marimo convert: variable renamed inside nested function bodies but not their parameters, producing wrong results

[damyantilev]

Describe the bug

When marimo convert privatizes a cell-level variable x to _x (because it is defined in multiple cells and would otherwise collide), it also renames every occurrence of x inside that cell — including references within nested def/lambda bodies whose own parameter is named x. The parameter is left as x, so the body now reads the cell-level _x closure instead of the argument.

This is silent when the enclosing cell defines _x: the function ignores its argument and reads the closure, so it runs without error and produces a wrong result (no crash, and marimo check reports nothing). When the enclosing cell does not define _x, it instead becomes a NameError / undefined-name at runtime.

Root problem: a function/lambda parameter introduces a new scope that shadows the cell-level name, so occurrences of that name inside the function body should not be rewritten by the cell-level rename.

Expected: the nested function's parameter and its uses stay consistent — either the parameter is renamed too, or (correct) the body is left alone because the parameter shadows the cell-level variable.

Actual: body uses are renamed to _x, parameter stays x → function silently ignores its argument.

Reproduction

bug.ipynb — two code cells (n appears in both cells, so convert privatizes it):

Cell 1:

def double(n):
    return n * 2

n = 5
print('double(3) =', double(3))   # correct: 6

Cell 2:

n = 999
print('n =', n)

Run:

marimo convert bug.ipynb -o bug.py

Produced bug.py (note def double(n) but body return _n * 2):

import marimo

__generated_with = "0.24.0"
app = marimo.App()


@app.cell
def _():
    def double(n):
        return _n * 2
    _n = 5
    print('double(3) =', double(3))
    return


@app.cell
def _():
    _n = 999
    print('n =', _n)
    return


if __name__ == "__main__":
    app.run()

double(3) now returns 10 (_n * 2 = 5 * 2) instead of 6. The function silently ignores its argument. This also affects lambda parameters, comprehension/for targets, and with ... as targets that share a name with a privatized cell-level variable.

Will you submit a PR?

• Yes

Environment

{
  "marimo": "0.24.0",
  "editable": false,
  "OS": "Linux",
  "OS Version": "6.17.0-1032-oem",
  "Processor": "x86_64",
  "Python Version": "3.12.13",
  "Locale": "en_US",
  "Binaries": {
    "Browser": "--",
    "Node": "v18.19.1",
    "uv": "0.10.8"
  },
  "Dependencies": {
    "click": "8.4.2",
    "docutils": "0.23",
    "itsdangerous": "2.2.0",
    "jedi": "0.19.2",
    "markdown": "3.10.3",
    "narwhals": "2.24.0",
    "packaging": "26.2",
    "psutil": "7.2.2",
    "pygments": "2.20.0",
    "pymdown-extensions": "11.0.2",
    "pyyaml": "6.0.3",
    "starlette": "1.2.1",
    "tomlkit": "0.15.1",
    "typing-extensions": "4.16.0",
    "uvicorn": "0.52.0",
    "websockets": "16.1.1"
  },
  "Optional Dependencies": {
    "loro": "1.13.2",
    "openai": "2.50.0",
    "pandas": "3.0.5",
    "pytest": "9.1.1",
    "ruff": "0.16.0"
  },
  "Experimental Flags": {}
}

Code to reproduce

No response