FastAPI Development

Quick start

Create a basic FastAPI application:

from fastapi import FastAPI

app = FastAPI()

@app.get("/") async def read_root(): return {"Hello": "World"}

@app.get("/items/{item_id}") async def read_item(item_id: int, q: str | None = None): return {"item_id": item_id, "q": q}

Run with:

uvicorn main:app --reload

Common patterns

Pydantic models for validation

from pydantic import BaseModel from typing import Optional

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

@app.post("/items/") async def create_item(item: Item): return item

Dependency injection

from typing import Annotated from fastapi import Depends

async def common_parameters( q: str | None = None, skip: int = 0, limit: int = 100 ): return {"q": q, "skip": skip, "limit": limit}

CommonsDep = Annotated[dict, Depends(common_parameters)]

@app.get("/items/") async def read_items(commons: CommonsDep): return commons

Database dependencies with cleanup

async def get_db(): db = connect_to_database() try: yield db finally: db.close()

@app.get("/query/") async def query_data(db: Annotated[dict, Depends(get_db)]): return {"data": "query results"}

Error handling

from fastapi import HTTPException

@app.get("/items/{item_id}") async def read_item(item_id: int): if item_id < 1: raise HTTPException(status_code=404, detail="Item not found") return {"item_id": item_id}

Path and query validation

from typing import Annotated from fastapi import Path, Query

@app.get("/items/{item_id}") async def read_item( item_id: Annotated[int, Path(gt=0, le=1000)], q: Annotated[str, Query(max_length=50)] = None ): return {"item_id": item_id, "q": q}

Response models

from pydantic import BaseModel

class ItemPublic(BaseModel): id: int name: str price: float

@app.get("/items/{item_id}", response_model=ItemPublic) async def read_item(item_id: int): return ItemPublic(id=item_id, name="Laptop", price=999.99)

Testing with TestClient

from fastapi.testclient import TestClient

client = TestClient(app)

def test_read_root(): response = client.get("/") assert response.status_code == 200 assert response.json() == {"Hello": "World"}

def test_read_item(): response = client.get("/items/42?q=test") assert response.status_code == 200 assert response.json() == {"item_id": 42, "q": "test"}

Requirements

pip install fastapi uvicorn pip install "fastapi[all]" # Includes all optional dependencies

Key concepts

• Async/await: Use async def for I/O operations

• Automatic validation: Request/response validation with Pydantic

• Dependency injection: Share logic across endpoints with Depends

• Type hints: Full editor support and validation

• Interactive docs: Auto-generated Swagger/OpenAPI at /docs

• Background tasks: Run tasks after response using BackgroundTasks