Cell internal variables not exposed to debugger #10269

Describe the bug

When debugging and trying to print variables in scope, the cell-internal variables (those starting with underscore) are not visible even when the debugger is viewing the scope or expression directly using them.

If we have a cell such as:

a = 7
_b = 3 + a
_b + "Hello"

Debugging the exception in that cell, we can do p a but we can't do p _b (that results in NameError: name '_b' is not defined).

Will you submit a PR?

• Yes

Environment

{
"marimo": "0.23.14",
"editable": false,
"location": "/Users/yairchu/dev/undersampled_add_sub/.venv/lib/python3.13/site-packages/marimo",
"OS": "Darwin",
"OS Version": "25.5.0",
"Processor": "arm",
"Python Version": "3.13.2",
"Locale": "C/en_US",
"Binaries": {
"Browser": "140.0.7339.134",
"Node": "v23.11.0",
"uv": "0.11.16 (135a36367 2026-05-21 aarch64-apple-darwin)"
},
"Dependencies": {
"click": "8.4.2",
"docutils": "0.23",
"itsdangerous": "2.2.0",
"jedi": "0.19.2",
"markdown": "3.10.2",
"narwhals": "2.24.0",
"packaging": "26.2",
"psutil": "7.2.2",
"pygments": "2.20.0",
"pymdown-extensions": "10.21.3",
"pyyaml": "6.0.3",
"starlette": "1.3.1",
"tomlkit": "0.15.1",
"typing-extensions": "4.16.0",
"uvicorn": "0.51.0",
"websockets": "16.1.1"
},
"Optional Dependencies": {
"anywidget": "0.11.0",
"loro": "1.13.2",
"mcp": "1.28.1",
"openai": "2.46.0",
"pandas": "3.0.3",
"python-lsp-ruff": "2.3.1",
"python-lsp-server": "1.14.0",
"ruff": "0.15.22",
"ty": "0.0.62"
},
"Experimental Flags": {}
}

Code to reproduce

a = 7
_b = 3 + a
_b + "Hello"

--- Debug exception

> p a
7
(Pdb)
> p _b
*** NameError: name '_b' is not defined
(Pdb)