I can't add reviewers, can someone add @gaogaotiantian and @iritkatriel ?

I'm not sure this needs a news as it's a fix for something that was added a week ago.

How do I reproduce the bug on pdb?

How do I reproduce the bug on pdb?

Oh, yes, sure:

Python 3.13.0a0 (heads/main-dirty:08727d0af61, Sep  4 2023, 11:11:14) [Clang 14.0.6 ] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> def f():
...     raise Exception() from Exception()
...
>>> f()
Exception

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 2, in f
Exception
>>> import pdb
>>> pdb.pm()
> <stdin>(2)f()
(Pdb) exceptions
    0 Exception()
>   1 Exception()
(Pdb) exceptions 0
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/bussonniermatthias/dev/cpython/Lib/pdb.py", line 2026, in pm
    post_mortem(sys.last_exc)
  File "/Users/bussonniermatthias/dev/cpython/Lib/pdb.py", line 2022, in post_mortem
    p.interaction(None, t)
  File "/Users/bussonniermatthias/dev/cpython/Lib/pdb.py", line 501, in interaction
    self._cmdloop()
  File "/Users/bussonniermatthias/dev/cpython/Lib/pdb.py", line 405, in _cmdloop
    self.cmdloop()
  File "/Users/bussonniermatthias/dev/cpython/Lib/cmd.py", line 138, in cmdloop
    stop = self.onecmd(line)
           ^^^^^^^^^^^^^^^^^
  File "/Users/bussonniermatthias/dev/cpython/Lib/pdb.py", line 592, in onecmd
    return cmd.Cmd.onecmd(self, line)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/bussonniermatthias/dev/cpython/Lib/cmd.py", line 217, in onecmd
    return func(arg)
           ^^^^^^^^^
  File "/Users/bussonniermatthias/dev/cpython/Lib/pdb.py", line 1178, in do_exceptions
    self.setup(None, self._chained_exceptions[number].__traceback__)
  File "/Users/bussonniermatthias/dev/cpython/Lib/pdb.py", line 298, in setup
    self.curframe = self.stack[self.curindex][0]
                    ~~~~~~~~~~^^^^^^^^^^^^^^^
IndexError: list index out of range

Is there any meaningful case where the latest exception (the one passed into pm() or sys.last_exc) does not have traceback, but the chained exceptions could be useful?

Here's what I thought:

• It's probably common where a chained exception does not have traceback, but it's normally not the exception raised and caught.
• When the exception pdb catches (through pm(), interaction is not documented API) does not have the traceback, there's normally nothing interesting.

However:

• We SHOULD keep the exceptions without tracebacks when possible, because they are part of the chain and it would confuse the users if there's an exception missing just because there's no trackback attached.
• We should try NOT to raise exceptions from a common function deep in pdb - it would be frustrating to users when pdb errors out and interaction is a function used else where. It may only raise the exception in this very specific situation now, but in the future it could be triggered under other circumstances when there are some code changes. If we want to raise an exception, we should raise early.

So, I think the better way to do it is:

• Check whether we can handle it (whether the raised exception has traceback) in post_mortem(), the same place as the existing ValueError. If not, raise the exception there.
• Once interaction() is called, we do not ever raise any exceptions. The code should make sure the latest exception has a trackback to display, and do_exceptions should check if the "target exception" has a traceback and display a message to user when it does not. No exception raised.

It would be much clearer to the future maintainers that pdb does not raise exception under a very complicated condition, and it would make the code in interaction() easier to understand - keep the exception related logic in do_exceptions.

Thoughts @iritkatriel ?

@gaogaotiantian I'll see what I can do, but it might probably be quite invasive to support exceptions w/o tb. I can maybe keep them but refuse to exceptions #tb-without-traceback and/or display them differently in exceptions.

Note that interaction() is fairly early in Pdb's life, were you maybe thinkingof _cmdloop maybe ?

Yes, I think it would be good if all exceptions were shown, and those without traceback would display that somehow (rather than erroring).

@gaogaotiantian I'll see what I can do, but it might probably be quite invasive to support exceptions w/o tb. I can maybe keep them but refuse to exceptions #tb-without-traceback and/or display them differently in exceptions.

Note that interaction() is fairly early in Pdb's life, were you maybe thinkingof _cmdloop maybe ?

Yes, as we can guarantee the default exception(latest one) is with traceback, we can simply decline all the access to the exceptions without tracebacks when the users do exceptions #exception_without_traceback. Send a message saying there's no traceback, and keep them in the current exception - so we don't need to take care of all the commands in a setup without the traceback.

interaction() is fairly early, but it's still called for every trace trigger - including the exception trigger which might take advantage of the exception feature in the future. We want to put the exception as closer to the surface as possible.

Ok, what about the following ? in do_exceptions I print - as number for exceptions that do not have a traceback (keeping the numbering though, so you would get something like

 (Pdb) exceptions
     - Exception()
     1 Exception()
     2 Exception()
     - Exception()
 >   4 Exception()
     5 Exception()
     - Exception()
     7 Exception()

And on attempt to jump to it, it says: This exception has not traceback, cannot jump to it

There is one caveat WRT testing, I can't properly test post_mortem() as the testing needs to pdb.Pdb(nosigint=True, readrc=False) and post_mortem() create its own instance I have no control over.

There is one caveat WRT testing, I can't properly test post_mortem() as the testing needs to pdb.Pdb(nosigint=True, readrc=False) and post_mortem() create its own instance I have no control over.

I added a private version of post-mortem that takes an instance so that I can use it in the test.