Body - 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:
from 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
from typing import Annotated, Union
from fastapi import Body, FastAPI
from pydantic import BaseModel, Field
app = FastAPI()
class Item(BaseModel):
name: str
description: Union[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: Union[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
from typing import Union
from fastapi import Body, FastAPI
from pydantic import BaseModel, Field
from typing_extensions import Annotated
app = FastAPI()
class Item(BaseModel):
name: str
description: Union[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: Union[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
Tip
Prefer to use the Annotated version if possible.
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: Item = Body(embed=True)):
results = {"item_id": item_id, "item": item}
return results
Tip
Prefer to use the Annotated version if possible.
from typing import Union
from fastapi import Body, FastAPI
from pydantic import BaseModel, Field
app = FastAPI()
class Item(BaseModel):
name: str
description: Union[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: Union[float, None] = None
@app.put("/items/{item_id}")
async def update_item(item_id: int, item: 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 attributes¶
You can then use Field with model attributes:
from 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
from typing import Annotated, Union
from fastapi import Body, FastAPI
from pydantic import BaseModel, Field
app = FastAPI()
class Item(BaseModel):
name: str
description: Union[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: Union[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
from typing import Union
from fastapi import Body, FastAPI
from pydantic import BaseModel, Field
from typing_extensions import Annotated
app = FastAPI()
class Item(BaseModel):
name: str
description: Union[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: Union[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
Tip
Prefer to use the Annotated version if possible.
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: Item = Body(embed=True)):
results = {"item_id": item_id, "item": item}
return results
Tip
Prefer to use the Annotated version if possible.
from typing import Union
from fastapi import Body, FastAPI
from pydantic import BaseModel, Field
app = FastAPI()
class Item(BaseModel):
name: str
description: Union[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: Union[float, None] = None
@app.put("/items/{item_id}")
async def update_item(item_id: int, item: 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.
Tip
Notice how each model's attribute with a type, default value and Field has the same structure as a path operation function's parameter, with Field instead of Path, Query and Body.
Add extra information¶
You can declare extra information in Field, Query, Body, etc. And it will be included in the generated JSON Schema.
You will learn more about adding extra information later in the docs, when learning to declare examples.
Warning
Extra keys passed to Field will also be present in the resulting OpenAPI schema for your application.
As these keys may not necessarily be part of the OpenAPI specification, some OpenAPI tools, for example the OpenAPI validator, may not work with your generated schema.
Recap¶
You can use Pydantic's Field to declare extra validations and metadata for model attributes.
You can also use the extra keyword arguments to pass additional JSON Schema metadata.