[Python] Pydantic function: assemble_cors_origins does not work as intented

I am having some problems with CORS in local deployment using Pydantic and FastAPI. It's a weird problem, because on how I import the domains on which I allow CORS, it works or not.

ERROR OBTAINED

Response to preflight request doesn't pass access control check: It does not have HTTP ok status.
​

When allowing all possible domains to be able requesting I don't have any problem:

if settings.BACKEND_CORS_ORIGINS:

app.add_middleware(
CORSMiddleware,
allow_origins=["*"],
allow_credentials=True,
allow_methods=["*"],
allow_headers=["*"],
)


The following code is the one I am currently using, and after doing debug I know that everything seems to be fine. And it works.

BACKEND_CORS_ORIGINS = ast.literal_eval(os.getenv("BACKEND_CORS_ORIGINS", "[]"))

if settings.BACKEND_CORS_ORIGINS:
app.add_middleware(
CORSMiddleware,
allow_origins=BACKEND_CORS_ORIGINS,
allow_credentials=True,
allow_methods=["*"],
allow_headers=["*"],
)


The problem is when I do the following, that is the way I am requested to do it:

if settings.BACKEND_CORS_ORIGINS:
BACKEND_CORS_ORIGINS =[str(origin) for origin in settings.BACKEND_CORS_ORIGINS]

app.add_middleware(
CORSMiddleware,
allow_origins=BACKEND_CORS_ORIGINS,
allow_credentials=True,
allow_methods=["*"],
allow_headers=["*"],
)


where the variable settings is located in the config.py file and has the following in-built function:

# BACKEND_CORS_ORIGINS is a JSON-formatted list of origins
# e.g: '["http://localhost", "http://localhost:4200", "http://localhost:3000", "http://localhost:8080"]'
BACKEND_CORS_ORIGINS: List[AnyHttpUrl]
@field_validator("BACKEND_CORS_ORIGINS", mode="before")
@classmethod
def assemble_cors_origins(cls, value: Union[str, List[str]]) -> Union[List[str], str]:
backend_cors_origins = None
if isinstance(value, str) and not value.startswith("["):
backend_cors_origins = [i.strip() for i in value.split(",")]
elif isinstance(value, (list, str)):
backend_cors_origins = value
if backend_cors_origins:
return backend_cors_origins
raise ValueError(value)


I have tried to do debug here, and the output list seems to be correct. That's why I don't understand what is happening.

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