Exceptions raised while tracing certain statements cannot be caught

[brandtbucher]

Bug report

So I found a really odd bug today. except blocks don't catch exceptions raised when tracing certain statements that themselves "cannot raise". For example:

def f():
    breakpoint()
    try:
        pass  # If you raise while tracing this line, the exception isn't caught.
    except:
        ...
    try:
        42  # Ditto.
    except:
        ...
    try:
        return  # Ditto.
    except:
        ...

A couple of observations:

• Exceptions raised while tracing other "normal" statements are handled the normal way.
• The code for the exception handling is still present in these examples, even though it's statically unreachable.
• If two or more of these "non-raising" statements occur together in the same block (like several pass statements in a row), only the last one will have the bug:

def f():
    breakpoint()
    try:
        pass  # No bug
        pass  # No bug
        pass  # Bug
    except:
        ...

I haven't invested any time into figuring out what the root cause is, I've just been poking at it with code examples. It's definitely a bug in the bytecode compiler though, not the interpreter (dis shows missing exception table entries). Probably something to do with how we emit exception tables for NOP or anything that unwinds the block stack, if I had to guess?

(This also affects 3.12, and maybe even earlier versions, but only tagging for 3.13 and newer since I don't think this is a "security" issue.)

[brandtbucher]

Here's a reproducer that doesn't involve tracing, just sending ^C:

>>> def f():
...     while True:
...         try:
...             continue  # This line is inside the try!
...         except KeyboardInterrupt:
...             break
...             
>>> f()
^CTraceback (most recent call last):
  File "<python-input-21>", line 1, in <module>
    f()
    ~^^
  File "<python-input-20>", line 4, in f
    continue  # This line is inside the try!
    ^^^^^^^^
KeyboardInterrupt

You can see the continue line in the traceback... which should mean that the exception is caught, but it isn't.

[MazinSharaf]

I can try working on this. Also just simplifying: Python sees the pass (taking as an example) as not apart of the try block even though it is while trailing?