AFAIU, the change forhttps://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 3.6I think the documentation inhttps://docs.python.org/3/library/subprocess.html#windows-popen-helpersdoes 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 alsohttps://bugs.python.org/issue19764#msg320784

New changeset483422f57e5d8c8bf8820fec29fc9b96bb15d4ef by Victor Stinner in branch 'master':bpo-34044: subprocess.Popen copies startupinfo (GH-8090)https://github.com/python/cpython/commit/483422f57e5d8c8bf8820fec29fc9b96bb15d4ef

New changeset29be3bd3c9aed0190e60096a603120cacda82375 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.