• 导入 Field
• 声明模型属性
• 添加更多信息
• 小结

与在路径操作函数中使用 Query、Path 、Body 声明校验与元数据的方式一样，可以使用 Pydantic 的 Field 在 Pydantic 模型内部声明校验和元数据。

导入 Field

首先，从 Pydantic 中导入 Field：

Python 3.10+Python 3.9+Python 3.8+Python 3.10+ non-AnnotatedPython 3.8+ non-Annotated

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

尽可能选择使用 Annotated 的版本。

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

尽可能选择使用 Annotated 的版本。

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`

警告

注意，与从 fastapi 导入 Query，Path、Body 不同，要直接从 pydantic 导入 Field 。

声明模型属性

然后，使用 Field 定义模型的属性：

Python 3.10+Python 3.9+Python 3.8+Python 3.10+ non-AnnotatedPython 3.8+ non-Annotated

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 的工作方式和 Query、Path、Body 相同，参数也相同。

技术细节

实际上，Query、Path 都是 Params 的子类，而 Params 类又是 Pydantic 中 FieldInfo 的子类。

Pydantic 的 Field 返回也是 FieldInfo 的类实例。

Body 直接返回的也是 FieldInfo 的子类的对象。后文还会介绍一些 Body 的子类。

注意，从 fastapi 导入的 Query、Path 等对象实际上都是返回特殊类的函数。

提示

注意，模型属性的类型、默认值及 Field 的代码结构与路径操作函数的参数相同，只不过是用 Field 替换了Path、Query、Body。

添加更多信息

Field、Query、Body 等对象里可以声明更多信息，并且 JSON Schema 中也会集成这些信息。

声明示例一章中将详细介绍添加更多信息的知识。

小结

Pydantic 的 Field 可以为模型属性声明更多校验和元数据。

传递 JSON Schema 元数据还可以使用更多关键字参数。