quitting a debugger based on the bdb module does not always stop code execution

Bug report

Bug description:

I encountered a bug while working with the Pyzo Python IDE, and the bug itself is located in the bdb module of Python's standard library. The same bug occurs when using the IDLE Python IDE, as it also uses bdb for the debugger.

When the debugger pauses code execution, e.g. at a breakpoint, the user can decide to quit debugging, which normally stops the execution of further code in the script to be debugged. But this does not work when the breakpoint is inside the "try" block of a try-except-clause and when exceptions are caught and ignored.

The Bdb class internally stops debugging by raising a BdbQuit error. But BdbQuit is like almost all exceptions derived from the Exception class, and therefore the script to be debugged can catch and dismiss the stop:

https://github.com/python/cpython/blob/1fc2b38d63170dd065efc2bc60e861f32c033b17/Lib/bdb.py#L15-L18

I think, it would be safer to change the definition of the BdbQuit class to:

class BdbQuit(BaseException):
    """Exception to give up completely."""

Though, it does not prevent the problem when there is an except clause like except: in the code to be debugged, but this is discouraged anyways.

As far as I have seen, the class definition of BdbQuit basically has not changed since Python v2.3, and so it predates BaseExceptions which where added to Python v2.5 via PEP 352.

Here is an example code to reproduce the problem and demonstrate the fix:

"""
* Save this code somewhere as a Python script (e.g. "/tmp/myscript.py").
* Launch IDLE (the Python IDE).
* Open this Python script in IDLE.
* Set breakpoints in the two code lines where indicated via comments.
    You can do this via the context menu when right-clicking on the line.
* Switch to the Python Shell window in IDLE (via menu "Run -> Python Shell".
* Open the Debugger (via menu "Debug -> Debugger" in the Python Shell).
* In the IDLE editor window, run the script (via menu "Run -> Run Module").
* Press the "Go" button in the "Debug Control" window.
* Execution stops at the line with the first breakpoint.
* Pressing the "Quit" button now would not execute any of the remaining lines.
* But we press "Go" instead of "Quit" to reach the second breakpoint.
* Now, really press "Quit", and see that script code execution is NOT stopped.
    There are some further text outputs from the print functions in the Shell
    window.
* Enable monkey-patching by changing "do_monkeypatch = False" to "do_monkeypatch = True".
    Or edit file "bdb.py" in Python's lib directory:
        change "class BdbQuit(Exception):" to "class BdbQuit(BaseException):"
* Do all the previous steps again, and you will see, that there is no more
    code execution in the script when quitting debugging at the second breakpoint.
"""

do_monkeypatch = False

if do_monkeypatch:
    import bdb

    if issubclass(bdb.BdbQuit, Exception):
        class BdbQuit(BaseException): pass

        bdb.BdbQuit = BdbQuit


print('this is before the first breakpoint')
a = 1  # <-- set a breakpoint in this line
print('this is after the first breakpoint')

try:
    print('this is before the second breakpoint')
    a = 2  # <-- set a breakpoint in this line
    print('this is after the second breakpoint, when continuing execution')
except Exception as e:
    print('this is after the second breakpoint, when quitting the debugger')
    print('got an exception, but ignoring it:', repr(e))

print('deleting important files :-(')

CPython versions tested on:

3.14

Operating systems tested on:

Linux

Linked PRs

• gh-149337

Start with Lib/bdb.py at the BdbQuit definition linked in the issue, then run the supplied IDLE reproduction script with the breakpoint inside the try block. Done means quitting the debugger prevents the remaining script code from executing even when the exception is caught; review linked PR gh-149337 before beginning.