DeprecationWarning at compile time is output multiple times to the stderr (on console from which IDLE was ran), but not to IDLE Shell window.
For example, when enter '\e':
$ ./python -m idlelib
Warning (from warnings module):
File "<pyshell#0>", line 1
'\e'
DeprecationWarning: invalid escape sequence \e
>>>
Warning (from warnings module):
File "<pyshell#0>", line 1
'\e'
DeprecationWarning: invalid escape sequence \e
>>>
Warning (from warnings module):
File "<pyshell#0>", line 1
'\e'
DeprecationWarning: invalid escape sequence \e
And when close IDLE, additional output is printed in 3.8+:
>>> Exception ignored in: <idlelib.run.PseudoInputFile object at 0x7f9e3b4a15a0>
Traceback (most recent call last):
File "/home/serhiy/py/cpython/Lib/idlelib/run.py", line 488, in close
File "/home/serhiy/py/cpython/Lib/idlelib/pyshell.py", line 1019, in close
File "/home/serhiy/py/cpython/Lib/idlelib/editor.py", line 1058, in close
File "/home/serhiy/py/cpython/Lib/idlelib/outwin.py", line 94, in maybesave
File "/home/serhiy/py/cpython/Lib/idlelib/editor.py", line 991, in get_saved
AttributeError: 'NoneType' object has no attribute 'get_saved'