`RequestValidationError` is raised when testing, instead of returing `422` response

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Example Code

import uuid

from fastapi import FastAPI, APIRouter, Header, HTTPException
from fastapi.testclient import TestClient
from pydantic import BaseModel


# Models and CRUD
class Item(BaseModel):
    id: int
    content: str


def crud_get_items():
    # This function lies in a separate module
    return [
        Item(id=1, content="First item"),
        Item(id=2, content="Second item"),
    ]


# App
app = APIRouter()


@app.get("/")
async def get_items(user_id: str = Header()):
    data = crud_get_items()
    if not len(data):  # This is redundant in the example, but exist in "normal" codebase
        raise HTTPException(status_code=404, detail="No items found")

    return data


# Tests
client = TestClient(app)


def test_get_items():
    headers = {"user-id": str(uuid.uuid4())}
    res = client.get("/", headers=headers)
    assert res.status_code == 200


def test_get_items_without_userid():
    res = client.get("/")
    assert res.status_code == 422

Description

When testing my app I noticed, that sending request to router without header parameter set, I get RequestValidationError instead of 422 status code. This doesn’t happen, when using FastAPI instead of APIRouter.

One could say, that an easy fix to this is to wrap the second test code in pytest.raises(), but this will only execute the client.get while skipping all asserts the test has.

from fastapi.exceptions import RequestValidationError

def test_get_items_without_userid():
    with pytest.raises(RequestValidationError):
        res = client.get("/")          # This line will execute
        assert res.status_code == 422  # This line won't (and any below) 

Operating System

Windows

Operating System Details

Windows 10, 21H2

FastAPI Version

0.85.0

Python Version

Python 3.10.4

Additional Context

No response

[JarroVGIT]

The TestClient is created to test Starlette applications, and by extension you can use it to test FastAPI applications. It is not designed to test Starlette’s Router or FastAPI’s APIRouter directly. As you have experienced, it can yield unexpected results. Most notably: The difference between sending a request to a FastAPI application and sending a request to the APIRouter directly (using it as an ASGI application on its own), is that no middleware is executed when doing the latter.

And by default (and design), there are always at least 2 middleware present in your FastAPI application (which will not be present in your APIRouter instance: one that catches and handles exceptions like HTTPException and one (the very last one) that handles any uncaught exception and returns a HTTP 500 Internal Server Error to ensure your server keeps running.

So, when using APIRouter directly in your TestClient, any raised HTTPExceptions will not result in a proper response, rather than actually raised (and unhandled) exception.

So, to answer your question:

But at this point I want to know if APIRouter is untestable on it’s own by design?

The behaviour that you are experiencing is by design, and when testing an APIRouter one should keep in mind that FastAPI + APIRouter has different behaviour than just the APIRouter as an ASGI application on its own.

[tiangolo]

Thanks for the help here @JarroVGIT ! 👏 You rock! 🙇

Thanks for reporting back and closing the issue @jkazimierczak 👍