"fixture is being applied more than once to the same function" in 3.6.0, not in 3.2.1

[simonw]

I'm getting the following test failure in my test suite running against pytest 3.6.0:

(venv) datasette $ pip install pytest==3.6.0
...
Successfully installed pytest-3.6.0
(venv) datasette $ pytest
=============================================== test session starts ================================================
platform darwin -- Python 3.6.5, pytest-3.6.0, py-1.5.3, pluggy-0.6.0
rootdir: /Users/simonw/Dropbox/Development/datasette, inifile:
collected 140 items / 1 errors                                                                                     

====================================================== ERRORS ======================================================
_______________________________________ ERROR collecting tests/test_html.py ________________________________________
tests/test_html.py:7: in <module>
    pytest.fixture(scope='module')(app_client)
venv/lib/python3.6/site-packages/_pytest/fixtures.py:857: in __call__
    "fixture is being applied more than once to the same function")
E   ValueError: fixture is being applied more than once to the same function
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! Interrupted: 1 errors during collection !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
============================================= 1 error in 1.22 seconds ==============================================

The exact same test suite runs without any errors under pytest 3.2.1:

(venv) datasette $ pip install pytest==3.2.1
Collecting pytest==3.2.1
...
Successfully installed pytest-3.2.1
(venv) datasette $ pytest
=============================================== test session starts ================================================
platform darwin -- Python 3.6.5, pytest-3.2.1, py-1.5.3, pluggy-0.4.0
rootdir: /Users/simonw/Dropbox/Development/datasette, inifile:
collected 175 items                                                                                                 

tests/test_api.py .............................................................................
tests/test_docs.py ............
tests/test_html.py ...................................
tests/test_inspect.py ..
tests/test_utils.py .................................................

=========================================== 175 passed in 21.16 seconds ============================================

This is the exact checkout of the code I ran the above tests against: https://github.com/simonw/datasette/tree/76d11eb768e2f05f593c4d37a25280c0fcdf8fd6

This appears to be a regression in 3.6.0.

The underlying issue is that in my test_html.py I have the following:

https://github.com/simonw/datasette/blob/76d11eb768e2f05f593c4d37a25280c0fcdf8fd6/tests/test_html.py#L1-L7

from .fixtures import app_client
import pytest

pytest.fixture(scope='module')(app_client)

And in my test_api.py file I have this:

https://github.com/simonw/datasette/blob/76d11eb768e2f05f593c4d37a25280c0fcdf8fd6/tests/test_api.py#L1-L14

from .fixtures import (
    app_client,
    app_client_shorter_time_limit,
    app_client_returend_rows_matches_page_size,
)
import pytest

pytest.fixture(scope='module')(app_client)
pytest.fixture(scope='module')(app_client_shorter_time_limit)
pytest.fixture(scope='module')(app_client_returend_rows_matches_page_size)

The pytest.fixture(scope='module')(app_client) lines in the two modules seem to be conflicting with each other in 3.6.0 even though they worked fine together in 3.2.1

[pytestbot]

GitMate.io thinks possibly related issues are #616 ((2.6.3) Conftest is ran, but fixtures not brought in.), #2782 (PyTest won't inject fixtures into decorated functions in Python 2.7), #1627 (Drop Python 3.0/3.1/3.2/2.6?), #1909 (Monkeypatch docs lost method reference between 3.0.1 and 3.0.2), and #1875 (Cannot create two fixtures based on the same function).

[nicoddemus]

Hi @simonw,

That's by design, we added this check intentionally in #2334, here's the relevant changelog entry:

Now when @pytest.fixture is applied more than once to the same function a ValueError is raised. This buggy behavior would cause surprising problems and if was working for a test suite it was mostly by accident.

The solution is to not apply the @fixture decorator twice to the function.

I'm closing this for now, feel free to follow up with any other questions you have.

[simonw]

Thanks - now that I understand the underlying issue I've fixed it in my code by moving my calls to @pytest.fixture to be decorators in my fixtures module instead: simonw/datasette@9697717

[RonnyPfannschmidt]

@simonw also please note that its problematic to import fixtures, in particular if they have a scope as pytest detects each import location as distinct fixture