Sys.path[0] (and os.getcwd()) for various startup conditions.
(What gets prepended before .../pythonxy.zip, which is the first file always
present on sys.path, at least on Windows)
Possibilities are '' (=cwd), some user dir, and .../idlelib.
Even with 'idlelib', proper startup can fail because PyShell line 1525
adds the current working directory anyway.

CONSOLE: by default, cwd (current working directory, os.getcwd())
is printed before the '>' prompt and is not changed by commands.

> python:  ''.  This is not changed by 'import idlelib.idle'.

> python -c "import idlelib.idle": ''.

> python -m idlelib: ''.

> python ...idlelib: ...idlelib, I presume.  Runs idlelib.__main__.

> python ...idlelib/idle.py:  ...idlelib.

> python -m idlelib userpath/x.py:  userpath, cwd?.

> python ...idlelib userpath/x.py:  ???.  IDLE starts, run may fail.

Summary: Any method of starting IDLE with python from the console can fail
if cwd contains conflicting .py files. 

ICONS on Windows:

python: '', where cwd is pythondir.  Idlelib import can fail.
3.5.0 changed cwd as C:\windows\system32, but this was changed back
in 3.5.1.

idle: 2,7 uses pythonw -m idlelib.idle.  3.5 uses pythonw .../idlelib.

files: right click, edit with idle.  Similar to editing from console.

TODO: I have not yet investigated any differences from using pythonw.exe
or idlew.exe or idle.bat, but I expect the results to be about the same.
I have also not looked at repository builds.

TOOLS: "import sys; print(sys.path)", "import os; print(os.getcwd())",
and test_path.py:

import sys
for dir in sys.path:
   print(dir)
import os

print('\ncwd', os.getcwd())

Also need to print from within idle.py or PyShell.py.
This is harder when there is no console.