marimo-notebook

name	marimo-notebook
description	Write a marimo notebook in a Python file in the right format.

Notes for marimo Notebooks

Running Marimo Notebooks

# Run as script (non-interactive, for testing)
uv run <notebook.py>

# Run interactively in browser
uv run marimo run <notebook.py>

# Edit interactively
uv run marimo edit <notebook.py>

Script Mode Detection

Use mo.app_meta().mode == "script" to detect CLI vs interactive:

@app.cell
def _(mo):
    is_script_mode = mo.app_meta().mode == "script"
    return (is_script_mode,)

Key Principle: Keep It Simple

Show all UI elements always. Only change the data source in script mode.

• Sliders, buttons, widgets should always be created and displayed
• In script mode, just use synthetic/default data instead of waiting for user input
• Don't wrap everything in if not is_script_mode conditionals
• Don't use try/except for normal control flow

Good Pattern

# Always show the widget
@app.cell
def _(ScatterWidget, mo):
    scatter_widget = mo.ui.anywidget(ScatterWidget())
    scatter_widget
    return (scatter_widget,)

# Only change data source based on mode
@app.cell
def _(is_script_mode, make_moons, scatter_widget, np, torch):
    if is_script_mode:
        # Use synthetic data for testing
        X, y = make_moons(n_samples=200, noise=0.2)
        X_data = torch.tensor(X, dtype=torch.float32)
        y_data = torch.tensor(y)
        data_error = None
    else:
        # Use widget data in interactive mode
        X, y = scatter_widget.widget.data_as_X_y
        # ... process data ...
    return X_data, y_data, data_error

# Always show sliders - use their .value in both modes
@app.cell
def _(mo):
    lr_slider = mo.ui.slider(start=0.001, stop=0.1, value=0.01)
    lr_slider
    return (lr_slider,)

# Auto-run in script mode, wait for button in interactive
@app.cell
def _(is_script_mode, train_button, lr_slider, run_training, X_data, y_data):
    if is_script_mode:
        # Auto-run with slider defaults
        results = run_training(X_data, y_data, lr=lr_slider.value)
    else:
        # Wait for button click
        if train_button.value:
            results = run_training(X_data, y_data, lr=lr_slider.value)
    return (results,)

Don't Guard Cells with if Statements

Marimo's reactivity means cells only run when their dependencies are ready. Don't add unnecessary guards:

# BAD - the if statement prevents the chart from showing
@app.cell
def _(plt, training_results):
    if training_results:  # WRONG - don't do this
        fig, ax = plt.subplots()
        ax.plot(training_results['losses'])
        fig
    return

# GOOD - let marimo handle the dependency
@app.cell
def _(plt, training_results):
    fig, ax = plt.subplots()
    ax.plot(training_results['losses'])
    fig
    return

The cell won't run until training_results has a value anyway.

Don't Use try/except for Control Flow

Don't wrap code in try/except blocks unless you're handling a specific, expected exception. Let errors surface naturally.

# BAD - hiding errors behind try/except
@app.cell
def _(scatter_widget, np, torch):
    try:
        X, y = scatter_widget.widget.data_as_X_y
        X = np.array(X, dtype=np.float32)
        # ...
    except Exception as e:
        return None, None, f"Error: {e}"

# GOOD - let it fail if something is wrong
@app.cell
def _(scatter_widget, np, torch):
    X, y = scatter_widget.widget.data_as_X_y
    X = np.array(X, dtype=np.float32)
    # ...

Only use try/except when:

• You're handling a specific, known exception type
• The exception is expected in normal operation (e.g., file not found)
• You have a meaningful recovery action

Cell Output Rendering

Marimo only renders the final expression of a cell. Indented or conditional expressions won't render:

# BAD - indented expression won't render
@app.cell
def _(mo, condition):
    if condition:
        mo.md("This won't show!")  # WRONG - indented
    return

# GOOD - final expression renders
@app.cell
def _(mo, condition):
    result = mo.md("Shown!") if condition else mo.md("Also shown!")
    result  # This renders because it's the final expression
    return

Marimo Variable Naming

Variables in for loops that would conflict across cells need underscore prefix:

# Use _name, _model to make them cell-private
for _name, _model in items:
    ...

PEP 723 Dependencies

# /// script
# requires-python = ">=3.12"
# dependencies = [
#     "marimo",
#     "torch>=2.0.0",
# ]
# ///

Prefer pathlib over os.path

Use pathlib.Path for file path operations instead of os.path:

# GOOD - use pathlib
from pathlib import Path
data_dir = Path(tempfile.mkdtemp())
parquet_file = data_dir / "data.parquet"

# BAD - avoid os.path
import os
parquet_file = os.path.join(temp_dir, "data.parquet")

marimo check

When working on a notebook it is important to check if the notebook can run. That's why marimo provides a check command that acts as a linter to find common mistakes.

uvx marimo check <notebook.py>

Make sure these are checked before handing a notebook back to the user.