Pydantic ValueError: [TypeError("'builtin_function_or_method' object is not iterable"), TypeError('vars() argument must have __dict__ attribute')]

Question

You have a simple pydantic model with datetime attribute, like below:

from datetime import datetime
from pydantic import BaseModel


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

When you try to serialize this model to json, that try to return it from fastapi end point. You get following error.

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')]

Answer 1

You are assigning a function to your field not a value. That is you are assigning datetime.now function to your field, not a current date and time value. Therefore, pydantic cannot serialize it. Change your default value like below to call now as function like now()

from datetime import datetime
from pydantic import BaseModel


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

Simple, but when you do not know a framework, it could be a stopper.