What is Annotated and how to use it with FastAPI?

Question

Accepted Answer

**Annotated** adds **metadata** to type hints. Used with FastAPI for parameter validation.

```python
from typing import Annotated
from fastapi import FastAPI, Query, Path, Body
from pydantic import Field

app = FastAPI()

# Query parameters
@app.get("/items/")
async def read_items(
    q: Annotated[str | None, Query(min_length=3, max_length=50)] = None,
    skip: Annotated[int, Query(ge=0)] = 0,
    limit: Annotated[int, Query(ge=1, le=100)] = 10,
):
    return {"q": q, "skip": skip, "limit": limit}

# Path parameters
@app.get("/items/{item_id}")
async def read_item(
    item_id: An[int, Path(ge=1, le=1000)]
):
    return {"item_id": item_id}

# Body parameters
@app.put("/items/{item_id}")
async def update_item(
    item_id: int,
    name: Annotated[str, Body(min_length=1)] = "Default",
    price: Annotated[float, Body(gt=0)] = 0.0,
):
    return {"item_id": item_id, "name": name, "price": price}

# With Pydantic models
from pydantic import BaseModel

class Item(BaseModel):
    name: str
    price: float = Field(gt=0)

@app.post("/items/")
async def create_item(item: Annotated[Item, Body()]):
    return item
```

**Benefits:**
- Reusable validation rules.
- Better type hints.
- Documentation generation.

Python: What is Annotated and how to use it with FastAPI?