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gh-153782: Make traceback.print_list() delegate to format_list()#153783

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gh-153782: Make traceback.print_list() delegate to format_list()#153783
basil wants to merge 1 commit into
python:mainfrom
basil:format-list

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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.

@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.

@bedevere-app bedevere-app Bot mentioned this pull request Jul 15, 2026
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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.

@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.

@picnixz

picnixz commented Jul 15, 2026

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

@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?

@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.

@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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Remove duplicate code by making traceback.print_list() delegate to format_list()

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@basil @picnixz @johnslavik