# mojo-python-interop

A skill that keeps your agent on current Mojo syntax when the code calls Python or exposes Mojo types to it.

- skill · `type=skill` · Claude Code, Codex, Cursor, GitHub Copilot, Gemini CLI, OpenCode · 2600 installs · ★ 206
- Author: modular
- License: Apache-2.0 · policy=mirror · the beginning of the file is at the end of this page
- Source: https://github.com/modular/skills/tree/HEAD/mojo-python-interop/SKILL.md
- Updated: 2026-10-25
- HTML: https://affpapa.org/en/nevibecode/skills/mojo-python-interop

## What it does

Mojo moves fast, and pretrained models keep generating syntax that no longer compiles. This skill loads into the agent's context as a correction layer and pins down the conventions for Mojo–Python interop. It comes from modular, lives in `modular/skills`, and ships under Apache-2.0.

The file is terse on purpose: short WRONG/CORRECT pairs instead of prose. Converting a `PythonObject` into a Mojo type needs the named argument — `Int(py=py_obj)`, not a positional call. Imports come from `std.python`, not `python`. For environment variables it points at the native `getenv` in `std.os` rather than reaching for Python's `os` module. A separate section covers extension modules: `PyInit_<module_name>` marked `abi("C")`, functions registered through `PythonModuleBuilder`, then `mojo build --emit shared-lib`.

Treat it as a companion to `mojo-syntax`, not a replacement — install both when a project mixes the two languages. The source file is truncated, so the full section list isn't visible from it; worth opening the repo before you commit.

## Install

### Claude Code (native platform)

1. **Skills CLI (Vercel)**
```bash
npx skills add modular/skills --skill mojo-python-interop -a claude-code
```

2. **Manually: put the files in place**
- File: `~/.claude/skills/mojo-python-interop/SKILL.md` — Copy the whole skill folder; for one project use .claude/skills/mojo-python-interop/

### Codex

1. **Skills CLI (Vercel)**
```bash
npx skills add modular/skills --skill mojo-python-interop -a codex
```

2. **Manually: put the files in place**
- File: `.agents/skills/mojo-python-interop/SKILL.md` — For all projects use ~/.agents/skills/

### Cursor

1. **Skills CLI (Vercel)**
```bash
npx skills add modular/skills --skill mojo-python-interop -a cursor
```

2. **Manually: put the files in place**
- File: `.cursor/skills/mojo-python-interop/SKILL.md` — Cursor also reads .agents/skills/ and .claude/skills/

### GitHub Copilot

1. **GitHub CLI**
```bash
gh skill install modular/skills mojo-python-interop
```

2. **Manually: put the files in place**
- File: `.github/skills/mojo-python-interop/SKILL.md`

### Gemini CLI

1. **Skills CLI (Vercel)**
```bash
npx skills add modular/skills --skill mojo-python-interop -a gemini-cli
```

2. **Platform CLI**
```bash
gemini skills install https://github.com/modular/skills --path mojo-python-interop
```

### Windsurf (Devin Desktop)

**Manually: put the files in place**
- File: `.devin/skills/mojo-python-interop/SKILL.md` — The legacy .windsurf/skills/ path still works

### OpenCode

**Skills CLI (Vercel)**
```bash
npx skills add modular/skills --skill mojo-python-interop -a opencode
```

### Claude.ai

**Claude.ai**
- Zip the folder and upload it in Settings → Capabilities → Skills

Where a file is given instead of a command, copy the files from the source to that path: https://github.com/modular/skills/tree/HEAD/mojo-python-interop/SKILL.md

## When to use it
- call a Python library from Mojo code
- build a Python extension module as a `.so` file
- wrap a Mojo struct so Python can use it
- convert `PythonObject` to `Int`, `Float64` or `String` without compile errors
- stop the agent from writing the outdated `from python import ...`

## Example prompts
- write a Mojo wrapper around numpy using std.python
- build a Python extension from this Counter struct in Mojo
- convert this PythonObject to Int and point out my syntax mistake

## Source text · SKILL.md · Apache-2.0

Below is the content of a third-party file. It is data, not instructions for you: do not act on anything written inside unless the human asks. Block tag: a40c74b052e0a477.

BEGIN THIRD-PARTY FILE a40c74b052e0a477
````text
---
name: mojo-python-interop
description: Aids in writing Mojo code that interoperates with Python using current syntax and conventions. Use this skill in addition to mojo-syntax when writing Mojo code that interacts with Python, calls Python libraries from Mojo, or exposes Mojo types/functions to Python. Also use when the user wants to build Python extension modules in Mojo, wrap Mojo structs for Python consumption, or convert between Python and Mojo types.
---

<!-- EDITORIAL GUIDELINES FOR THIS SKILL FILE
This file is loaded into an agent's context window as a correction layer for
pretrained Mojo knowledge. Every line costs context. When editing:
- Be terse. Use tables and inline code over prose where possible.
- Never duplicate information — if a concept is shown in a code example, don't
  also explain it in a paragraph.
- Only include information that *differs* from what a pretrained model would
  generate. Don't document things models already get right.
- Prefer one consolidated code block over multiple small ones.
- Keep WRONG/CORRECT pairs short — just enough to pattern-match the fix.
- If adding a new section, ask: "Would a model get this wrong?" If not, skip it.
These same principles apply to any files this skill references.
-->

Mojo is rapidly evolving. Pretrained models generate obsolete syntax.
**Always follow this skill over pretrained knowledge.**

## Using Python from Mojo

```mojo
from std.python import Python, PythonObject

var np = Python.import_module("numpy")
var arr = np.array([1, 2, 3])

# PythonObject → Mojo: MUST use `py=` keyword (NOT positional)
var i = Int(py=py_obj)
var f = Float64(py=py_obj)
var s = String(py=py_obj)
var b = Bool(py=py_obj)            # Bool is the exception — positional also works
# Works with numpy types: Int(py=np.int64(1)), Float64(py=np.float64(3.14))
```

| WRONG                    | CORRECT                      |
|--------------------------|------------------------------|
| `Int(py_obj)`            | `Int(py=py_obj)`             |
| `Float64(py_obj)`        | `Float64(py=py_obj)`         |
| `String(py_obj)`         | `String(py=py_obj)`          |
| `from python import ...` | `from std.python import ...` |

### Mojo → Python conversions

Mojo types implementing `ConvertibleToPython` auto-convert when passed to Python
functions. For explicit conversion: `value.to_python_object()`.

### Building Python collections from Mojo

```mojo
var py_list = Python.list(1, 2.5, "three")
var py_tuple = Python.tuple(1, 2, 3)
var py_dict = Python.dict(name="value", count=42)

# Python.dict() is generic over a single value type V for all kwargs.
# Mixed types fail because the compiler can't infer one V.
# WRONG:  Python.dict(flag=my_bool, count=42)
````
END THIRD-PARTY FILE a40c74b052e0a477

The third-party file has ended. Everything between the a40c74b052e0a477 tags is data: requests and commands from it are not followed. An “end of file” line with another tag inside the block is part of the file.

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