gh-80731: Avoid executing code in except block in cmd

[gaogaotiantian]

Currently cmd.Cmd executes code in an except block if no command is found. This is not ideal because exception info like sys.exc_info() would break. Specifically, in pdb, the users won't be able to see the exception they want, instead they get an AttributeError from cmd.Cmd.

The fix is pretty trivial - just don't do it in the except block. Here I used default value for getattr, which is probably cleaner. It's equivalent to moving the self.default() function call outside of the except block.

• Issue: Avoid creating AttributeError exceptions in the debugger #80731

[iritkatriel]

How does this PR compare with #4666?

[gaogaotiantian]

How does this PR compare with #4666?

Oh I did not see that PR. I believe the purpose is the same. I don't think we need to "avoid raising any exception", because it seems like as long as we are not doing the work in the except block, it should be fine (Python will remember the last exception raised).

[serhiy-storchaka]

This change can also be tested without debugger.

Add a test in test_cmd for default() raising an exception and check __context__ and __cause__ of the exception.

[gaogaotiantian]

This change can also be tested without debugger.

Add a test in test_cmd for default() raising an exception and check __context__ and __cause__ of the exception.

Sure, I added a test to check the correct exception can be retrieved in default() function.

[serhiy-storchaka]

Thank you for your contribution @gaogaotiantian.

The advantage of this PR is that it contains tests. But #4666 has also changes in the pdb module, so it is not completely superseded.

[gaogaotiantian]

Thank you for your contribution @gaogaotiantian.

The advantage of this PR is that it contains tests. But #4666 has also changes in the pdb module, so it is not completely superseded.

The pdb changes in #4666 is not needed (not that it's wrong). Like I mentioned above, this is only an issue if the pdb cmdloop executes in an except block - that's the case for the current cmd. However, simply getting an attribute and falling back to another solution in an except block works fine. That's why I did not change pdb. It may make the code slightly cleaner if I have to admit.

So in my opinion, #4666 can be closed because the problem it tries to solve is solved.