Pydantic模型datetime默认值配置错误导致FastAPI序列化失败求助

我定义了一个带有datetime属性的简单Pydantic模型，代码如下：

from datetime import datetime
from pydantic import BaseModel


class Model(BaseModel):
    dt: datetime = datetime.now

当尝试将该模型序列化为JSON，或从FastAPI端点返回该模型时，出现了如下错误：

INFO:     Application startup complete.
INFO:     Uvicorn running on http://0.0.0.0:8000 (Press CTRL+C to quit)
INFO:     127.0.0.1:52831 - "POST /yard-manager/check_rule?id_request=1 HTTP/1.1" 500 Internal Server Error
ERROR:    Exception in ASGI application
Traceback (most recent call last):
  File "WINDOWS_USERNAME\scoop\apps\python310\current\lib\site-packages\anyio\streams\memory.py", line 94, in receive
    return self.receive_nowait()
  File "WINDOWS_USERNAME\scoop\apps\python310\current\lib\site-packages\anyio\streams\memory.py", line 89, in receive_nowait
    raise WouldBlock
anyio.WouldBlock

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "WINDOWS_USERNAME\scoop\apps\python310\current\lib\site-packages\starlette\middleware\base.py", line 43, in call_next
    message = await recv_stream.receive()
  File "WINDOWS_USERNAME\scoop\apps\python310\current\lib\site-packages\anyio\streams\memory.py", line 114, in receive
    raise EndOfStream
anyio.EndOfStream

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "WINDOWS_USERNAME\scoop\apps\python310\current\lib\site-packages\uvicorn\protocols\http\httptools_impl.py", line 404, in run_asgi
    result = await app(  # type: ignore[func-returns-value]
  File "WINDOWS_USERNAME\scoop\apps\python310\current\lib\site-packages\uvicorn\middleware\proxy_headers.py", line 78, in __call__
    return await self.app(scope, receive, send)
  File "WINDOWS_USERNAME\scoop\apps\python310\current\lib\site-packages\fastapi\applications.py", line 269, in __call__
    await super().__call__(scope, receive, send)
  File "WINDOWS_USERNAME\scoop\apps\python310\current\lib\site-packages\starlette\applications.py", line 124, in __call__
    await self.middleware_stack(scope, receive, send)
  File "WINDOWS_USERNAME\scoop\apps\python310\current\lib\site-packages\starlette\middleware\errors.py", line 184, in __call__
    raise exc
  File "WINDOWS_USERNAME\scoop\apps\python310\current\lib\site-packages\starlette\middleware\errors.py", line 162, in __call__
    await self.app(scope, receive, _send)
  File "WINDOWS_USERNAME\scoop\apps\python310\current\lib\site-packages\starlette\middleware\cors.py", line 92, in __call__
    await self.simple_response(scope, receive, send, request_headers=headers)
  File "WINDOWS_USERNAME\scoop\apps\python310\current\lib\site-packages\starlette\middleware\cors.py", line 147, in simple_response
    await self.app(scope, receive, send)
  File "WINDOWS_USERNAME\scoop\apps\python310\current\lib\site-packages\starlette\middleware\base.py", line 68, in __call__
    response = await self.dispatch_func(request, call_next)
  File "c:\Users\ati_o\Projects\sms-digital\SSAB\ssab-ox-mes-pss-alg\rest_service\middleware.py", line 36, in check_header_token
    response = await call_next(request)
  File "WINDOWS_USERNAME\scoop\apps\python310\current\lib\site-packages\starlette\middleware\base.py", line 46, in call_next
    raise app_exc
  File "WINDOWS_USERNAME\scoop\apps\python310\current\lib\site-packages\starlette\middleware\base.py", line 36, in coro
    await self.app(scope, request.receive, send_stream.send)
  File "WINDOWS_USERNAME\scoop\apps\python310\current\lib\site-packages\starlette\exceptions.py", line 93, in __call__
    raise exc
  File "WINDOWS_USERNAME\scoop\apps\python310\current\lib\site-packages\starlette\exceptions.py", line 82, in __call__
    await self.app(scope, receive, sender)
  File "WINDOWS_USERNAME\scoop\apps\python310\current\lib\site-packages\fastapi\middleware\asyncexitstack.py", line 21, in __call__
    raise e
  File "WINDOWS_USERNAME\scoop\apps\python310\current\lib\site-packages\fastapi\middleware\asyncexitstack.py", line 18, in __call__
    await self.app(scope, receive, send)
  File "WINDOWS_USERNAME\scoop\apps\python310\current\lib\site-packages\starlette\routing.py", line 670, in __call__
    await route.handle(scope, receive, send)
  File "WINDOWS_USERNAME\scoop\apps\python310\current\lib\site-packages\starlette\routing.py", line 266, in handle
    await self.app(scope, receive, send)
  File "WINDOWS_USERNAME\scoop\apps\python310\current\lib\site-packages\starlette\routing.py", line 65, in app
    response = await func(request)
  File "WINDOWS_USERNAME\scoop\apps\python310\current\lib\site-packages\fastapi\routing.py", line 249, in app
    content = await serialize_response(
  File "WINDOWS_USERNAME\scoop\apps\python310\current\lib\site-packages\fastapi\routing.py", line 149, in serialize_response
    return jsonable_encoder(response_content)
  File "WINDOWS_USERNAME\scoop\apps\python310\current\lib\site-packages\fastapi\encoders.py", line 157, in jsonable_encoder
    return jsonable_encoder(
  File "WINDOWS_USERNAME\scoop\apps\python310\current\lib\site-packages\fastapi\encoders.py", line 113, in jsonable_encoder
    encoded_value = jsonable_encoder(
  File "WINDOWS_USERNAME\scoop\apps\python310\current\lib\site-packages\fastapi\encoders.py", line 156, in jsonable_encoder
    raise ValueError(errors)
ValueError: [TypeError("'builtin_function_or_method' object is not iterable"), TypeError('vars() argument must have __dict__ attribute')]

问题根源在于模型定义中的dt: datetime = datetime.now——你把datetime.now函数本身赋值给了dt的默认值，而非调用该函数生成的datetime对象。当Pydantic尝试序列化模型时，无法处理函数对象，因此抛出序列化错误。

有两种修复方式：

方式1：直接调用函数作为默认值 #

如果希望模型类定义时就确定默认时间，后续所有实例复用该时间，直接调用datetime.now()：

from datetime import datetime
from pydantic import BaseModel


class Model(BaseModel):
    dt: datetime = datetime.now()

方式2：使用Pydantic的Field和默认工厂 #

如果希望每次创建模型实例时都生成最新的当前时间，推荐使用default_factory：

from datetime import datetime
from pydantic import BaseModel, Field


class Model(BaseModel):
    dt: datetime = Field(default_factory=datetime.now)

default_factory会在每次实例化模型时调用传入的函数，确保每个实例的dt都是最新时间；而直接调用datetime.now()的话，默认值会在类定义时就固定，所有实例共享同一个时间。

内容的提问来源于stack exchange，提问作者Atilla Ozgur