Testing custom error handlers #6248
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Example Codeclass BaseError(Exception):
def __init__(self, message, status_code) -> None:
self.message = message
self.status_code = status_code
async def error_handler(request: Request, exc: BaseError):
return JSONResponse(content={"message": exc.message}, status_code=exc.status_code)DescriptionGiven the example code, which is only a custom exception class and its handler, what I was trying while making the tests was to cover the handler lines but it keeps being uncovered, even when I force the exception to be raised. By the time, it's not that relevant because it's only one line but it'd get a pain if one handler is larger... Operating SystemWindows Operating System DetailsNo response FastAPI Version0.78.0 Python Version3.9.13 Additional ContextNo response |
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Replies: 11 comments 5 replies
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What tool are you using for testing the application? Also, can you show how are you testing it? |
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Btw, this code is similar to yours from fastapi import FastAPI, Request
from fastapi.responses import JSONResponse
class BaseError(Exception):
def __init__(self, message: str, status_code: int) -> None:
self.message = message
self.status_code = status_code
app = FastAPI()
@app.exception_handler(BaseError)
async def error_handler(request: Request, exc: BaseError):
print("Hello")
return JSONResponse(content={"message": exc.message}, status_code=exc.status_code)
@app.get("/")
def index():
raise BaseError(message="Error", status_code=404)The output I get when visiting Hello
INFO: 127.0.0.1:63867 - "GET / HTTP/1.1" 404 Not FoundMaybe you're missing the |
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Thanks for replying! I think I'm not missing the decorator, actually the example code is just a snipper, the actual code is divided into different files, and the decorator is for the app. When I run the app it works and handles the exception properly but my problem is when I want to write the unit tests. Please tell me if I was clear. |
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Understood. How are you writing the test and what is the behavior that you expect from the |
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I'm running the tests with pytest and I'm patching some random app function with a BaseError side effect so it triggers the handler. What I expect is for the test to execute the handler code lines. FYI I also tried to call the handler method directly but it returns me a coroutine object instead of the JSONResponse. |
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I don’t think this works the way you think it works. Especially if you say that when testing the handler directly, and you didn’t expect a coroutine to be returned by an asynchronous function🤔 your (async) test client needs to await the function for it to return a value. Provide the testcase that you tried, including any fixtures and mocking, maybe that clarifies. |
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After several tries, I could achieve to cover the file with tests. Thank you guys for replying! |
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The best way to test custom exception handlers would be to test an endpoint that triggers that exception, and verify that the response matches what the custom exception handler is doing. |
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Hello, would you have a full working example, please? I would like to catch any exception, that will normally turn into HTTPException with status_code 500. But during testing, the original exception is raised and not handled. Test file: |
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where is get_client defined? |
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@Kludex have there been any updates on this issue? I also was not able to directly test the behaviour of the exception handler, it works when running the app and an endpoint raises an unexpected error, but testing it just raises the initial error without going through the handler. My code is similar to @blenzi 's |
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this case fails even though my custom handler catches the error and replies (proven by breakpoint and integration tests): Pytest reports: |
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here's a more complete minimal repro that fails in python 3.12: #######################
# temp_test.py
#######################
import pytest
from fastapi import FastAPI, Request
from fastapi.responses import JSONResponse
from starlette.testclient import TestClient
#
# Define the FastAPI application in this same file
#
app = FastAPI()
@app.exception_handler(Exception)
async def catch_all_exceptions(request: Request, exc: Exception):
"""
Catch-all exception handler.
Returns HTTP 500 and a JSON body with the error detail.
"""
return JSONResponse(
status_code=500,
content={"detail": str(exc)},
)
@app.get("/error")
async def error_endpoint():
"""
This endpoint raises a raw Python Exception("Database error").
The catch_all_exceptions should handle it and return HTTP 500.
"""
raise Exception("Database error")
#
# Tests in the same file
#
@pytest.fixture
def client():
"""
Creates a fresh TestClient instance for each test.
"""
return TestClient(app)
def test_error_endpoint(client):
"""
Test that verifies the /error endpoint, which raises a Python exception,
is converted by the global exception handler to an HTTP 500 response.
"""
response = client.get("/error")
# We expect a 500 response, not an uncaught Python exception.
assert response.status_code == 500
# The response JSON should contain the exception string.
data = response.json()
assert "detail" in data
assert data["detail"] == "Database error" |
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I found a solution! modify the test client this way: and the repro will pass |
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worked -- if you use the AsyncClient add async with AsyncClient(transport=ASGITransport(app=app, raise_app_exceptions=False), base_url='http://test') as client:
response = await client.get('/test/exception')
assert response.status_code == 500 |
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Thank you @Vandivier. Crazy that the best FastAPI docs seem to be tidbits I find buried in discussion threads. |
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The best way to test custom exception handlers would be to test an endpoint that triggers that exception, and verify that the response matches what the custom exception handler is doing.