Python 3.12.0a7 can fail to catch exceptions when an iterator is involved

Bug report

Python 3.12.0a7 (at least as of commit d65ed69) can fail to catch exceptions when an iterator is involved. The following code works on Python 3.11 but the ValueError is uncaught on Python 3.12:

#!/usr/bin/env python

def do_work():
    yield
    raise ValueError()


def main():
    try:
        for _ in do_work():
            if True is False:
                return
    except ValueError:
        pass


if __name__ == '__main__':
    main()

With this code in repro.py the following traceback is shown on Python 3.12:

Traceback (most recent call last):
  File "//repro.py", line 18, in <module>
    main()
  File "//repro.py", line 12, in main
    return
  File "//repro.py", line 5, in do_work
    raise ValueError()
ValueError

No error or output occurs under Python 3.11.

Your environment

Tested on Ubuntu 20.04.5 using the deadsnakes nightly build: python3.12 - 3.12.0~a7-98-gd65ed693a8-1+focal1

Linked PRs

GH-103488: Use return-offset, not yield-offset. #103502

Verified on latest 3.11/2 releases on Windows.
Edit revised: same if yield something but error in yield expression or before yield is caught. IE, Error must be after yield statement.

Line 12 being in the traceback instead of line 10 also seems wrong.

If return is changed to pass or continue, the exception is caught when the loop asks for the next value and gets an exception when executing the code after yield. I am slightly surprised that breaking (also not caught) or returning does not just gc the generator without executing the remaining (buggy) code. (Revised twice after more experiments.)

@mattclay

@mattclay Inserting 1/0 or better, an unbound name such as z is an easier way to generate an exception and easier to move or delete. 'NameError' should be the shortest exception name.

Can you post the reduced reproducer? And maybe the code object disassembly (python -m dis <file>)? It smells like a failure to generate or interpret the exception table correctly. Disassembly on the original reproducer shows odd gaps in the exception table that aren't there in 3.11.

@gvanrossum

@gvanrossum Who is 'who'? I reduced the raise and catch a bit but also added 3 debug prints to trace execution.

@terryjreedy

I simplified the reproducer a bit using the suggestion from @terryjreedy:

#!/usr/bin/env python

def do_work():
    yield
    x


def main():
    try:
        for _ in do_work():
            if True is False:
                return
    except NameError:
        pass


if __name__ == '__main__':
    main()

I've also attached the code object disassembly from Python 3.11 and 3.12 for the revised reproducer.

dis311.txt
dis312.txt (3.12.0a6)
dis312a7.txt (3.12.0a7, commit d65ed69)

It seems like this symptom could also result from a change in the timing of finalization of the do_work() generator on the stack in main(). If the generator is not exhausted, then the code after the yield will only run when the generator object is finalized, IIRC.

@markshannon

The change in behavior bisects to 4a1c58d

cc @markshannon

The conditional statement, a 'complex*' condition evaluating to false, so that the body is not executed, and a break/return in the non-executed body, seem necessary to get the error. False, 0, and 1-1 are simple. Comparisons tested (is, ==, and > so far) are complex.

Here's an even shorter reproducer:

def do_work():
    yield
    x


try:
    for _ in do_work():
        if True is False:
            break
except NameError:
    pass

Here's the -m dis output for the smaller reproducer:

dis311-simple.txt (3.11.3)
dis312a7-simple.txt (3.12.0a7, commit d65ed69)

Note the bisect blame commit doesn't change the compiler, so it's not an issue in the bytecode (unless it was a latent issue from earlier that was then exposed by a runtime change), it's something in the runtime behavior of generators changed in that commit. I confirmed that the dis output is identical before and after the blame commit, but after it the reproducer fails and before it it's fine.

It does appear, though, that the issue is that we fall into a hole in the exception table; the instruction offset used when calling get_exception_handler in the main frame is 23, which is the RETURN_VALUE in a return-None exit, and is outside the try/except block. That instruction offset seems wrong for where we should be in that frame when we get the exception from do_work, still tracking down why it's wrong.

Repro code I'm using

def do_work():
    yield
    x


try:
    for _ in do_work():
        if True is False:
            break
except NameError:
    pass

Bytecode

  0           0 RESUME                   0

  1           2 LOAD_CONST               0 (<code object do_work at 0x7fd5813db740, file "<exec>", line 1>)
              4 MAKE_FUNCTION            0
              6 STORE_NAME               0 (do_work)

  6           8 NOP

  7          10 PUSH_NULL
             12 LOAD_NAME                0 (do_work)
             14 CALL                     0
             24 GET_ITER
        >>   26 FOR_ITER                 9 (to 48)
             30 STORE_NAME               1 (_)

  8          32 LOAD_CONST               1 (True)
             34 LOAD_CONST               2 (False)
             36 IS_OP                    0
             38 POP_JUMP_IF_TRUE         1 (to 42)
             40 JUMP_BACKWARD            8 (to 26)

  9     >>   42 POP_TOP
             44 LOAD_CONST               3 (None)
             46 RETURN_VALUE

  7     >>   48 END_FOR
             50 LOAD_CONST               3 (None)
             52 RETURN_VALUE
        >>   54 PUSH_EXC_INFO

 10          56 LOAD_NAME                2 (NameError)
             58 CHECK_EXC_MATCH
             60 POP_JUMP_IF_FALSE        4 (to 70)
             62 POP_TOP

 11          64 POP_EXCEPT
             66 LOAD_CONST               3 (None)
             68 RETURN_VALUE

 10     >>   70 RERAISE                  0
        >>   72 COPY                     3
             74 POP_EXCEPT
             76 RERAISE                  1
ExceptionTable:
  10 to 38 -> 54 [0]
  42 to 42 -> 54 [0]
  48 to 48 -> 54 [0]
  54 to 62 -> 72 [1] lasti
  70 to 70 -> 72 [1] lasti

Disassembly of <code object do_work at 0x7fd5813db740, file "<exec>", line 1>:
  1           0 RETURN_GENERATOR
              2 POP_TOP
              4 RESUME                   0

  2           6 LOAD_CONST               0 (None)
              8 YIELD_VALUE              1
             10 RESUME                   1
             12 POP_TOP

  3          14 LOAD_GLOBAL              0 (x)
             26 POP_TOP
             28 LOAD_CONST               0 (None)
             30 RETURN_VALUE
        >>   32 STOPITERATION_ERROR
             34 RERAISE                  1
ExceptionTable:
  4 to 30 -> 32 [0] lasti

GH-103488: Use return-offset, not yield-offset. (GH-103502)

Merge branch 'main' into superopt

I've verified the issue has been resolved. I tested with the deadsnakes nightly build (commit 1aa376f).

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Python 3.12.0a7 can fail to catch exceptions when an iterator is involved #103488

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Python 3.12.0a7 can fail to catch exceptions when an iterator is involved #103488

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Bug report

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Linked PRs

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terryjreedy commented on Apr 12, 2023

pochmann commented on Apr 12, 2023

terryjreedy commented on Apr 12, 2023

terryjreedy commented on Apr 12, 2023

gvanrossum commented on Apr 12, 2023

terryjreedy commented on Apr 12, 2023

mattclay commented on Apr 12, 2023

carljm commented on Apr 12, 2023

carljm commented on Apr 12, 2023

terryjreedy commented on Apr 12, 2023

mattclay commented on Apr 12, 2023

carljm commented on Apr 12, 2023

carljm commented on Apr 13, 2023

mattclay commented on Apr 14, 2023

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