FastAPI Setup

You are a Python backend developer. The user wants to scaffold a FastAPI application with async endpoints and auto-generated API documentation.

What to check first

• Run python --version to ensure Python 3.7+ is installed
• Check if pip is available with pip --version
• Verify no existing FastAPI project in the current directory

Steps

1. Create a new project directory and initialize a virtual environment with python -m venv venv
2. Activate the virtual environment: source venv/bin/activate (on Windows: venv\Scripts\activate)
3. Install FastAPI and Uvicorn with pip install fastapi uvicorn[standard]
4. Create main.py and import FastAPI from the fastapi module
5. Instantiate a FastAPI app with app = FastAPI(title="Your API", version="1.0.0")
6. Define async route handlers using @app.get(), @app.post(), etc. decorators with async def functions
7. Add Pydantic BaseModel classes for request/response validation with type hints
8. Run the development server with uvicorn main:app --reload to enable hot-reload and auto-docs at /docs

Code

from fastapi import FastAPI, HTTPException
from pydantic import BaseModel
from typing import Optional
import uvicorn

app = FastAPI(
    title="FastAPI App",
    description="A scalable async API with auto-generated documentation",
    version="1.0.0"
)

class Item(BaseModel):
    id: int
    name: str
    description: Optional[str] = None
    price: float

class ItemUpdate(BaseModel):
    name: Optional[str] = None
    description: Optional[str] = None
    price: Optional[float] = None

items_db = {}

@app.get("/", tags=["Root"])
async def read_root():
    """Root endpoint that returns a welcome message."""
    return {"message": "Welcome to FastAPI"}

@app.get("/items/{item_id}", tags=["Items"])
async def get_item(item_id: int):
    """Fetch a single item by ID."""
    if item_id not in items_db:
        raise HTTPException(status_code=404, detail="Item not found")
    return items_db[item_id]

@app.post("/items", tags=["Items"], response_model=Item)
async def create_item(item: Item):
    """Create a new item with validation."""
    items_db[item.id] = item
    return item

@app.put("/items/{item_id}", tags=["Items"])
async def update_item(item_id: int, item_update: ItemUpdate):
    """Update an existing item partially."""
    if item_id not in items_db:
        raise HTTPException(status_code=404, detail="Item not found")
    existing =

Note: this example was truncated in the source. See the GitHub repo for the latest full version.

Common Pitfalls

• Treating this skill as a one-shot solution — most workflows need iteration and verification
• Skipping the verification steps — you don't know it worked until you measure
• Applying this skill without understanding the underlying problem — read the related docs first

When NOT to Use This Skill

• When a simpler manual approach would take less than 10 minutes
• On critical production systems without testing in staging first
• When you don't have permission or authorization to make these changes

How to Verify It Worked

• Run the verification steps documented above
• Compare the output against your expected baseline
• Check logs for any warnings or errors — silent failures are the worst kind

Production Considerations

• Test in staging before deploying to production
• Have a rollback plan — every change should be reversible
• Monitor the affected systems for at least 24 hours after the change