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.