qtbot fails in github actions if cv2 is imported

[JSS95]

What I encountered

I imported cv2 module in my testing file, and github action fails with similar error in #293 .
However, when I lazy-import cv2 by loading it inside function block, the error disappears.

Reproducing the error

I made a repository to reproduce this error : https://github.com/JSS95/pytestqt-githubaction

In commit 976f3af, I imported cv2 in function block and the test does not fail.
In commit f23f189, I imported cv2 at the top of the file and the test fails.

My environment

I am using PyQt5 with pytest-qt. My CI ran on ubuntu, Python 3.7.
I followed the troubleshooting guide in the document, and all was well before I import cv2.

Error log

============================= test session starts ==============================
platform linux -- Python 3.7.12, pytest-6.2.5, py-1.11.0, pluggy-1.0.0
PyQt5 5.15.6 -- Qt runtime 5.15.2 -- Qt compiled 5.15.2
rootdir: /home/runner/work/pytestqt-githubaction/pytestqt-githubaction
plugins: xvfb-2.0.0, qt-4.0.2
collected 1 item

Fatal Python error: Aborted

Current thread 0x00007fb4a1414740 (most recent call first):
  File "/opt/hostedtoolcache/Python/3.7.12/x64/lib/python3.7/site-packages/pytestqt/plugin.py", line 41 in qapp
  File "/opt/hostedtoolcache/Python/3.7.12/x64/lib/python3.7/site-packages/_pytest/fixtures.py", line 932 in call_fixture_func
  File "/opt/hostedtoolcache/Python/3.7.12/x64/lib/python3.7/site-packages/_pytest/fixtures.py", line 1126 in pytest_fixture_setup
  File "/opt/hostedtoolcache/Python/3.7.12/x64/lib/python3.7/site-packages/pluggy/_callers.py", line 39 in _multicall
  File "/opt/hostedtoolcache/Python/3.7.12/x64/lib/python3.7/site-packages/pluggy/_manager.py", line 80 in _hookexec
  File "/opt/hostedtoolcache/Python/3.7.12/x64/lib/python3.7/site-packages/pluggy/_hooks.py", line 265 in __call__
  File "/opt/hostedtoolcache/Python/3.7.12/x64/lib/python3.7/site-packages/_pytest/fixtures.py", line 1072 in execute
  File "/opt/hostedtoolcache/Python/3.7.12/x64/lib/python3.7/site-packages/_pytest/fixtures.py", line 687 in _compute_fixture_value
  File "/opt/hostedtoolcache/Python/3.7.12/x64/lib/python3.7/site-packages/_pytest/fixtures.py", line 601 in _get_active_fixturedef
  File "/opt/hostedtoolcache/Python/3.7.12/x64/lib/python3.7/site-packages/_pytest/fixtures.py", line 581 in getfixturevalue
  File "/opt/hostedtoolcache/Python/3.7.12/x64/lib/python3.7/site-packages/_pytest/fixtures.py", line 568 in _fillfixtures
  File "/opt/hostedtoolcache/Python/3.7.12/x64/lib/python3.7/site-packages/_pytest/python.py", line 1647 in setup
  File "/opt/hostedtoolcache/Python/3.7.12/x64/lib/python3.7/site-packages/_pytest/runner.py", line 449 in prepare
  File "/opt/hostedtoolcache/Python/3.7.12/x64/lib/python3.7/site-packages/_pytest/runner.py", line 150 in pytest_runtest_setup
  File "/opt/hostedtoolcache/Python/3.7.12/x64/lib/python3.7/site-packages/pluggy/_callers.py", line 39 in _multicall
  File "/opt/hostedtoolcache/Python/3.7.12/x64/lib/python3.7/site-packages/pluggy/_manager.py", line 80 in _hookexec
  File "/opt/hostedtoolcache/Python/3.7.12/x64/lib/python3.7/site-packages/pluggy/_hooks.py", line 265 in __call__
  File "/opt/hostedtoolcache/Python/3.7.12/x64/lib/python3.7/site-packages/_pytest/runner.py", line 255 in <lambda>
  File "/opt/hostedtoolcache/Python/3.7.12/x64/lib/python3.7/site-packages/_pytest/runner.py", line 311 in from_call
  File "/opt/hostedtoolcache/Python/3.7.12/x64/lib/python3.7/site-packages/_pytest/runner.py", line 255 in call_runtest_hook
  File "/opt/hostedtoolcache/Python/3.7.12/x64/lib/python3.7/site-packages/_pytest/runner.py", line 215 in call_and_report
  File "/opt/hostedtoolcache/Python/3.7.12/x64/lib/python3.7/site-packages/_pytest/runner.py", line 120 in runtestprotocol
  File "/opt/hostedtoolcache/Python/3.7.12/x64/lib/python3.7/site-packages/_pytest/runner.py", line 109 in pytest_runtest_protocol
  File "/opt/hostedtoolcache/Python/3.7.12/x64/lib/python3.7/site-packages/pluggy/_callers.py", line 39 in _multicall
  File "/opt/hostedtoolcache/Python/3.7.12/x64/lib/python3.7/site-packages/pluggy/_manager.py", line 80 in _hookexec
  File "/opt/hostedtoolcache/Python/3.7.12/x64/lib/python3.7/site-packages/pluggy/_hooks.py", line 265 in __call__
  File "/opt/hostedtoolcache/Python/3.7.12/x64/lib/python3.7/site-packages/_pytest/main.py", line 348 in pytest_runtestloop
  File "/opt/hostedtoolcache/Python/3.7.12/x64/lib/python3.7/site-packages/pluggy/_callers.py", line 39 in _multicall
  File "/opt/hostedtoolcache/Python/3.7.12/x64/lib/python3.7/site-packages/pluggy/_manager.py", line 80 in _hookexec
  File "/opt/hostedtoolcache/Python/3.7.12/x64/lib/python3.7/site-packages/pluggy/_hooks.py", line 265 in __call__
  File "/opt/hostedtoolcache/Python/3.7.12/x64/lib/python3.7/site-packages/_pytest/main.py", line 323 in _main
  File "/opt/hostedtoolcache/Python/3.7.12/x64/lib/python3.7/site-packages/_pytest/main.py", line 269 in wrap_session
  File "/opt/hostedtoolcache/Python/3.7.12/x64/lib/python3.7/site-packages/_pytest/main.py", line 316 in pytest_cmdline_main
  File "/opt/hostedtoolcache/Python/3.7.12/x64/lib/python3.7/site-packages/pluggy/_callers.py", line 39 in _multicall
  File "/opt/hostedtoolcache/Python/3.7.12/x64/lib/python3.7/site-packages/pluggy/_manager.py", line 80 in _hookexec
  File "/opt/hostedtoolcache/Python/3.7.12/x64/lib/python3.7/site-packages/pluggy/_hooks.py", line 265 in __call__
  File "/opt/hostedtoolcache/Python/3.7.12/x64/lib/python3.7/site-packages/_pytest/config/__init__.py", line 163 in main
  File "/opt/hostedtoolcache/Python/3.7.12/x64/lib/python3.7/site-packages/_pytest/config/__init__.py", line 185 in console_main
  File "/opt/hostedtoolcache/Python/3.7.12/x64/bin/pytest", line 8 in <module>
/home/runner/work/_temp/035e6ce0-402d-47c0-9d15-d62f155e6d0d.sh: line 1:  1728 Aborted                 (core dumped) pytest

