Develop Reference

WARNING: Although PyQt5 is already imported, the PyQt5 backend could not be used

After installing Ubuntu 22.04 and trying to resurrect a FlatCam build, I ran into this bizarre PyQt5 error:
WARNING: Although PyQt5 is already imported, the PyQt5 backend could not
be used ("cannot import name 'QtOpenGL' from 'PyQt5' (/usr/lib/python3/dist-packages/PyQt5/init.py)").
Note that running multiple GUI toolkits simultaneously can cause side effects.
pyopengl is already installed so I'm not sure what dependency is missing...

The problem magically resolved itself after installing pyqtgraph. After inspecting the dependent packages, I now believe the missing import was python3-pyqt5.qtopengl

How to support alternate dependencies in a Python package?

I have written a utility library in Python that works with the Qt framework. My code is pure Python and is compatible with both PyQt5 and PySide2. My main module could either be run on its own from the command line with python -m or it could be imported into another project. Is there a clean way to specify that the project needs either PyQt5 or PySide2 in its a wheel distribution?
Here is what I have found in my research but I am asking in case there is a better way to package the project than these options:
I could add logic to setup.py in a source distribution of the project to check for PyQt5 and PySide2. However, wheels are recommended way to distribute Python projects, and from what I can tell this kind of install-time logic is not possible with wheels. Alternatively, I could not specify either PySide2 or PyQt5 as dependencies and recommend in the install instructions that one of them be installed together with my project.

Use extras_require:
setup(
…
extras_require={
'pyqt5': ['PyQt5'],
'pyside2': ['PySide2'],
},
)
and teach your users to run either
pip install 'yourpackage[pyqt5]'
or
pip install 'yourpackage[pyside2]'

If you don't want to make either a strict requirement (which makes sense), I'd just throw a runtime error if neither is available.
For example
try:
import PyQt5 as some_common_name
except ImportError:
try:
import PySide2 as some_common_name
except ImportError:
raise ImportError('Please install either PyQt5 or PySide2') from None

My particular case is somewhat niche (so I am not accepting this as the answer). I realized the package was really doing two things: acting as a library and as a command line tool. I decided to split it into two packages: package and package-cli. package does not explicitly depend on PyQt5 or PySide2 but specifies that one of them must be installed in the documentation. Since package is a library, it is intended to be integrated into another project where it is easy to list package and PyQt5 together in the requirements.txt. For package-cli, I just choose one of PyQt5 or PySide2 to be the explicit dependency. package-cli depends on package and PyQt5 and just adds a console_script to call the main module in package.

PyQt5 gets "No module named 'PyQt4'" error calling matplotlib.pyplot in Python 3.5

I am using Anaconda with Python 3.5.2, Matplotlib 2.0.2, PyQt5.6 on a windows 10 machine. When I import matplotlib.pyplot as plt I get the following error:
...
File "C:...\Anaconda3\Lib\site-packages\matplotlib\backends\qt_compat.py",
line 137, in <module> from PyQt4 import QtCore, QtGui
ImportError: No module named 'PyQt4'
I don't know why it would want to import from PyQt4 when it has never been installed on my machine.
This question has the same error, but on a machine that actually has PyQt4 installed.
I have checked my matplotlibrc file, I've used matplotlib.use('qt5agg') in the program, also matplotlib.rcParams['backend'] = "Qt5Agg". I've uninstalled and reinstalled all the above packages to no avail. I have even attempted to install PyQt4 just to get things going. I'm completely stumped. None of the various possible causes or remedies that I've been able to find on either SO or github have helped.

I just had the same error (even though on Windows 7). For me the problem was that there was an old matplotlibrc file in C:\Users\<username>\.matplotlib\matplotlibrc which overwrote the settings from my environment's matplotlibrc file. Deleting that file solved the issue for me.

It seems the API version is PyQt5 but the default is PyQt4,just open Tools -> Preferences -> IPython console -> Graphics -> backend,change QT4 to QT5

Looking at the file and line number in your error it looks that you have the same issue that I answered here.
That is, I think your issue is caused by having a QT_API environment variable that still is set to pyqt4 (or pyside).

