Blocked conftest loads anyway; raises "not supposed to happen"

[osvenskan]

I'm using pytest 8.4.1, Python 3.11. I'm on a Mac, but I also see this problem in our Linux-based CI.

My test suite contains several conftest.py files in the file tree. In one context, I want to run a test suite without using a top-level conftest.py, so when I run the test suite, I do so with -p no:/Users/xxx/src/watthouse/watthouse/conftest.py.

With versions of pytest up to and including 7.4.4, this works fine. With versions ≥ 8.0.0, I get this --

AssertionError: While trying to load conftest path /Users/xxx/src/watthouse/watthouse/conftest.py, found that the module <module 'watthouse.conftest' from '/Users/xxx/src/watthouse/watthouse/conftest.py'> is already loaded with path /Users/xxx/src/watthouse/watthouse/conftest.py. This is not supposed to happen. Please report this issue to pytest.

In the debugger, I can see in consider_pluginarg() in _pytest/config/__init__.py that my -p no: option is interpreted correctly, and self.set_blocked(name) is executed. I'm not sure where to debug from there. If I can help, LMK.

This project is many years old and has accumulated a lot of pieces and parts over time, so it's far from a scenario that I can share with you so you can reproduce the problem on your end. 😕 I created a simple project with a file layout that has conftest.py in the same places, but was not able to reproduce the problem with that simple project.

pip list output

$ pip list
Package                  Version
------------------------ ------------
alembic                  1.11.2
ameff_util               1.0.66
amqp                     5.3.1
beautifulsoup4           4.12.2
billiard                 4.2.1
black                    24.4.2
blinker                  1.9.0
boto3                    1.34.65
botocore                 1.34.162
build                    1.2.2.post1
cachetools               5.5.2
celery                   5.3.4
certifi                  2025.7.14
cffi                     1.17.1
charset-normalizer       3.4.2
click                    8.2.1
click-didyoumean         0.3.1
click-plugins            1.1.1.2
click-repl               0.3.0
cryptography             44.0.2
et_xmlfile               2.0.0
factory-boy              3.3.0
Faker                    37.4.2
fastcore                 1.8.5
Flask                    2.3.3
Flask-Admin              1.6.1
Flask-Login              0.6.3
Flask-WTF                1.2.1
GeoAlchemy2              0.14.2
ghapi                    1.0.4
gitdb                    4.0.12
GitPython                3.1.44
google-api-core          2.25.1
google-api-python-client 2.106.0
google-auth              2.40.3
google-auth-httplib2     0.2.0
googleapis-common-protos 1.70.0
greenlet                 3.0.1
gunicorn                 23.0.0
httplib2                 0.22.0
idna                     3.10
iniconfig                2.1.0
itsdangerous             2.2.0
Jinja2                   3.1.6
jmespath                 1.0.1
kombu                    5.5.4
Mako                     1.3.10
MarkupSafe               3.0.2
mypy_extensions          1.1.0
openpyxl                 3.1.2
packaging                25.0
pathspec                 0.12.1
pdfminer.six             20231228
pdfplumber               0.11.0
pillow                   11.3.0
pip                      25.2
pip-tools                7.4.1
platformdirs             4.3.8
pluggy                   1.6.0
prompt_toolkit           3.0.51
proto-plus               1.26.1
protobuf                 6.31.1
psycopg2                 2.9.9
pyasn1                   0.6.1
pyasn1_modules           0.4.2
pycparser                2.22
Pygments                 2.19.2
PyJWT                    2.8.0
pyparsing                3.2.3
pypdfium2                4.30.1
pyproject_hooks          1.2.0
pytest                   8.4.1
pytest-ruff              0.4.1
python-dateutil          2.9.0.post0
pytz                     2023.3.post1
pyxlsb                   1.0.10
PyYAML                   6.0.1
redis                    5.0.1
requests                 2.32.4
rsa                      4.9.1
ruff                     0.12.3
s3transfer               0.10.4
setuptools               80.9.0
six                      1.17.0
slack-sdk                3.23.0
smmap                    5.0.2
soupsieve                2.7
SQLAlchemy               2.0.23
tenacity                 8.2.3
typing_extensions        4.14.1
tzdata                   2025.2
uritemplate              4.2.0
urllib3                  2.5.0
vine                     5.1.0
wcwidth                  0.2.13
Werkzeug                 3.0.6
wheel                    0.45.1
WTForms                  3.1.2