New submission from Steve Barnes: When running under pythonw, or pyinstaller with the -w flag, modules that use subprocess calls such as popen, run, etc. will crash if the default `stdout=None, stderr=None` behaviour is used rather than PIPE. This is an obscure problem which is very hard to debug yet there is no warning in the documentation on this. I would like to suggest adding a :warning:`stdout=None, stderr=None` must not be used in any of the calls in this module when running under pythonw due to the lack of sys.stdout & sys.stderr in that case. Please use `stdout=PIPE, stderr=PIPE` instead. A patch against the default branch would be: diff -r 4243df51fe43 Doc/library/subprocess.rst --- a/Doc/library/subprocess.rst Fri Feb 10 14:19:36 2017 +0100 +++ b/Doc/library/subprocess.rst Thu Mar 16 16:56:24 2017 +0000 @@ -33,6 +33,13 @@ function for all use cases it can handle. For more advanced use cases, the underlying :class:`Popen` interface can be used directly. +.. warning:: Do not use default parameters on Windows with pythonw. + + As pythonw deletes `sys.stdout` & `sys.stderr` the use of the default + parameters, `stdout=None, stderr=None,`, which defaults to being + `stdout=sys.stdout, stderr=sys.stderr,` may cause unexpected crashes + it is recommended to use `stdout=PIPE, stderr=PIPE,` instead. + The :func:`run` function was added in Python 3.5; if you need to retain compatibility with older versions, see the :ref:`call-function-trio` section. ---------- assignee: docs@python components: Documentation messages: 289722 nosy: Steve Barnes, docs@python priority: normal severity: normal status: open title: Documentation lacks clear warning of subprocess issue with pythonw type: behavior versions: Python 2.7, Python 3.6, Python 3.7 _______________________________________ Python tracker http://bugs.python.org/issue29829 _______________________________________