Fields

The same way you can declare additional validation and metadata in path operation function parameters with Query, Path and Body, you can declare validation and metadata inside of Pydantic models using Pydantic’s Field.

Import Field

First, you have to import it:Python 3.10+Python 3.9+Python 3.8+Python 3.10+ non-AnnotatedPython 3.8+ non-Annotatedfrom typing import Annotated from fastapi import Body, FastAPI from pydantic import BaseModel, Field app = FastAPI() class Item(BaseModel): name: str description: str | None = Field( default=None, title="The description of the item", max_length=300 ) price: float = Field(gt=0, description="The price must be greater than zero") tax: float | None = None @app.put("/items/{item_id}") async def update_item(item_id: int, item: Annotated[Item, Body(embed=True)]): results = {"item_id": item_id, "item": item} return results

Warning

Notice that Field is imported directly from pydantic, not from fastapi as are all the rest (Query, Path, Body, etc).

Declare model attribute

You can then use Field with model attributes:Python 3.10+Python 3.9+Python 3.8+Python 3.10+ non-AnnotatedPython 3.8+ non-Annotatedfrom typing import Annotated from fastapi import Body, FastAPI from pydantic import BaseModel, Field app = FastAPI() class Item(BaseModel): name: str description: str | None = Field( default=None, title="The description of the item", max_length=300 ) price: float = Field(gt=0, description="The price must be greater than zero") tax: float | None = None @app.put("/items/{item_id}") async def update_item(item_id: int, item: Annotated[Item, Body(embed=True)]): results = {"item_id": item_id, "item": item} return results

Field works the same way as Query, Path and Body, it has all the same parameters, etc.

Technical Details

Actually, Query, Path and others you’ll see next create objects of subclasses of a common Param class, which is itself a subclass of Pydantic’s FieldInfo class.

And Pydantic’s Field returns an instance of FieldInfo as well.

Body also returns objects of a subclass of FieldInfo directly. And there are others you will see later that are subclasses of the Body class.

Remember that when you import Query, Path, and others from fastapi, those are actually functions that return special classes.