Marimo

Contents

• Editing and Verification Enforcement
• Key Concepts
• Cell Structure
• Editing Rules
• Core CLI Commands
• Export Commands
• Data and Visualization
• Debugging Workflow
• Common Issues
• Additional Resources

Marimo Reactive Notebooks

Marimo is a reactive Python notebook where cells form a DAG and auto-execute on dependency changes. Notebooks are stored as pure .py files.

Editing and Verification Enforcement

IRON LAW #1: NEVER MODIFY CELL DECORATORS OR SIGNATURES

Only edit code INSIDE @app.cell function bodies. This is not negotiable.

NEVER modify:

• Cell decorators (@app.cell)
• Function signatures (def _(deps):)
• Return statements structure (trailing commas required)

ALWAYS verify:

• All used variables are in function parameters
• All created variables are in return statement
• Trailing comma for single returns: return var,

IRON LAW #2: NO EXECUTION CLAIM WITHOUT OUTPUT VERIFICATION

Before claiming ANY marimo notebook works:

1. VALIDATE syntax and structure: marimo check notebook.py
2. EXECUTE with outputs: marimo export ipynb notebook.py -o __marimo__/notebook.ipynb --include-outputs
3. VERIFY using notebook-debug skill's verification checklist
4. CLAIM success only after verification passes

This is not negotiable. Claiming "notebook works" without executing and inspecting outputs is LYING to the user.

Rationalization Table - STOP If You Think:

Red Flags - STOP Immediately If You Think:

• "Let me add this variable to the function signature" → NO. Marimo manages signatures.
• "I'll just run marimo check and call it done" → NO. Execute with outputs required.
• "The code looks correct" → NO. Marimo's reactivity must be verified at runtime.
• "I can redefine this variable in another cell" → NO. One variable = one cell.

Editing Checklist

Before every marimo edit:

Structure Validation:

• Only edit code INSIDE @app.cell function bodies
• Do NOT modify decorators or signatures
• Verify all used variables are in function parameters
• Verify all created variables are in return statement
• Ensure trailing comma used for single returns
• Ensure no variable redefinitions across cells

Syntax Validation:

• Execute marimo check notebook.py
• Verify no syntax errors reported
• Verify no undefined variable warnings
• Verify no redefinition warnings

Runtime Verification:

• Execute with marimo export ipynb notebook.py -o __marimo__/notebook.ipynb --include-outputs
• Verify export succeeded (exit code 0)
• Verify output ipynb exists and is non-empty
• Apply notebook-debug verification checklist
• Verify no tracebacks in any cell
• Verify all cells executed (execution_count not null)
• Verify outputs match expectations

Only after ALL checks pass:

• Claim "notebook works"

Gate Function: Marimo Verification

Follow this sequence for EVERY marimo task:

1. EDIT     → Modify code inside @app.cell function bodies only
2. CHECK    → marimo check notebook.py
3. EXECUTE  → marimo export ipynb notebook.py -o __marimo__/notebook.ipynb --include-outputs
4. INSPECT  → Use notebook-debug verification
5. VERIFY   → Outputs match expectations
6. CLAIM    → "Notebook works" only after all gates passed

NEVER skip verification gates. Marimo's reactivity means changes propagate unpredictably.

Honesty Framing

Claiming a marimo notebook works without executing it with --include-outputs and inspecting the results is LYING.

Syntax checks and code inspection prove nothing about reactive execution correctness. The user expects a working notebook where all cells execute correctly with proper dependency tracking.

Key Concepts

• Reactive execution: Cells auto-update when dependencies change
• No hidden state: Each variable defined in exactly one cell
• Pure Python: .py files, version control friendly
• Cell structure: @app.cell decorator pattern

Cell Structure

import marimo

app = marimo.App()

@app.cell
def _(pl):  # Dependencies as parameters
    df = pl.read_csv("data.csv")
    return df,  # Trailing comma required for single return

@app.cell
def _(df, pl):
    summary = df.describe()
    filtered = df.filter(pl.col("value") > 0)
    return summary, filtered  # Multiple returns

Editing Rules

• Edit code INSIDE @app.cell functions only
• Never modify cell decorators or function signatures
• Variables cannot be redefined across cells
• All used variables must be returned from their defining cell
• Markdown cells: Always wrap $ in backticks - mo.md("Cost: $50") not mo.md("Cost: $50")

Core CLI Commands

Export Commands

# marimo: Export to ipynb with code only
marimo export ipynb notebook.py -o __marimo__/notebook.ipynb

# marimo: Export to ipynb with outputs (runs notebook first)
marimo export ipynb notebook.py -o __marimo__/notebook.ipynb --include-outputs

# marimo: Export to HTML (runs notebook by default)
marimo export html notebook.py -o __marimo__/notebook.html

# marimo: Export to HTML with auto-refresh on changes (live preview)
marimo export html notebook.py -o __marimo__/notebook.html --watch

Key difference: HTML export runs the notebook by default. ipynb export does NOT - use --include-outputs to run and capture outputs.

Tip: Use __marimo__/ folder for all exports (ipynb, html). The editor can auto-save there.

Data and Visualization

• Prefer polars over pandas for performance
• Use mo.ui for interactive widgets
• SQL cells: mo.sql(df, "SELECT * FROM df")
• Display markdown: mo.md("# Heading")

Debugging Workflow

1. Pre-execution validation:

# scripts: Validate notebook syntax and cell structure
scripts/check_notebook.sh notebook.py

Runs syntax check, marimo validation, and cell structure overview in one command.

2. Runtime errors: Export with outputs, then use notebook-debug skill:

# marimo: Export to ipynb with outputs for inspection
marimo export ipynb notebook.py -o __marimo__/notebook.ipynb --include-outputs

Common Issues

Additional Resources

Reference Files

For detailed patterns and advanced techniques, consult:

• references/reactivity.md - DAG execution, variable rules, dependency detection patterns
• references/debugging.md - Error patterns, runtime debugging, environment-specific issues
• references/widgets.md - Interactive UI components and mo.ui patterns
• references/sql.md - SQL cells and database integration techniques

Examples

Working examples available in examples/:

• examples/basic_notebook.py - Minimal marimo notebook structure
• examples/data_analysis.py - Data loading, filtering, and visualization patterns
• examples/interactive_widgets.py - Interactive UI component usage

Scripts

Validation utilities in scripts/:

• scripts/check_notebook.sh - Primary validation: syntax check, marimo validation, cell structure overview
• scripts/get_cell_map.py - Extract cell metadata (invoked by check_notebook.sh)

Related Skills

• notebook-debug - Debugging executed ipynb files with tracebacks and output inspection