I ran into a similar thing and found that the problem was having a 64-bit Anaconda installation that couldn't load the PyPt5 DLLs that were perhaps 32-bit. The short answer is that I uninstalled Anaconda 64-bit and installed the 32-bit version instead.
To work through this, I stepped through qt_compat.py where the error originates. For most of the time through this module, it’s trying to work with PyQt5, as that’s what it finds in the environment. However, when it gets to the lines below, the import fails so it tries to fall back to PyQt4, which isn’t installed with Anaconda, and thus issues the error.
if QT_API == QT_API_PYQT5:
try:
from PyQt5 import QtCore, QtGui, QtWidgets
_getSaveFileName = QtWidgets.QFileDialog.getSaveFileName
except ImportError:
# fell through, tried PyQt5, failed fall back to PyQt4
QT_API = rcParams['backend.qt4']
QT_RC_MAJOR_VERSION = 4
Testing the import statement outside of the file gave the message “DLL load failed: %1 is not a valid Win32 application.” That's what suggests the DLL mismatch, and also answers why it was trying to fall back to PyQt4.

3 steps can solve this problem:
pip uninstall pyqt5
pip uninstall matplotlib
pip install matplotlib

I had faced the same problem and here is how I fixed it.
Look into your matplotlib configuration file which is often located at
path-to/site-packages/matplotlib/mpl-data/matplotlibrc
or
path-to/Anaconda3/envs/your_env_name/Lib/site-packages/matplotlib/mpl-data/matplotlibrc
See an example of matplotlibrc here
https://github.com/daler/matplotlibrc/blob/master/rc/default
If you have pyqt5.x.x installed and have the statement 'backend : Qt5Agg' in matplotlibrc, then to use '%matplotlib qt', change '#backend.qt4 : PyQt4' to '#backend.qt4 : PyQt5'.
For an Anaconda environment, reactivate the environment.
Note if you update matplotlib in the future, your matplotlibrc will automatically be overwritten.

QT Webengine for Ubuntu

I have several different qt packages installed for my development environment on Ubuntu 15.04 (3.19.0-51-generic), and I can't seem to get my imports correct. The error I am running into in PyCharm is:
from PyQt5.QtWebEngineWidgets import QWebEnginePage
ImportError: No module named 'PyQt5.QtWebEngineWidgets'
The following packages I have installed already, which I believe to be relevant are as follows:
python3-pyqt5
python3-pyqt5-dbg
python-pyqt5
python-pyqt5-dbg
pyqt5-dev-tools
pyqt5-dev
python-pyqt5.qtwebkit
python3-pyqt5.qtwebkit
python-pyqt5.qtwebkit-dbg
python3-pyqt5.qtwebkit-dbg
What am I missing here? Note that the project explicitly requires the following in its README:
Qt 5.5+
PyQt5.6+
Furthermore, I understand that webengine and webkit are two different things (I was giving it a stab). I can't find any Webengine for PyQT5 anywhere in the official Vivid repository. I tried installing libqt5webengine5-dev and libqt5webengine5 from https://launchpad.net/~ethereum/+archive/ubuntu/ethereum-qt packages with the same result, perhaps I need to add something to Python path. Can anyone lend a hand (Im normally a .NET developer :/)?

PyQT5 Python 3.3 Pycharm

I am new to python and I have an issue running PyQT5 for my Python 3.3 on PyCharm.
I have read several posts about it, some speaking about doing a build which I am not sure to get.
Here is what I have done :
Downloaded and run the .exe found here.
Installing normally, the same way as my Pillow for instance (which is running with my Pycharm).
aaaand, that's its.
from PyQt5.QtCore import *
from PyQt5.QtWidgets import *
== from PyQt5.QtCore import *
ImportError: No module named 'PyQt5.QtCore'
When I setup the module in PyCharm it simply didn't find it. So is the .exe found on the official PyQT5 dead ?
Thanks
Bloby

I also met the same problem recently, maybe you also install PyQt5 by pip？
It seems that the after using pip, I can import the PyQt5 but not any packages of it, so I have to download the old version with an installer.
Then Pycharm can runs without error.