IDLE fails to start or raises when an editor window's file does not exist

Bug report

Bug description:

IDLE raises an uncaught traceback when a file that is open in the editor is deleted or renamed by another program and the IDLE window is then refocused.

EditorWindow.focus_in_event runs on every <FocusIn> event and calls EditorWindow.last_mtime, which calls os.path.getmtime(self.io.filename) with no error handling. When the open file has been removed or renamed, getmtime raises FileNotFoundError, which escapes the Tk event handler and breaks the modified-on-disk reload check.

Reproduction

Open a file in the IDLE editor, delete or rename it in another program, then click back into the IDLE window. A traceback like this reaches the console and the reload check stops working:

FileNotFoundError: [Errno 2] No such file or directory: '.../somefile.py'

The same call reproduced without a GUI:

import os, tempfile, types
from idlelib.editor import EditorWindow

with tempfile.TemporaryDirectory() as d:
    path = os.path.join(d, 'gone.py')
    open(path, 'w').close()
    mtime = os.path.getmtime(path)
    os.remove(path)
    stub = types.SimpleNamespace(io=types.SimpleNamespace(filename=path), mtime=mtime)
    EditorWindow.last_mtime(stub)   # raises FileNotFoundError on the main branch

Suggested fix

Guard os.path.getmtime in last_mtime with try/except OSError and return the last known mtime, so focus_in_event sees no change and does not prompt to reload a file that is gone.

CPython versions tested on:

3.16, CPython main branch

Operating systems tested on:

macOS

Linked PRs

• gh-153481

idlelib.editor の EditorWindow.last_mtime と、その呼び出し元である focus_in_event から始めます。issue にあるファイル欠落のケースを再現し、その後、IDLE に再びフォーカスを移しても例外が発生しないこと、またリロードチェックで削除されたファイルについて確認を求められないことを確認します。