In the first section of the doc for site.py, after

"After these path manipulations, an attempt is made to import a module named sitecustomize, which can perform arbitrary site-specific customizations. It is typically created by a system administrator in the site-packages directory. If this import fails with an ImportError exception, it is silently ignored."

I propose to add (something like)

"If python is started without output streams available, as with pythonw on Windows (used by default to start Idle), attempted output from sitecustomize is ignored. Any exception other that ImportError causes a silent and perhaps mysterious failure of the process."

This issue was stimulated by someone asking on python-list about print() outout appearing when starting the console interpreter but not when starting Idle (on Windows).  I then tested the result of an excecption other than ImportError. At the console, nothing visible happens until a new prompt appears.

C:\Programs\Python34>pythonw -m idlelib.idle

C:\Programs\Python34>

If Idle is already running, the attempt to run a file with F5 gives a messages about not being able to connect to the subprocess (which silently failed).

Updated documentation using Terry Reedy's suggested addition.

New changeset 79a4560a702f by Terry Jan Reedy in branch '2.7':
Closes #21026: Augment site doc based on experiments. Patch by Carol Willing.
http://hg.python.org/cpython/rev/79a4560a702f

New changeset 3fef95842314 by Terry Jan Reedy in branch '3.4':
Issue #21026: Augment site doc based on experiments. Patch by Carol Willing.
http://hg.python.org/cpython/rev/3fef95842314