[JoaoMarcos44]

Why

dict | ChainMap can replace a value exposed by the right-hand mapping with a deeper map's stored value. For example, ChainMap(Counter(), {'x': 5})['x'] is 0, but its reverse union returned 5.

What changed

• Update the result from the ChainMap view so its lookup semantics determine the values.
• Cover Counter and defaultdict, including collisions, key order, and the original maps' state.
• Add a NEWS entry.

Scope

The change is limited to reverse union and its regression tests.

Tradeoffs

The union now performs per-key chain lookups, as dict(chain) does. Long disjoint chains can require quadratic lookup work. Default factories can run and insert missing keys, as they do when reading the same ChainMap directly.

Verification

• Built native CPython 3.16.0a1+dev on Ubuntu 24.04. Both regression tests fail against the original module and pass with the fix; the minimal reproduction changes from the wrong 5 to the expected 0.
• Ran test_collections test_userdict test_userlist test_userstring test_numeric_tower with ten hash seeds. Each pass reports 286 tests, 284 passes, two skips, and zero failures or errors. Both new tests are present and pass in every run. Native validation and results.
• Repository lint, make patchcheck, and git diff --check pass. Lint result. Contribution checklist result.

Illustrated explanation

The first Counter() layer returns 0 for the missing key. The old reverse union copied the deeper stored 5; the fix preserves the value returned by the ChainMap view.

Accessible text explanation and validation notes. The illustration is supplemental; it is hosted outside this source patch.

Fixes #160929.

AI tools assisted the investigation, patch, tests, and supplemental illustration. I have personally reviewed the AI-assisted material in this PR.

• Issue: ChainMap reverse union bypasses missing-key lookup semantics #160929

[python-cla-bot]

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[chatgpt-codex-connector]

Codex Review Summary

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Review	Status	Commit	Review trigger
📝 Code Review	⚠️ Failed 2026-10-29T21:50:57.754299Z We couldn’t complete this review. Comment @codex review to try again.	aa59123	PR opened

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