`AssertionError`: `assert _xterm_term_title_saved`

In some (unknown) situation, it is possible that the `_xterm_term_title_saved` is unset,
but the code would make a call to `_restore_term_title_xterm`, resulting in `AssertionError`.

At least on replicatable reproduction is
returning from `ipython` to `pudb` via `^D^D` on an empty cell.

See more details in https://github.com/ipython/ipython/pull/14480

Signed-off-by: Stavros Ntentos <[e-mail]>

IPython/utils/terminal.py

@@ -1,125 +1,131 b''
 # encoding: utf-8
 """
 Utilities for working with terminals.
 
 Authors:
 
 * Brian E. Granger
 * Fernando Perez
 * Alexander Belchenko (e-mail: bialix AT ukr.net)
 """
 
 # Copyright (c) IPython Development Team.
 # Distributed under the terms of the Modified BSD License.
 
 import os
 import sys
 import warnings
 from shutil import get_terminal_size as _get_terminal_size
 
 # This variable is part of the expected API of the module:
 ignore_termtitle = True
 
 
 
 if os.name == 'posix':
     def _term_clear():
         os.system('clear')
 elif sys.platform == 'win32':
     def _term_clear():
         os.system('cls')
 else:
     def _term_clear():
         pass
 
 
 
 def toggle_set_term_title(val):
     """Control whether set_term_title is active or not.
 
     set_term_title() allows writing to the console titlebar.  In embedded
     widgets this can cause problems, so this call can be used to toggle it on
     or off as needed.
 
     The default state of the module is for the function to be disabled.
 
     Parameters
     ----------
     val : bool
         If True, set_term_title() actually writes to the terminal (using the
         appropriate platform-specific module).  If False, it is a no-op.
     """
     global ignore_termtitle
     ignore_termtitle = not(val)
 
 
 def _set_term_title(*args,**kw):
     """Dummy no-op."""
     pass
 
 
 def _restore_term_title():
     pass
 
 
 _xterm_term_title_saved = False
 
 
 def _set_term_title_xterm(title):
     """ Change virtual terminal title in xterm-workalikes """
     global _xterm_term_title_saved
     # Only save the title the first time we set, otherwise restore will only
     # go back one title (probably undoing a %cd title change).
     if not _xterm_term_title_saved:
         # save the current title to the xterm "stack"
         sys.stdout.write("\033[22;0t")
         _xterm_term_title_saved = True
     sys.stdout.write('\033]0;%s\007' % title)
 
 
 def _restore_term_title_xterm():
     # Make sure the restore has at least one accompanying set.
     global _xterm_term_title_saved
-    assert _xterm_term_title_saved
+    if not _xterm_term_title_saved:
+        warnings.warn(
+            "Expecting xterm_term_title_saved to be True, but is not; will not restore terminal title.",
+            stacklevel=1,
+        )
+        return
+
     sys.stdout.write('\033[23;0t') 
     _xterm_term_title_saved = False
 
 
 if os.name == 'posix':
     TERM = os.environ.get('TERM','')
     if TERM.startswith('xterm'):
         _set_term_title = _set_term_title_xterm
         _restore_term_title = _restore_term_title_xterm
 elif sys.platform == 'win32':
     import ctypes
 
     SetConsoleTitleW = ctypes.windll.kernel32.SetConsoleTitleW
     SetConsoleTitleW.argtypes = [ctypes.c_wchar_p]
 
     def _set_term_title(title):
         """Set terminal title using ctypes to access the Win32 APIs."""
         SetConsoleTitleW(title)
 
 
 def set_term_title(title):
     """Set terminal title using the necessary platform-dependent calls."""
     if ignore_termtitle:
         return
     _set_term_title(title)
 
 
 def restore_term_title():
     """Restore, if possible, terminal title to the original state"""
     if ignore_termtitle:
         return
     _restore_term_title()
 
 
 def freeze_term_title():
     warnings.warn("This function is deprecated, use toggle_set_term_title()")
     global ignore_termtitle
     ignore_termtitle = True
 
 
 def get_terminal_size(defaultx=80, defaulty=25):
     return _get_terminal_size((defaultx, defaulty))