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GeezerVison is showing github.com — gh-159184: Do not compare set elements when marshalling a set by gpshead · Pull Request #159196 · python/cpython

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gpshead commented Oct 11, 2026

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marshal writes the elements of a set in the order of their dumps, so that the output does not depend on hash order. It did that by sorting (dump, element) pairs, which compares the elements whenever two of them have the same dump. That happens for NaNs that are distinct objects, and complex numbers cannot be ordered:

>>> marshal.dumps(frozenset({complex('nan'), complex('nan')}))
TypeError: '<' not supported between instances of 'complex' and 'complex'

>>> code = compile("x in {(1e999-1e999)*1j, (1e999-1e999)*2j}", "m.py", "exec")
>>> marshal.dumps(code)       # so no .pyc can be written for this module
TypeError: '<' not supported between instances of 'complex' and 'complex'

Change

flowchart TB
  subgraph before["main"]
    direction TB
    b1["list of (dump, element)"] --> b2["PyList_Sort()"]
    b2 --> b3{"equal dumps?"}
    b3 -- no --> b4["ordered by dump"]
    b3 -- yes --> b5["compares the elements"]
    b5 -- complex --> b6["TypeError"]
    b5 -- "float NaN" --> b7["neither is smaller:<br>left in set order"]
  end
  subgraph after["this PR"]
    direction TB
    a1["list of (dump, index, element)<br>index = position in the set"] --> a2["PyList_Sort()"]
    a2 --> a3{"equal dumps?"}
    a3 -- no --> a4["ordered by dump"]
    a3 -- yes --> a5["compares the indexes,<br>which always differ:<br>left in set order"]
  end

The output is unchanged wherever marshalling worked before: elements with different dumps are ordered by dump, and float NaNs with equal dumps stay in set order as they did. The only difference is that elements are never compared.

This is the small part of gh-159184 that can be backported. The other part, that the order of elements with equal dumps follows their addresses, needs #159192 and is a separate PR.

This touches the same block as the PR for gh-159185; whichever is merged second needs a one-line conflict resolved.

Tests

test_unorderable_set_items and test_unorderable_set_items_in_code replace test_error_in_set_item, which asserted the TypeError.

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gpshead self-assigned this

Oct 11, 2026

bedevere-app Bot mentioned this pull request

Oct 11, 2026

marshal.dumps() raises TypeError for a set containing several complex NaNs #159184

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bedevere-app Bot added the awaiting core review label

Oct 11, 2026

gpshead added needs backport to 3.14

bugs and security fixes

needs backport to 3.15 bugs and security fixes labels

Oct 11, 2026

gpshead mentioned this pull request

Oct 11, 2026

gh-159184: Make the marshal order of set elements with equal dumps stable #159197

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          pythongh-159184: Do not compare set elements when marshalling a set

910690f

Set elements are written in the order of their dumps.  They were sorted
as (dump, element) pairs, so elements with the same dump were compared.
That raised TypeError for a set of several complex NaNs, which the
compiler can produce by constant folding, making the module impossible
to marshal.

Sort (dump, index, element) tuples, in which the indexes are all
different.  Elements with the same dump stay in the order of the set,
as float NaNs did before.

gpshead force-pushed the marshal-set-sort-bytes branch from 9ae5920 to 910690f Compare

October 11, 2026 22:00

gpshead merged commit 1967a87 into python:main

Oct 11, 2026

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miss-islington-app Bot commented Oct 11, 2026

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Thanks @gpshead for the PR 🌮🎉.. I'm working now to backport this PR to: 3.14, 3.15.
🐍🍒⛏🤖

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bedevere-app Bot removed the awaiting core review label

Oct 11, 2026

miss-islington-app Bot commented Oct 11, 2026

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Sorry, @gpshead, I could not cleanly backport this to 3.15 due to a conflict.

Please backport manually with cherry_picker, see the devguide for more information.

cherry_picker 1967a87ad395552dbd1e16fd3f13d3b4b2f694de 3.15

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miss-islington-app Bot commented Oct 11, 2026

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Sorry, @gpshead, I could not cleanly backport this to 3.14 due to a conflict.

Please backport manually with cherry_picker, see the devguide for more information.

cherry_picker 1967a87ad395552dbd1e16fd3f13d3b4b2f694de 3.14

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gpshead added the type-bug An unexpected behavior, bug, or error label

Oct 11, 2026

gpshead mentioned this pull request

Oct 11, 2026

[3.15] gh-159184: Do not compare set elements when marshalling a set (GH-159196) #159205

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bedevere-app Bot commented Oct 11, 2026

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GH-159205 is a backport of this pull request to the 3.15 branch.

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bedevere-app Bot removed the needs backport to 3.15 bugs and security fixes label

Oct 11, 2026

gpshead mentioned this pull request

Oct 11, 2026

[3.14] gh-159184: Do not compare set elements when marshalling a set (GH-159196) #159206

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bedevere-app Bot commented Oct 11, 2026

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GH-159206 is a backport of this pull request to the 3.14 branch.

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bedevere-app Bot removed the needs backport to 3.14 bugs and security fixes label

Oct 11, 2026

gpshead added a commit that referenced this pull request

Oct 11, 2026


          [3.14] gh-159184: Do not compare set elements when marshalling a set (G…

758278c

…H-159196) (#159206)

Set elements are written in the order of their dumps.  They were sorted
as (dump, element) pairs, so elements with the same dump were compared.
That failed for a set of several complex NaNs, which the compiler can
produce by constant folding, making the module impossible to marshal.
On this branch the failure was reported as MemoryError.

Sort (dump, index, element) tuples, in which the indexes are all
different.  Elements with the same dump stay in the order of the set,
as float NaNs did before.

(cherry picked from commit 1967a87)

gpshead added a commit that referenced this pull request

Oct 11, 2026


          [3.15] gh-159184: Do not compare set elements when marshalling a set (G…

8c7ac57

…H-159196) (#159205)

Set elements are written in the order of their dumps.  They were sorted
as (dump, element) pairs, so elements with the same dump were compared.
That failed for a set of several complex NaNs, which the compiler can
produce by constant folding, making the module impossible to marshal.
On this branch the failure was reported as MemoryError.

Sort (dump, index, element) tuples, in which the indexes are all
different.  Elements with the same dump stay in the order of the set,
as float NaNs did before.

(cherry picked from commit 1967a87)

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