What is the typing.Annotated with Field?

Question

Accepted Answer

**Annotated + Field** adds **validation metadata** to type hints.

```python
from typing import Annotated
from pydantic import Field, BaseModel

# Basic usage
def create_user(
    name: Annotated[str, Field(min_length=1, max_length=100)],
    age: Annotated[int, Field(ge=0, le=150)],
    email: Annotated[str, Field(pattern=r"^\S+@\S+$")],
):
    return {"name": name, "age": age, "email": email}

# With Pydantic models
class User(BaseModel):
    name: Annotated[str, Field(min_length=1)]
    age: Annotated[int, Field(ge=0)]
    tags: Annotated[list[str], Field(max_length=10)] = []

# FastAPI query parameters
from fastapi import Query

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

# Field with default_factory
class Config(BaseModel):
    items: Annotated[list[str], Field(default_factory=list)]
    metadata: Annotated[dict, Field(default_factory=dict)]

# With alias
class APIResponse(BaseModel):
    user_name: Annotated[str, Field(alias="userName")]
    created_at: Annotated[str, Field(alias="createdAt")]
```

**Field options:**
- `default`: default value.
- `default_factory`: callable for mutable defaults.
- `alias`: field name in serialized data.
- `description`: field description.
- `examples`: example values.

Python: What is the typing.Annotated with Field?