[The-Compiler]

I forked your repository and replaced the CI by a simple script which just does:

from PyQt5.QtWidgets import QApplication
import cv2

app = QApplication([])

and that fails as well:

qt.qpa.plugin: Could not load the Qt platform plugin "xcb" in "/opt/hostedtoolcache/Python/3.7.12/x64/lib/python3.7/site-packages/cv2/qt/plugins" even though it was found.
11

. It looks like import cv2 patches Qt stuff in some weird way so that existing Qt code fails to run. Doesn't seem like pytest-qt has anything to do with it though, so I'm closing this.

[JSS95]

Thanks. I found opencv/opencv-python#386, and using opencv-python-headless module resolved this error.

For anyone that might encounter same issue:

1. The reason why qtbot with PySide2 fails in github actions #293 raised error was that the server cannot initialize UI session. (qt.qpa.plugin: Could not load the Qt platform plugin "xcb" in "/home/surbhi/.local/lib/python3.8/site-packages/cv2/qt/plugins" even though it was found. opencv/opencv-python#447) Using pytest-xvfb resolves this.
2. The reason of this error is that cv2 and PyQt5 conflicts on Qt. Installing opencv-python-headless, which has no binding to Qt, can fix this.

[JSS95]

@The-Compiler Although this is not related to pytest-qt, I think someone else might fall into same trap. I would like to document this if you think it's ok.

[The-Compiler]

I guess it wouldn't hurt to add this to the troubleshooting page if you want to. Though you got me curious: How come you use that combination of Qt and cv2 in the tests, but not in your application code?

[JSS95]

I used them in my application as well. But strangely, nothing went wrong in my Windows local machine when I ran my app and tests.

[eyllanesc]

If you still want to use python-opencv and not opencv-python-headless then a workaround is:

import os

from PyQt5.QtCore import QLibraryInfo
# from PySide2.QtCore import QLibraryInfo

import cv2

os.environ["QT_QPA_PLATFORM_PLUGIN_PATH"] = QLibraryInfo.location(
    QLibraryInfo.PluginsPath
)