fix: avoid double-mangling names inside walrus comprehension

[nojaf]

📝 Summary

Closes #9274

Walrus expressions at module scope that hold a comprehension as their value were having names inside the comprehension mangled twice, producing errors like:

NameError: name '_cell_MJUe_cell_MJUe_values' is not defined

Root cause

In visit_NamedExpr (marimo/_ast/visitor.py), node.value was visited explicitly, and then — when the walrus was not inside a comprehension — self.generic_visit(node) was called, which visits all children of the node and therefore re-visited node.value. For a walrus whose value is a comprehension (e.g. _matches := {k for k, v in _values.items() if v > 1}), this entered the comprehension's scope twice. After #8762 extended private-name mangling to nested-scope lookups, the second visit re-mangled the already-mangled private name, producing the double prefix.

Fix

In the non-comprehension branch of visit_NamedExpr, only visit node.target — the value was already visited earlier in the method.

Reproduction (from the issue)

items = {"a": {"x": 1, "y": 2}, "b": {"x": 3, "y": 4}}
for _key, _values in items.items():
    if _matches := {k for k, v in _values.items() if v > 1}:
        pass

Before: NameError: name '_cell_XXX_cell_XXX_values' is not defined. After: runs successfully.

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✅ Merge Checklist

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• Tests have been added for the changes made.

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Project	Deployment	Actions	Updated (UTC)
marimo-docs	Ready	Preview, Comment	Apr 20, 2026 3:23pm

[Copilot]

Pull request overview

Fixes a scoping/mangling bug in the AST ScopedVisitor where module-scope walrus expressions whose value is a comprehension could cause names inside that comprehension to be mangled twice (regression for #9274).

Changes:

• Adjust visit_NamedExpr traversal to avoid re-visiting node.value (and thus re-entering comprehension scope).
• Add a regression test to ensure comprehension-in-walrus values do not produce double-mangled names.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
marimo/_ast/visitor.py	Prevents double traversal of walrus value by visiting only target in the non-comprehension branch.
tests/_ast/test_visitor.py	Adds regression coverage to ensure names in comprehension values are mangled exactly once.

[manzt]

Thank you for fixing! I also put up the same fix in #9277. Would you mind copying over the other parameterized regression test from that PR and we can get yours merged? Added in 93dcbaf.

[codecov]

Bundle Report

Changes will increase total bundle size by 225.07kB (0.9%) ⬆️. This is within the configured threshold ✅

Detailed changes

Bundle name	Size	Change
marimo-esm	25.1MB	225.07kB (0.9%) ⬆️

Affected Assets, Files, and Routes:

view changes for bundle: marimo-esm

Assets Changed:

Asset Name	Size Change	Total Size	Change (%)
assets/cells-*.js	59 bytes	704.0kB	0.01%
assets/index-*.js	3.6kB	605.9kB	0.6%
assets/index-*.css	386 bytes	362.67kB	0.11%
assets/JsonOutput-*.js	2.15kB	342.27kB	0.63%
assets/edit-*.js	1 bytes	329.61kB	0.0%
assets/add-*.js	7 bytes	192.76kB	0.0%
assets/layout-*.js	-311 bytes	185.6kB	-0.17%
assets/cell-*.js	3 bytes	183.15kB	0.0%
assets/reveal-*.js (New)	165.6kB	165.6kB	100.0% 🚀
assets/swiper-*.js (New)	116.4kB	116.4kB	100.0% 🚀
assets/reveal-*.css (New)	54.08kB	54.08kB	100.0% 🚀
assets/file-*.js	50 bytes	49.4kB	0.1%
assets/panels-*.js	6 bytes	45.36kB	0.01%
assets/session-*.js	-9 bytes	24.99kB	-0.04%
assets/state-*.js	46 bytes	24.45kB	0.19%
assets/home-*.js	132 bytes	21.86kB	0.61%
assets/purify.es-*.js	16 bytes	21.05kB	0.08%
assets/swiper-*.css (New)	15.12kB	15.12kB	100.0% 🚀
assets/run-*.js	-735 bytes	9.74kB	-7.02%
assets/column-*.js	-659 bytes	6.53kB	-9.16%
assets/mermaid-*.core-B0EhnNjP.js (New)	2.38kB	2.38kB	100.0% 🚀
assets/slide-*.js (New)	615 bytes	615 bytes	100.0% 🚀
assets/slides-*.css	-15.1kB	305 bytes	-98.02%
assets/slides-*.js	-116.37kB	0 bytes	-100.0%
assets/mermaid-*.core-Blvy2Lmj.js (Deleted)	-2.38kB	0 bytes	-100.0% 🗑️

[manzt]

Added the additional regression test from #9277

[nojaf]

Thanks @manzt !

[manzt]

thank you!

[dmadisetti]

CI is flaky, but the runtime and AST tests passed. Thanks @nojaf !