Incorrect traceback when future's exception is raised multiple times

[iritkatriel]

BPO	45924
Nosy	@gvanrossum, @asvetlov, @lilydjwg, @1st1, @iritkatriel, @kumaraditya303, @vishalpandeyvip
PRs	bpo-45924: Fix asyncio incorrect traceback when future's exception is raised multiple times #30274

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

Show more details

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2021-11-29.11:55:26.419>
labels = ['easy', 'type-bug', '3.9', '3.10', '3.11', 'expert-asyncio']
title = "Incorrect traceback when future's exception is raised multiple times"
updated_at = <Date 2022-03-18.09:58:44.181>
user = 'https://github.com/iritkatriel'

bugs.python.org fields:

activity = <Date 2022-03-18.09:58:44.181>
actor = 'lilydjwg'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['asyncio']
creation = <Date 2021-11-29.11:55:26.419>
creator = 'iritkatriel'
dependencies = []
files = []
hgrepos = []
issue_num = 45924
keywords = ['patch', 'easy']
message_count = 7.0
messages = ['407265', '407308', '407485', '407486', '407488', '414813', '415478']
nosy_count = 7.0
nosy_names = ['gvanrossum', 'asvetlov', 'lilydjwg', 'yselivanov', 'iritkatriel', 'kumaraditya', 'vishalpandeyvip']
pr_nums = ['30274']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue45924'
versions = ['Python 3.9', 'Python 3.10', 'Python 3.11']

[iritkatriel]

Background:
#29780 (comment)
#29780 (comment)

######################################################

import asyncio, traceback

async def raise_after(fut, delay):
    await asyncio.sleep(delay)
    fut.set_exception(TypeError(42))

async def main():
    loop = asyncio.get_running_loop()
    fut = loop.create_future()
    loop.create_task(
        raise_after(fut, 1))

    print('hello ...')

    for i in range(3):
        try:
            print(await fut)
        except Exception as e:
            traceback.print_exception(e)

asyncio.run(main())
######################################################

Output (traceback accumulates a frame each time):

hello ...
Traceback (most recent call last):
  File "/Users/iritkatriel/src/cpython-1/as.py", line 17, in main
    print(await fut)
          ^^^^^^^^^
TypeError: 42
Traceback (most recent call last):
  File "/Users/iritkatriel/src/cpython-1/as.py", line 17, in main
    print(await fut)
          ^^^^^^^^^
  File "/Users/iritkatriel/src/cpython-1/as.py", line 17, in main
    print(await fut)
          ^^^^^^^^^
TypeError: 42
Traceback (most recent call last):
  File "/Users/iritkatriel/src/cpython-1/as.py", line 17, in main
    print(await fut)
          ^^^^^^^^^
  File "/Users/iritkatriel/src/cpython-1/as.py", line 17, in main
    print(await fut)
          ^^^^^^^^^
  File "/Users/iritkatriel/src/cpython-1/as.py", line 17, in main
    print(await fut)
          ^^^^^^^^^
TypeError: 42

[gvanrossum]

There's a similar issue with concurrent.futures.Future, and really, anything that stores exceptions and later raises them can get in trouble if there's a way to get the exception raised multiple times. This is rarely noticed because usually the exception isn't raised more than once.

We need separate bpo issues for the other cases.

The fix is easy enough (separately store a traceback and raise using e.with_traceback(tb)) so I'm marking this as an easy issue.

[vishalpandeyvip]

hey Irit, Is this issue platform-dependent? I am on ubuntu 20.04, and running python 3.10. I am not getting the same error that you have mentioned below.
This is what I am getting.
###################################

hello ...
Traceback (most recent call last):
  File "error.py", line 17, in main
    print(await fut)
TypeError: 42

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "error.py", line 21, in <module>
    asyncio.run(main())
  File "/usr/lib/python3.8/asyncio/runners.py", line 44, in run
    return loop.run_until_complete(main)
  File "/usr/lib/python3.8/asyncio/base_events.py", line 616, in run_until_complete
    return future.result()
  File "error.py", line 19, in main
    traceback.print_exception(e)
TypeError: print_exception() missing 2 required positional arguments: 'value' and 'tb'

###########################################################
and if I replace traceback.print_exception method with the print method, It runs perfectly fine, printing

hello ...
42
42
42

Can you please help me with this issue?

[iritkatriel]

Vishal, your output shows you’re running 3.8, not 3.10.

traceback.print_exception() used to require 3 parameters, now it can make do with just one. See the docs for the details.

[vishalpandeyvip]

thanks, It was my mistake, bug reproduced.

[iritkatriel]

Closed bpo-46954 as a duplicate of this.

[iritkatriel]

Closed bpo-42682 as duplicate of this.

[kumaraditya303]

Fixed by #30274