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One issue I've encountered is someone else's software setting PYTHONPATH to their install directory of python. We have some old software that installs and uses python 2.3 scripts and unfortunately this prevents the IDLE shortcuts for newer versions of python from working correctly on the system. This could be remedied with -E being added to the shortcut, but windows doesn't allow me to directly edit the IDLE shortcut, and it hides how IDLE is being launched via Python in the first place, making it a challenge to create my own. I'd remove PYTHONPATH from the system environment variables except I don't know why it's set in the first place.
Suggestion: IDLE safe-mode which runs with -E OR include a batch file for IDLE so that I can easily append my own command line arguments and create my own shortcuts.

I'm guessing that 'batch file' version of this request is answered by the new 'py' launcher command in Python3, which can also be used with other versions of Python. The obscurity of configuring things on Windows isn't directly a Python problem, but perhaps there are docs we could add somewhere?

Look in the registry at "Computer\HKEY_CLASSES_ROOT\Python.File\shell\" and then inspect the launch commands under "open\command" and "Edit with IDLE\command".

Something on Windows configuration here https://docs.python.org/3/using/windows.html#excursus-setting-environment-variables 

There already is an idlelib/idle.bat that starts the corresponding python with
set CURRDIR=%~dp0
start "IDLE" "%CURRDIR%..\..\pythonw.exe" "%CURRDIR%idle.pyw" %1 %2 %3 %4 %5 %6 %7 %8 %9
Arguments added to the .bat command are passed on. So the enhancement requested already exists.
C:\Programs>python34\lib\idlelib\idle.bat, for instance, starts 3.4, and changing '34' to '33' or '27' starts 3.3 or 2.7.
python -E ignores PYTHON* environment variables. Unfortunately, this seems to prevent python from running idle. Adding -E to the above or directly to
C:\Programs\Python34>pythonw.exe Lib/idlelib/idle.pyw -E
results in a new prompts with no visible action. Since I do not have any PYTHON* vars to ignore, this puzzle me. Does this indicate a bug in the startup handling of -E? (Nick?)
Except for this puzzle, I would close this as a third party issue.

`idle.pyw -E` passes an invalid argument, and pythonw.exe doesn't inherit the console to print the usage text to stderr. The -E option needs to be passed to the interpreter:
 C:\Python34>pythonw.exe -E Lib/idlelib/idle.pyw
Or run idlelib as a script:
 pyw -3 -Em idlelib

Thanks for the correction.