The error message shown by a model validation error is worse in FastAPI 0.100.0 and above. For the following test code, the route will raise a 500 (Internal Server Error) since the data being returned cannot be validated by Pydantic.
from fastapi import FastAPI
from pydantic import BaseModel
app = FastAPI()
class RootSchema(BaseModel):
foo: str
@app.get("/", response_model=RootSchema)
def get_root():
return {}
In fastapi==0.99.1, the error message is:
INFO: 127.0.0.1:58889 - "GET / HTTP/1.1" 500 Internal Server Error
ERROR: Exception in ASGI application
Traceback (most recent call last):
File "/Users/harrylees/Documents/fastapi-test/venv/lib/python3.10/site-packages/uvicorn/protocols/http/h11_impl.py", line 408, in run_asgi
result = await app( # type: ignore[func-returns-value]
File "/Users/harrylees/Documents/fastapi-test/venv/lib/python3.10/site-packages/uvicorn/middleware/proxy_headers.py", line 84, in __call__
return await self.app(scope, receive, send)
File "/Users/harrylees/Documents/fastapi-test/venv/lib/python3.10/site-packages/fastapi/applications.py", line 290, in __call__
await super().__call__(scope, receive, send)
File "/Users/harrylees/Documents/fastapi-test/venv/lib/python3.10/site-packages/starlette/applications.py", line 122, in __call__
await self.middleware_stack(scope, receive, send)
File "/Users/harrylees/Documents/fastapi-test/venv/lib/python3.10/site-packages/starlette/middleware/errors.py", line 184, in __call__
raise exc
File "/Users/harrylees/Documents/fastapi-test/venv/lib/python3.10/site-packages/starlette/middleware/errors.py", line 162, in __call__
await self.app(scope, receive, _send)
File "/Users/harrylees/Documents/fastapi-test/venv/lib/python3.10/site-packages/starlette/middleware/exceptions.py", line 79, in __call__
raise exc
File "/Users/harrylees/Documents/fastapi-test/venv/lib/python3.10/site-packages/starlette/middleware/exceptions.py", line 68, in __call__
await self.app(scope, receive, sender)
File "/Users/harrylees/Documents/fastapi-test/venv/lib/python3.10/site-packages/fastapi/middleware/asyncexitstack.py", line 20, in __call__
raise e
File "/Users/harrylees/Documents/fastapi-test/venv/lib/python3.10/site-packages/fastapi/middleware/asyncexitstack.py", line 17, in __call__
await self.app(scope, receive, send)
File "/Users/harrylees/Documents/fastapi-test/venv/lib/python3.10/site-packages/starlette/routing.py", line 718, in __call__
await route.handle(scope, receive, send)
File "/Users/harrylees/Documents/fastapi-test/venv/lib/python3.10/site-packages/starlette/routing.py", line 276, in handle
await self.app(scope, receive, send)
File "/Users/harrylees/Documents/fastapi-test/venv/lib/python3.10/site-packages/starlette/routing.py", line 66, in app
response = await func(request)
File "/Users/harrylees/Documents/fastapi-test/venv/lib/python3.10/site-packages/fastapi/routing.py", line 259, in app
content = await serialize_response(
File "/Users/harrylees/Documents/fastapi-test/venv/lib/python3.10/site-packages/fastapi/routing.py", line 145, in serialize_response
raise ValidationError(errors, field.type_)
pydantic.error_wrappers.ValidationError: 1 validation error for RootSchema
response -> foo
field required (type=value_error.missing)
which is helpful as the error from pydantic is shown and can be used to debug the issue. However, in fastapi==0.100.0 and above, the error message is:
INFO: 127.0.0.1:58970 - "GET / HTTP/1.1" 500 Internal Server Error
ERROR: Exception in ASGI application
Traceback (most recent call last):
File "/Users/harrylees/Documents/fastapi-test/venv/lib/python3.10/site-packages/uvicorn/protocols/http/h11_impl.py", line 408, in run_asgi
result = await app( # type: ignore[func-returns-value]
File "/Users/harrylees/Documents/fastapi-test/venv/lib/python3.10/site-packages/uvicorn/middleware/proxy_headers.py", line 84, in __call__
return await self.app(scope, receive, send)
File "/Users/harrylees/Documents/fastapi-test/venv/lib/python3.10/site-packages/fastapi/applications.py", line 289, in __call__
await super().__call__(scope, receive, send)
File "/Users/harrylees/Documents/fastapi-test/venv/lib/python3.10/site-packages/starlette/applications.py", line 122, in __call__
await self.middleware_stack(scope, receive, send)
File "/Users/harrylees/Documents/fastapi-test/venv/lib/python3.10/site-packages/starlette/middleware/errors.py", line 184, in __call__
raise exc
File "/Users/harrylees/Documents/fastapi-test/venv/lib/python3.10/site-packages/starlette/middleware/errors.py", line 162, in __call__
await self.app(scope, receive, _send)
File "/Users/harrylees/Documents/fastapi-test/venv/lib/python3.10/site-packages/starlette/middleware/exceptions.py", line 79, in __call__
raise exc
File "/Users/harrylees/Documents/fastapi-test/venv/lib/python3.10/site-packages/starlette/middleware/exceptions.py", line 68, in __call__
await self.app(scope, receive, sender)
File "/Users/harrylees/Documents/fastapi-test/venv/lib/python3.10/site-packages/fastapi/middleware/asyncexitstack.py", line 20, in __call__
raise e
File "/Users/harrylees/Documents/fastapi-test/venv/lib/python3.10/site-packages/fastapi/middleware/asyncexitstack.py", line 17, in __call__
await self.app(scope, receive, send)
File "/Users/harrylees/Documents/fastapi-test/venv/lib/python3.10/site-packages/starlette/routing.py", line 718, in __call__
await route.handle(scope, receive, send)
File "/Users/harrylees/Documents/fastapi-test/venv/lib/python3.10/site-packages/starlette/routing.py", line 276, in handle
await self.app(scope, receive, send)
File "/Users/harrylees/Documents/fastapi-test/venv/lib/python3.10/site-packages/starlette/routing.py", line 66, in app
response = await func(request)
File "/Users/harrylees/Documents/fastapi-test/venv/lib/python3.10/site-packages/fastapi/routing.py", line 291, in app
content = await serialize_response(
File "/Users/harrylees/Documents/fastapi-test/venv/lib/python3.10/site-packages/fastapi/routing.py", line 154, in serialize_response
raise ResponseValidationError(
fastapi.exceptions.ResponseValidationError
which doesn't show the error from Pydantic, and makes the error quite a bit harder to debug (in my opinion).
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