marimo-pair-vscode

marimo is a reactive Python runtime for building reproducible Python programs (marimo notebooks). Cells are connected by the variables they define and reference. Running a cell re-executes dependents in dataflow order. The active runtime holds the kernel namespace, cell state, and dataflow graph. The notebook (`.py` file) is the artifact the kernel writes from that state while a session is running. A user interacts with the same runtime via a notebook UI with cells, outputs, and widgets. **WARNING. The active runtime is the source of truth.** While the user is pairing on an open notebook, you SHOULD NOT modify the associated `.py` file directly, and you SHOULD NOT use VS Code's `NotebookEdit` to change cells. File edits WILL NOT reach the active kernel or user, and the kernel may overwrite them on save; `NotebookEdit` bypasses the kernel's dataflow graph. Use `marimo._code_mode` (`cm`) for all notebook changes. Reading disk is fine, but prefer `ctx.cells[...].code` for current cell code. **WARNING. Every notebook edit goes through code mode — no exceptions.** When the user asks you to change the notebook (add, edit, delete, or run a cell; rename; change a value or a package), you MUST make that change through `marimo._code_mode` (`cm`). DO NOT edit the notebook `.py` file with `Edit` or `Write`, and DO NOT use VS Code's notebook-editing tools (`NotebookEdit`) — those bypass the live kernel, never reach the user's running session, and get clobbered when the kernel saves. Running ad-hoc Python with `marimo_executeCode` to explore or test is fine; _persisting any change to the notebook_ is only ever done via `cm`. ## Connect to a Notebook The notebook is already open in VS Code, and the extension manages its kernel. Run Python in that kernel with the `marimo_executeCode` tool, passing the `notebookUri` of the notebook the user is working in (VS Code provides the active notebook as context; if several are open or you are unsure which one, ask rather than guessing). The tool runs your `code` in the scratchpad — see below. ## Scratchpad Scope `marimo_executeCode` evaluates Python in marimo's scratchpad: a temporary namespace with a shallow copy of the kernel globals. Notebook variables are available by name, but new top-level bindings and rebindings are discarded after each call. In-place mutations to notebook-owned objects can persist because those names still reference live objects. Each call reports stdout and stderr from the scratchpad, plus console output from notebook cells it causes to run, including reactive descendants. ### Ordinary Python Use ordinary Python in the scratchpad to inspect variables, sample data, test transformations, probe APIs, check imports, and read widget state. ```python print(df.head()) x = 10 print(x) ``` Here `df` comes from notebook globals, while `x` is a scratchpad-local binding. `x` exists for this call only and WILL NOT be added to notebook globals. ### Persist with `cm` Top-level scratchpad assignments and rebindings are temporary. To persist work, including new variables, you MUST submit changes through `marimo._code_mode` (`cm`). `marimo._code_mode` is a PRIVATE, UNSTABLE agent API (note the leading underscore). It exists for tools like this skill to drive a live kernel from the scratchpad. DO NOT import it from notebook cells, library code, or anything a user would run — methods can change or disappear across marimo versions and kernels. Treat every `import marimo._code_mode as cm` as scratchpad-only. At session start, inspect what `cm` exposes in the active kernel: ```python import marimo._code_mode as cm help(cm) ``` Open a code-mode context to queue notebook changes. ```python import marimo._code_mode as cm async with cm.get_context() as ctx: cid = ctx.create_cell("x = df.head()") ctx.run_cell(cid) ``` The scratchpad supports top-level async code. Use `async with` directly; wrapping it in `asyncio.run(...)` is unnecessary and can conflict with the kernel's event loop. After this block exits and the new cell runs, `x` is notebook state. Later scratchpad calls can read `x` by name. Code later in the same scratchpad call should read `ctx.globals["x"]`, because the scratchpad namespace was copied before the cell ran. Inside the context, queued mutation methods are synchronous. Call them directly; do not `await` them. Each call queues an operation for marimo to apply when the context exits normally. If the block raises, the queue is discarded. On clean exit, marimo applies packages, validates and applies structural cell changes, runs queued cells, then may run dependents. Validation is only structural since queued cell runs can still error. `create_cell` and `edit_cell` change notebook structure only. Use `run_cell` to execute. `create_cell` currently defaults to `hide_code=True`, which collapses the code editor in the UI. Pass `hide_code=False` if the user wants created cells to be visible without manually expanding them. ## Marimo Rules marimo imposes a small contract on notebook code so it can keep the notebook as a directed acyclic