When I try to run a really simple script like:
def fun(a):
    print a
    return

I get this error:

Exception in Tkinter callback
Traceback (most recent call last):
  File "C:\Python27\lib\lib-tk\Tkinter.py", line 1541, in __call__
    return self.func(*args)
  File "C:\Python27\lib\idlelib\MultiCall.py", line 166, in handler
    r = l[i](event)
  File "C:\Python27\lib\idlelib\ScriptBinding.py", line 149, in run_module_event

    if PyShell.use_subprocess:
AttributeError: 'module' object has no attribute 'use_subprocess'

It basically means Python is only usable by copy-pasting code into Shell, any kind of script fails with that error.