82375 by Victor Stinner in branch '3.7':
bpo-34044: subprocess.Popen copies startupinfo (GH-8090) (GH-8121)
https://github.com/python/cpython/commit/29be3bd3c9aed0190e60096a603120cacda82375

Sebastian Bank: Thank you for your bug report and your example! I used your example to write an unit test. I fixed the bug in 3.7 and master branches. The fix will be part of the future 3.7.1 release.

Perfect, thanks for the quick fix.