@Marvin; why is my unit test failing? I have a testcase written like:

from prefect.testing.utilities import prefect_test_harness
from my_project import flow_to_test

def test_flow():
    with prefect_test_harness():
        flow_to_test()

when I try testing this, it results in this error:

E                   anyio._backends._asyncio.ExceptionGroup: 2 exceptions were raised in the task group:
E                   ----------------------------
E                   Traceback (most recent call last):
E                     File "/Users/psktam/Library/Caches/pypoetry/virtualenvs/project-RcQVLUoH-py3.10/lib/python3.10/site-packages/prefect/engine.py", line 1389, in create_task_run_then_submit
E                       task_run = await create_task_run(
E                     File "/Users/psktam/Library/Caches/pypoetry/virtualenvs/project-RcQVLUoH-py3.10/lib/python3.10/site-packages/prefect/engine.py", line 1434, in create_task_run
E                       task_run = await flow_run_context.client.create_task_run(
E                     File "/Users/psktam/Library/Caches/pypoetry/virtualenvs/project-RcQVLUoH-py3.10/lib/python3.10/site-packages/prefect/client/orchestration.py", line 1980, in create_task_run
E                       response = await <http://self._client.post|self._client.post>(
E                     File "/Users/psktam/Library/Caches/pypoetry/virtualenvs/project-RcQVLUoH-py3.10/lib/python3.10/site-packages/httpx/_client.py", line 1848, in post
E                       return await self.request(
E                     File "/Users/psktam/Library/Caches/pypoetry/virtualenvs/project-RcQVLUoH-py3.10/lib/python3.10/site-packages/httpx/_client.py", line 1530, in request
E                       return await self.send(request, auth=auth, follow_redirects=follow_redirects)
E                     File "/Users/psktam/Library/Caches/pypoetry/virtualenvs/project-RcQVLUoH-py3.10/lib/python3.10/site-packages/prefect/client/base.py", line 312, in send
E                       response.raise_for_status()
E                     File "/Users/psktam/Library/Caches/pypoetry/virtualenvs/project-RcQVLUoH-py3.10/lib/python3.10/site-packages/prefect/client/base.py", line 164, in raise_for_status
E                       raise PrefectHTTPStatusError.from_httpx_error(exc) from exc.__cause__
E                   prefect.exceptions.PrefectHTTPStatusError: Server error '500 Internal Server Error' for url '<http://ephemeral-prefect/api/task_runs/>'
E                   Response: {'exception_message': 'Internal Server Error'}
E                   For more information check: <https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/500>
E                   ----------------------------
E                   Traceback (most recent call last):
E                     File "/Users/psktam/Library/Caches/pypoetry/virtualenvs/project-RcQVLUoH-py3.10/lib/python3.10/site-packages/prefect/engine.py", line 1389, in create_task_run_then_submit
E                       task_run = await create_task_run(
E                     File "/Users/psktam/Library/Caches/pypoetry/virtualenvs/project-RcQVLUoH-py3.10/lib/python3.10/site-packages/prefect/engine.py", line 1434, in create_task_run
E                       task_run = await flow_run_context.client.create_task_run(
E                     File "/Users/psktam/Library/Caches/pypoetry/virtualenvs/project-RcQVLUoH-py3.10/lib/python3.10/site-packages/prefect/client/orchestration.py", line 1980, in create_task_run
E                       response = await <http://self._client.post|self._client.post>(
E                     File "/Users/psktam/Library/Caches/pypoetry/virtualenvs/project-RcQVLUoH-py3.10/lib/python3.10/site-packages/httpx/_client.py", line 1848, in post
E                       return await self.request(
E                     File "/Users/psktam/Library/Caches/pypoetry/virtualenvs/project-RcQVLUoH-py3.10/lib/python3.10/site-packages/httpx/_client.py", line 1530, in request
E                       return await self.send(request, auth=auth, follow_redirects=follow_redirects)
E                     File "/Users/psktam/Library/Caches/pypoetry/virtualenvs/project-RcQVLUoH-py3.10/lib/python3.10/site-packages/prefect/client/base.py", line 312, in send
E                       response.raise_for_status()
E                     File "/Users/psktam/Library/Caches/pypoetry/virtualenvs/project-RcQVLUoH-py3.10/lib/python3.10/site-packages/prefect/client/base.py", line 164, in raise_for_status
E                       raise PrefectHTTPStatusError.from_httpx_error(exc) from exc.__cause__
E                   prefect.exceptions.PrefectHTTPStatusError: Server error '500 Internal Server Error' for url '<http://ephemeral-prefect/api/task_runs/>'
E                   Response: {'exception_message': 'Internal Server Error'}
E                   For more information check: <https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/500>

../Library/Caches/pypoetry/virtualenvs/project-RcQVLUoH-py3.10/lib/python3.10/site-packages/anyio/_backends/_asyncio.py:595: ExceptionGroup

The error you're encountering seems to be related to the way you're calling your flow in the test. In Prefect 2.0, the way to define and run flows has changed. The

with Flow()

syntax and

flow.run()

are now obsolete. In your test case, you're using

prefect_test_harness()

which is also not the recommended way to test flows in Prefect 2.0. Instead, you should define your flows with the

@flow

decorator and tasks with the

@task

decorator. To run a flow, you just call the flow directly instead of using

my_flow.run()

. So, your test case should look something like this:

from my_project import flow_to_test

def test_flow():
    flow_to_test()

This should resolve the issue you're facing. If you're still encountering problems, it might be helpful to check the Prefect documentation or community resources for more information.

I don't think that's correct? Looking at the documentation, it recommends to use the

prefect_test_harness()

command to test flows with: https://docs.prefect.io/2.14.4/guides/testing/

do I need to start a local server in order to run this? I was hoping that I wouldn't have to do that

no you dont, i would just have an autouse fixture like this

oh interesting

im not sure off the top why your example above isnt working tho 🧐

I was going to ask how this is different from using

prefect_test_harness()

with the

with

statement though?

it shouldnt be, it should be roughly the same thing

I see

ahh - think dask might be the issue. is it important that your unit tests use dask?

it's not important that the unit tests use dask, although it'd definitely help speed it up