# sanic

Guides Cursor when writing async Python code with Sanic: enforces the app factory pattern, Blueprints, non-blocking I/O, and background tasks.

- rule · `type=rule` · Cursor · ★ 3580
- Author: sanjeed5
- License: CC0-1.0 · policy=mirror · the beginning of the file is at the end of this page
- Source: https://github.com/sanjeed5/awesome-cursor-rules-mdc/tree/HEAD/rules-mdc/sanic.mdc
- Updated: 2026-05-19
- HTML: https://affpapa.org/en/nevibecode/rules/sanic

## What it does

The rule structures Sanic applications targeting `**/*.py` files. Instead of relying on a global app instance, it instructs Cursor to build modular services using a `create_app` factory and Blueprints. Configurations are separated into dedicated config classes. Shared state, such as database connection pools, attaches to `app.ctx` inside the `@app.listener("before_server_start")` lifecycle hook.

Non-blocking execution is strictly enforced. Calling synchronous functions like `time.sleep` in a handler stalls the entire event loop and ruins performance. The rule directs the agent to offload heavy sync tasks to a `ThreadPoolExecutor` via `request.app.loop.run_in_executor`. For secondary operations like user emails, it specifies `request.app.add_task` to return HTTP responses immediately.

## Install

### Cursor (native platform)

**Manually: put the files in place**
- File: `.cursor/rules/sanic.mdc` — Frontmatter: description, globs, alwaysApply

### Claude Code

**Manually: put the files in place**
- File: `.claude/rules/sanic.md` — The paths frontmatter field limits the files: move globs from the source into paths

### Codex

**AGENTS.md**
- Codex reads AGENTS.md: paste the rule text there

### GitHub Copilot

**Manually: put the files in place**
- File: `.github/instructions/sanic.instructions.md` — The applyTo field sets which files it applies to: move globs from the source into applyTo

### Windsurf (Devin Desktop)

**Manually: put the files in place**
- File: `.devin/rules/sanic.md`

### Cline

**Manually: put the files in place**
- File: `.clinerules/sanic.md`

Where a file is given instead of a command, copy the files from the source to that path: https://github.com/sanjeed5/awesome-cursor-rules-mdc/tree/HEAD/rules-mdc/sanic.mdc

## When to use it
- structure Sanic applications with the app factory pattern and Blueprints
- offload blocking I/O calls to a ThreadPoolExecutor
- initialize database pools inside a before_server_start listener
- run fire-and-forget background jobs using app.add_task

## Example prompts
- Refactor user routes into a Sanic Blueprint using the application factory pattern
- Create a signup endpoint that sends welcome emails in the background with app.add_task
- Offload a blocking file operation in Sanic using run_in_executor and a thread pool

## Source text · sanic.mdc · CC0-1.0

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: 24b80f772e96bb76.

BEGIN THIRD-PARTY FILE 24b80f772e96bb76
````text
---
description: Definitive guidelines for building high-performance, maintainable, and secure Sanic applications using modern Python async best practices.
globs: **/*.py
---
# sanic Best Practices

Sanic is built for speed and scalability. To harness its full power and maintain a robust codebase, adhere strictly to these guidelines.

## 1. Code Organization and Structure

Always structure your Sanic application using the **factory pattern** and **Blueprints** for modularity. Keep configuration separate.

❌ BAD: Global app instance, monolithic file
```python
# app.py
from sanic import Sanic, response

app = Sanic("my_app") # Global instance is hard to test and configure

@app.route("/")
async def hello_world(request):
    return response.json({"message": "Hello!"})

# Configuration directly in code
app.config.DB_URL = "sqlite:///db.sqlite"
```

✅ GOOD: Factory pattern, Blueprints, external config
```python
# config.py
import os

class Config:
    DB_URL = os.getenv("DB_URL", "sqlite:///db.sqlite")
    SECRET_KEY = os.getenv("SECRET_KEY", "super-secret-key")

# app.py
from sanic import Sanic
from .config import Config
from .blueprints.users import bp as users_bp
from .blueprints.auth import bp as auth_bp

def create_app() -> Sanic:
    app = Sanic("my_app")
    app.config.update(Config.__dict__) # Load config from a dedicated object

    app.blueprint(users_bp)
    app.blueprint(auth_bp)

    # Add listeners, middleware, etc. here
    @app.listener("before_server_start")
    async def setup_db(app, loop):
        # Example: Initialize a database connection pool
        app.ctx.db = "AsyncDatabaseConnection" # Placeholder for actual DB setup
        print(f"Connecting to DB: {app.config.DB_URL}")

    return app

# blueprints/users.py
from sanic import Blueprint, Request, response
from typing import Dict, Any

bp = Blueprint("users", url_prefix="/users")

@bp.route("/", methods=["GET"])
async def get_all_users(request: Request) -> response.HTTPResponse:
    # Simulate async DB call using app.ctx.db
    # await request.app.ctx.db.fetch_all("SELECT * FROM users")
    return response.json([{"id": 1, "name": "Alice"}])

@bp.route("/<user_id:int>", methods=["GET"])
async def get_user(request: Request, user_id: int) -> response.HTTPResponse:
    # Simulate async DB call
    # user = await request.app.ctx.db.fetch_one(f"SELECT * FROM users WHERE id={user_id}")
    user = {"id": user_id, "name": "Alice"} # Placeholder
    if user:
        return response.json(user)
    return response.json({"message": "User not found"}, status=404)
```

## 2. Common Patterns and Anti-patterns

**Never block the event loop.** Sanic's performance hinges on non-blocking I/O. Offload heavy computations or blocking I/O to background tasks or external services.

❌ BAD: Blocking I/O in a handler
```python
import time
from sanic import Request, response

@app.route("/blocking")
async def blocking_endpoint(request: Request) -> response.HTTPResponse:
    time.sleep(5) # Blocks the entire event loop!
    return response.json({"message": "Done after 5s"})
```

✅ GOOD: Use `asyncio.sleep` or offload blocking work
```python
import asyncio
from sanic import Request, response
from concurrent.futures import ThreadPoolExecutor # For I/O-bound blocking tasks
from typing import Dict, Any

# Use a global executor or pass it via app.ctx
executor = ThreadPoolExecutor()

def heavy_blocking_io_task(data: int) -> Dict[str, int]:
    # Simulate I/O-bound blocking task
    time.sleep(2)
    return {"result": data * 2}

@app.route("/non_blocking")
async def non_blocking_endpoint(request: Request) -> response.HTTPResponse:
    await asyncio.sleep(0.1) # Yields control, doesn't block
    return response.json({"message": "Done quickly"})

@app.route("/heavy_io_task")
async def heavy_io_task_endpoint(request: Request) -> response.HTTPResponse:
````
END THIRD-PARTY FILE 24b80f772e96bb76

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