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GeezerVison is showing github.com — gh-153782: Make traceback.print_list() delegate to format_list() by basil · Pull Request #153783 · python/cpython

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@basil

@basil basil commented Jul 15, 2026 •
edited by bedevere-app Bot
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Fixes #153782

In Lib/traceback.py, the public helpers print_list() and format_list() independently contain the same formatting expression, so their outputs agree only by duplication. This PR routes print_list() through format_list() so that they share a single formatting path, with no change in behavior.

Problem

format_list() returns the formatted lines:

def format_list(extracted_list):
    return StackSummary.from_list(extracted_list).format()

print_list() writes those same lines, but re-derives them with the identical expression instead of reusing format_list():

def print_list(extracted_list, file=None):
    if file is None:
        file = sys.stderr
    for item in StackSummary.from_list(extracted_list).format():
        print(item, file=file, end="")

The outputs match only because StackSummary.from_list(extracted_list).format() is duplicated in both. Any future change to how a frame list is formatted has to be applied in both places to prevent the two public helpers from silently diverging.

Solution

Have print_list() iterate format_list():

def print_list(extracted_list, file=None):
    if file is None:
        file = sys.stderr
    for item in format_list(extracted_list):
        print(item, file=file, end="")

This eliminates the possibility of drift and mirrors the delegation already used in this module; for example, print_tb() calls print_list() rather than re-deriving extract_tb(...).format().

Testing Done

format_list() is exactly the same expression, so output is identical for every documented input type. Therefore, there is no change in behavior. This PR adds a regression test asserting this equality for tuple, FrameSummary, and StackSummary inputs.

Documentation

I have not proposed an entry in Misc/NEWS.d, as the developer guide says that strictly internal changes with no user-visible effect do not require one. I am happy to add one if a maintainer prefers.

  • Issue: Remove duplicate code by making traceback.print_list() delegate to format_list() #153782

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@basil
pythongh-153782: Make traceback.print_list() delegate to format_list()
d151f33 
print_list() and format_list() each formatted a StackSummary with the
same expression, so their outputs agreed only by duplication. Route
print_list() through format_list() so print_list(x), keeping the two
public helpers from drifting, and add a regression test that
print_list() matches format_list() for tuple, FrameSummary, and
StackSummary inputs.
@bedevere-app

bedevere-app Bot commented Jul 15, 2026

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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.
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@bedevere-app bedevere-app Bot added the awaiting review label Jul 15, 2026
@bedevere-app bedevere-app Bot mentioned this pull request Jul 15, 2026
Remove duplicate code by making traceback.print_list() delegate to format_list() #153782
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@basil

basil commented Jul 15, 2026

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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.

Per my comment from the PR description:

I have not proposed an entry in Misc/NEWS.d, as the developer guide says that strictly internal changes with no user-visible effect do not require one. I am happy to add one if a maintainer prefers.

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

I will wait for a maintainer to apply the skip news label.
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@picnixz

picnixz commented Jul 15, 2026

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I do not think it is really worth it. The current code is working and such change would create a drift in backports. We do not accept such changes in general. I think it is fine as is.
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@picnixz

picnixz commented Jul 15, 2026

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However the tests, if not already there, are worth it.
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@picnixz

picnixz commented Jul 15, 2026

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If tests do not yet exist, we can accept such changes. Can you confirm that?
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@johnslavik

johnslavik commented Jul 15, 2026

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I think the current code is OK.

I understand your reasoning. However, these functions are essentially kept together and they are simple functions. They have not drifted for 11 years.

Test coverage would be cool to have, yes.
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@basil

basil commented Jul 15, 2026

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Thank you, that makes sense. This PR adds test_print_list_matches_format_list, covering tuple input, FrameSummary input, and StackSummary input. I confirmed that there was not an existing test checking that traceback.print_list() produces the same output as traceback.format_list().
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@basil

basil commented Aug 19, 2026

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If tests do not yet exist, we can accept such changes.

@picnixz Are there any next steps for this PR from your perspective?
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@picnixz

picnixz commented Aug 19, 2026 •
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  1. This PR is unlikely to be merged to reduce friction with other branches.
  2. If you can add improve the coverage of traceback.py that related to these functions we can accept auch PR. But this must be a separate one and you likely won't need an issue unless you plan to work on large parts of traceback.py (i.e if we are severly lacking coverage if would be good to have an issue and know which interface is poorly or not tested)
  3. We can keep your PR if you only keep the tests.
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@basil

basil commented Aug 19, 2026

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This PR is unlikely to be merged to reduce friction with other branches.

Thanks @picnixz, would that friction still exist if I proposed this PR against those other branches as well?
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@picnixz

picnixz commented Aug 19, 2026

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This is not our policy, so no. We do not backport refactoring changes.
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basil added 2 commits August 19, 2026 10:09
@basil
Merge remote-tracking branch 'origin/main' into format-list
744dd2a
@basil
https://github.com/python/cpython/pull/153783#issuecomment-5344367230
eb96153
@bedevere-app

bedevere-app Bot commented Aug 19, 2026

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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.
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@basil

basil commented Aug 19, 2026

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We can keep your PR if you only keep the tests.

Thanks @picnixz, I have done so in commit eb96153.
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Remove duplicate code by making traceback.print_list() delegate to format_list()

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@basil @picnixz @johnslavik
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