[docs] [issue29829] Documentation lacks clear warning of subprocess issue with pythonw

[Steve Barnes]

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 at python
components: Documentation
messages: 289722
nosy: Steve Barnes, docs at 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

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue29829>
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