gen.throw() with multiple yield froms skips intermediate exceptions

[cjerdonek]

BPO	40694
Nosy	@cjerdonek

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2020-05-20.04:47:05.315>
labels = ['interpreter-core', '3.10']
title = 'gen.throw() with multiple yield froms skips intermediate exceptions'
updated_at = <Date 2020-05-20.04:47:05.315>
user = 'https://github.com/cjerdonek'

bugs.python.org fields:

activity = <Date 2020-05-20.04:47:05.315>
actor = 'chris.jerdonek'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Interpreter Core']
creation = <Date 2020-05-20.04:47:05.315>
creator = 'chris.jerdonek'
dependencies = []
files = []
hgrepos = []
issue_num = 40694
keywords = []
message_count = 1.0
messages = ['369416']
nosy_count = 1.0
nosy_names = ['chris.jerdonek']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = None
url = 'https://bugs.python.org/issue40694'
versions = ['Python 3.10']

[cjerdonek]

Here is another gen.throw() exception chain example similar to the examples in bpo-29587: https://bugs.python.org/issue29587

def f():
    yield

def g():
    try:
        raise RuntimeError('a')
    except Exception as exc:
        print(f'handling: {exc!r}')
        yield from f()

def h():
    try:
        raise RuntimeError('b')
    except Exception as exc:
        print(f'handling: {exc!r}')
        yield from g()

gen = h()
gen.send(None)
gen.throw(ValueError)

Output:

handling: RuntimeError('b')
handling: RuntimeError('a')
Traceback (most recent call last):
  File "/.../test.py", line 13, in h
    raise RuntimeError('b')
RuntimeError: b

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/.../test.py", line 20, in <module>
    gen.throw(ValueError)
  File "/.../test.py", line 16, in h
    yield from g()
  File "/.../test.py", line 9, in g
    yield from f()
  File "/.../test.py", line 2, in f
    yield
ValueError

The issue is that "RuntimeError: a" is skipped. It should also appear in the exception chain.

I believe this has the same root cause as bpo-29590: https://bugs.python.org/issue29590

[gschaffner]

calling outer, middle, inner = h, g, f for clarity, it looks like at the end of yield from in outer(), exc_info[1].__context__ is getting reset to RuntimeError("raised by outer") when it should remain as RuntimeError("raised by middle"):

click to expand

import sys


def outer():
    try:
        raise RuntimeError("raised by outer")
    except Exception:
        try:
            yield from middle()
        finally:
            val_err = sys.exc_info()[1]
            assert isinstance(val_err, ValueError)
            print(
                "outer:"
                f"\n\t{val_err.__context__=} (should be RE('raised by middle'))"
                f"\n\t{val_err.__context__.__context__=} (should be RE('raised by outer'))"
            )


def middle():
    try:
        raise RuntimeError("raised by middle")
    except Exception:
        try:
            yield from inner()
        finally:
            val_err = sys.exc_info()[1]
            assert isinstance(val_err, ValueError)
            print(
                "middle:"
                f"\n\t{val_err.__context__=} (should be RE('raised by middle'))"
                f"\n\t{val_err.__context__.__context__=} (should be RE('raised by outer'))"
            )


def inner():
    yield


gen = outer()
gen.send(None)
gen.throw(ValueError)

output:

middle:
        val_err.__context__=RuntimeError('raised by middle') (should be RE('raised by middle'))
        val_err.__context__.__context__=RuntimeError('raised by outer') (should be RE('raised by outer'))
outer:
        val_err.__context__=RuntimeError('raised by outer') (should be RE('raised by middle'))
        val_err.__context__.__context__=None (should be RE('raised by outer'))
Traceback (most recent call last):
  File "/.../eg.py", line 6, in outer
    raise RuntimeError("raised by outer")
RuntimeError: raised by outer

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/.../eg.py", line 42, in <module>
    gen.throw(ValueError)
  File "/.../eg.py", line 9, in outer
    yield from middle()
  File "/.../eg.py", line 25, in middle
    yield from inner()
ValueError