graph (DAG): - **No cycles** - cells cannot depend on each other in a cycle. - **No public redefinitions across cells** - each name has one owning cell. - **No wildcard imports** - `import *` prevents static analysis of definitions. These rules keep the kernel, UI, and saved artifact consistent. When `cm` submits a cell body, marimo parses its top-level definitions and references. A top-level name enters the graph unless it is private with a leading underscore. ```python # Public definitions: values, total, i, value, mean values = np.array([1, 2, 3]) total = 0 for i, value in enumerate(values): total += value mean = total / len(values) mean ``` ```python # Public definition: mean _values = np.array([1, 2, 3]) _total = 0 for _i, _value in enumerate(_values): _total += _value mean = _total / len(_values) mean ``` Use private names for intermediates that no other cell should read. Public names define the notebook-level dataflow. If a `cm` edit violates the contract, marimo rejects the structural change and returns the validation error. ## The Notebook's Shape A notebook is an ordered collection of cells. `ctx.cells` is the document view and `ctx.graph` is the dataflow view. ```python for cell in ctx.cells: cell # .id, .code, .name, .config, .status, .errors ctx.cells["setup"] # by name ctx.cells[0] # by position list(ctx.cells.keys()) # all IDs, in notebook order ``` Cell IDs are opaque strings which can be queried from the notebook or captured from `cm` return values: ```python cid = ctx.create_cell("df = pd.read_csv('data.csv')") print(cid) # e.g. 'Hbol' ``` Alternatively, cells can be assigned and referenced by `name`. The graph can be used to understand its role in the dataflow. ```python for cid, impl in ctx.graph.cells.items(): impl # .defs, .refs (sets of public names) ctx.graph.descendants(cid) # cells that re-run when this one changes ctx.graph.ancestors(cid) # cells this one depends on ``` In marimo, deletes are _destructive_ so it can be useful to query the descendants prior to deleting to understand it's impact. ## Writing Notebook Changes The graph contract keeps marimo able to run and save the notebook. Passing those checks alone does not guarantee a useful artifact. Committed cells should still be readable, rerunnable, and editable. Make durable edits that reuse the notebook's existing names, imports, dependencies, and UI model. Don't be lazy. Avoid one-off workarounds that pass `cm` validation but leave a brittle notebook. ### Cell Bodies Submit the code that belongs in the cell. - **Submit cell contents** - `create_cell` and `edit_cell` take cell contents, not saved-file `@app.cell` wrappers. - **Read before replacing** - for now, another editor may change a cell between scratchpad calls. Before `edit_cell`, read the current body from `ctx.cells[...]` and submit the full replacement. - **Reuse notebook imports** - if `np` already exists, use it or edit the owning import cell. DO NOT add `import numpy as _np` just to bypass the graph. - **Define public names intentionally** - use public names for values later cells should reference. Use private `_name` bindings or function locals for same-cell intermediates. - **Define each public name once** - a public name has one owning cell. Reassigning it in another cell fails with `Multiply-defined names`; edit the owning cell or give the result a new name. See [gotchas.md](reference/gotchas.md). - **Run cells deliberately** - `create_cell` and `edit_cell` change structure only. Queue `ctx.run_cell(...)` when the cell should execute. ### Prefer `cm`-Managed Changes Use `cm` APIs when they exist. Avoid direct file edits, shell package commands, and scratchpad-only state for changes that should persist. - **Do not edit the `.py` artifact** - DO NOT use `Edit`, `Write`, or `NotebookEdit` on the notebook file during a live session. Use `ctx.edit_cell(...)` even for small changes. - **Manage packages through `cm`** - use `ctx.packages.add()` or `ctx.packages.remove()` instead of direct `uv` or `pip`; confirm non-obvious dependency changes. - **Avoid transient paths** - persisted cells should not depend on `/tmp/...` unless the work is intentionally transient. - **Delete deliberately** - deleting a cell removes globals it defines. Reuse empty cells when convenient and delete cells left empty after edits. ### UI and Widgets Inspect the object before changing it. Different UI objects update through different paths. - **Set `mo.ui.*` through `cm`** - use `ctx.set_ui_value(element, value)` inside `cm.get_context()`. - **Set anywidget traitlets directly** - synced traitlets are Python attributes, for example `widget.value = 5`. For designing custom visual or interactive output, see [rich-representations.md](reference/rich-representations.md). ## References - [gotchas.md](reference/gotchas.md) — name redefinition, cached module proxies, and notebook traps - [rich-representations.md](reference/rich-representations.md) — custom widgets and visualizations - [notebook-improvements.md](reference/notebook-improvements.md) — improving existing notebooks