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gh-141982: Fix pdb can't set breakpoints on async functions#141983

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gaogaotiantian merged 9 commits into
python:mainfrom
cocolato:fix_pdb_async
Dec 2, 2025
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gh-141982: Fix pdb can't set breakpoints on async functions#141983
gaogaotiantian merged 9 commits into
python:mainfrom
cocolato:fix_pdb_async

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@cocolato

@cocolato cocolato commented Nov 26, 2025 •
edited by bedevere-app Bot
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Fix: #141982

I found that while debugging with pdb you currently cannot set a breakpoint on an async function. Reproducer:

import asyncio

async def main():
    print(f"Hello")
    await asyncio.sleep(1)
    print(f"World!")

asyncio.run(main())

If you run pdb and try to set a breakpoint on main you get:

λ › ./python -m pdb main.py                                                                                                                                          
> /cpython/main.py(1)<module>()
-> import asyncio
(Pdb) break main
*** The specified object 'main' is not a function or was not found along sys.path.
(Pdb) quit

This happens because the regular expression used by find_function does not match async functions:

cpython/Lib/pdb.py

Lines 132 to 133 in 2ff8608

def find_function(funcname, filename):
cre = re.compile(r'def\s+%s(\s*\[.+\])?\s*[(]' % re.escape(funcname))

After the fix:

λ › ./python -m pdb main.py                                                                                                                                          
> /cpython/main.py(1)<module>()
-> import asyncio
(Pdb) break main
Breakpoint 1 at /cpython/main.py:4
(Pdb) continue
> /cpython/main.py(4)main()
-> print(f"Hello")
(Pdb) quit

Now we can set breakpoints on async functions as expected.

@bedevere-app

bedevere-app Bot commented Nov 26, 2025

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Most changes to Python require a NEWS entry. Add one using the blurb_it web app or the blurb command-line tool.

If this change has little impact on Python users, wait for a maintainer to apply the skip news label instead.

@bedevere-app bedevere-app Bot mentioned this pull request Nov 26, 2025
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@aisk aisk added stdlib Standard Library Python modules in the Lib/ directory and removed stdlib Standard Library Python modules in the Lib/ directory labels Nov 26, 2025
aisk
aisk reviewed Nov 26, 2025
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Comment thread Misc/NEWS.d/next/Library/2025-11-26-15-36-55.gh-issue-141982.-8yHA3.rst Outdated
@aisk

aisk commented Nov 27, 2025 •
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I'm not familiar with the pdb module, but I think that even after this change, functions defined within an if branch or other indented blocks still can't be found by this function. Perhaps we could use another approach, like walking the AST in the find_function to handle this situation.

For now, however, I think it's acceptable to just handle the async function case described in the original issue because this is the most common situation.

gaogaotiantian
gaogaotiantian reviewed Nov 29, 2025
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@gaogaotiantian gaogaotiantian left a comment

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Let's optimize the test case a bit. So we want a test case that is accurate and comprehensive, while being concise. The sleep and World part did not do anything - we don't need them.

Checking Hello is not the best practice because there could be many ways that Hello appears in stdout - it's part of the source code and you are printing it. Say the breakpoint is never set, you'd still have Hello in stdout.

One way is to check World is not printed out - which could be an indicator that a breakpoint is set. For Hello I think we need to check it in a better way.

@bedevere-app

bedevere-app Bot commented Nov 30, 2025

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Most changes to Python require a NEWS entry. Add one using the blurb_it web app or the blurb command-line tool.

If this change has little impact on Python users, wait for a maintainer to apply the skip news label instead.

@cocolato

cocolato commented Nov 30, 2025

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Thanks for review, @gaogaotiantian ! Do you think the current tests are sufficient?

yihong0618
yihong0618 reviewed Nov 30, 2025
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Comment thread Lib/test/test_pdb.py Outdated
gaogaotiantian
gaogaotiantian reviewed Nov 30, 2025
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gaogaotiantian
gaogaotiantian reviewed Dec 1, 2025
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@gaogaotiantian

gaogaotiantian commented Dec 2, 2025

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Thank you for fixing this.

@gaogaotiantian
Add skip condition for async break tests
e543320 
Skip coroutine tests if SKIP_CORO_TESTS is True.
@gaogaotiantian gaogaotiantian merged commit fddc24e into python:main Dec 2, 2025
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StanFromIreland pushed a commit to StanFromIreland/cpython that referenced this pull request Dec 6, 2025
@cocolato @gaogaotiantian @StanFromIreland
pythongh-141982: Fix pdb can't set breakpoints on async functions (py…
9f664dd 
…thon#141983)

Co-authored-by: Tian Gao <gaogaotiantian@hotmail.com>
@cocolato cocolato deleted the fix_pdb_async branch January 20, 2026 14:11
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Support pdb breakpoints on async functions

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@cocolato @aisk @gaogaotiantian @yihong0618 @nazeerali4325-commits