fix: pretty format hidden variable behavior in stack traces

[dmadisetti]

📝 Summary

This PR renames "hidden" variables like _a (saved in globals as _cell_<id>_<variable>) in stack traces to the expected variable name.

closes #9623

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Project	Deployment	Actions	Updated (UTC)
marimo-docs	Ready	Preview, Comment	May 22, 2026 6:40pm

[cubic-dev-ai]

1 issue found across 8 files

Architecture diagram

sequenceDiagram
    participant User
    participant UI as Frontend UI
    participant Traceback as Traceback/Traceback Renderer
    participant MarimoError as MarimoErrorOutput
    participant MangledChip as MangledLocalChip
    participant LocalVars as local-variables Utils
    participant Runtime as Python Runtime
    participant Variables as _ast/variables
    participant Pytest as _runtime/pytest

    Note over User,Pytest: Error display flow for mangled cell-local variables

    User->>UI: Interacts with notebook cell
    UI->>Runtime: Execute cell code
    Runtime->>Variables: Compile cell (rewrites `_a` → `_cell_Hbol_a`)
    Runtime-->>UI: Returns error with mangled name (e.g., `name '_cell_Hbol_a' is not defined`)

    alt Standard output/message
        UI->>MarimoError: Render NameError message
        MarimoError->>LocalVars: splitMangledLocals(msg)
        LocalVars-->>MarimoError: Segments [text, {cellId, name}, text]
        MarimoError->>MangledChip: renderMangledSegments(segments)
        MangledChip->>MangledChip: Render variable name + tooltip
        MangledChip-->>MarimoError: React elements
        MarimoError-->>UI: Display demangled variable with link
    end

    alt Traceback display
        UI->>Traceback: Render full traceback
        Traceback->>LocalVars: replaceMangledLocal(domNode)
        LocalVars->>LocalVars: containsMangledLocal(text)
        alt Has mangled name
            LocalVars->>LocalVars: splitMangledLocals(text)
            LocalVars-->>Traceback: Segments
            Traceback->>MangledChip: renderMangledSegments(segments)
            MangledChip-->>Traceback: React elements
        else No mangled name
            LocalVars-->>Traceback: Return unchanged
        end
        Traceback-->>UI: Display demangled traceback
    end

    alt Pytest output
        Runtime->>Pytest: Run tests, capture report
        Pytest->>Variables: demangle_locals_in_text(lines)
        Variables-->>Pytest: Demangled exception/source lines
        Pytest->>Variables: demangle_locals_in_text(reprcrash.message)
        Variables-->>Pytest: Demangled summary message
        Pytest-->>Runtime: Rewritten longrepr
        Runtime-->>UI: Display demangled test output
    end

    Note over User,UI: User sees friendly variable name (e.g., `_a`) with tooltip showing defining cell

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[Copilot]

Pull request overview

This PR improves the readability of stack traces and error output by converting compiler-mangled “cell-local” variable names (e.g. _cell_<id>_a) back into the user-facing underscore-prefixed names (e.g. _a) in both backend pytest output and the frontend error/traceback UI.

Changes:

• Added a Python demangling helper (demangle_locals_in_text) and applied it to pytest longrepr formatting.
• Added a frontend utility to detect/split mangled locals and render them as user-friendly chips linking to the defining cell.
• Added unit tests on both Python and frontend sides to cover basic demangling/splitting behavior.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
tests/_ast/test_variables.py	Adds Python unit tests for demangling mangled locals in free-form text.
marimo/_runtime/pytest.py	Rewrites pytest longrepr traceback output to use marimo URIs and demangle locals.
marimo/_ast/variables.py	Introduces demangle_locals_in_text() via a compiled regex.
frontend/src/utils/local-variables.ts	Adds frontend parsing helpers to detect/split mangled locals for display.
frontend/src/utils/tests/local-variables.test.ts	Adds vitest coverage for frontend mangled-local parsing/splitting.
frontend/src/components/editor/output/MarimoTracebackOutput.tsx	Demangles mangled locals inside traceback HTML and the header message.
frontend/src/components/editor/output/MarimoErrorOutput.tsx	Demangles NameError messages for underscore-prefixed locals in the error UI.
frontend/src/components/editor/errors/mangled-local-chip.tsx	Adds UI rendering for mangled locals as chips with tooltips and cell links.

[dmadisetti]

Created #9671 for flaky tests