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--- technical:recipes:pyqt5-in-virtualenv [2019-05-13 16:02] – frey
+++ technical:recipes:pyqt5-in-virtualenv [2020-01-06 10:52] (current) – [Setup a virtual environment] anita
@@ Line 1: / Line 1: @@
+====== Building PyQt5 in a Python Virtual Environment ======
+By its construction, the PyQt5 library contains very little native Python code and consists primarily of compiled shared libraries that expose C++ Qt5 APIs via Python interfaces.  Any pre-built binary copy of PyQt5 (e.g. a wheel) thus has quite a few dependencies that PyPI cannot track:  what Qt5, libc, libxml, et al. libraries were used to build that wheel.  Users of our clusters (built on CentOS/RedHat that tend to use older releases at the OS level) often find that using ''pip'' to add binary packages to a Python virtualenv results in compatibility issues.
+One such instance came up today when a user complained that a virtualenv was crashing with the following error message:
+<code>
+This application failed to start because it could not find or load the Qt platform plugin "xcb"
+in "".
+Available platform plugins are: minimal, offscreen, xcb.
+Reinstalling the application may fix this problem.
+Aborted (core dumped)
+</code>
+Searching for this sort of error online yielded plenty of the typical responses:
+  - Recreate your virtualenv
+  - Reinstall everything in your virtualenv
+  - Use ''ldd'' to check that the shared libraries installed by ''pip'' don't have missing dependencies
+None of this helped.  In the end, asking the underlying Qt5 library to verbosely log its behavior was the key:
+<code>
+$ QT_DEBUG_PLUGINS=1 python qt-test.py
+   :
+Got keys from plugin meta data ("xcb")
+QFactoryLoader::QFactoryLoader() checking directory path "/home/1001/qt-test/bin/platforms" ...
+Cannot load library /home/1001/qt-test/lib/python3.6/site-packages/PyQt5/Qt/plugins/platforms/libqxcb.so: (/home/1001/qt-test/lib/python3.6/site-packages/PyQt5/Qt/lib/libQt5Core.so.5: version `Qt_5.9' not found (required by /opt/shared/anaconda/5.2.0-python3/lib/libQt5XcbQpa.so.5))
+QLibraryPrivate::loadPlugin failed on "/home/1001/qt-test/lib/python3.6/site-packages/PyQt5/Qt/plugins/platforms/libqxcb.so" : "Cannot load library /home/1001/qt-test/lib/python3.6/site-packages/PyQt5/Qt/plugins/platforms/libqxcb.so: (/home/1001/qt-test/lib/python3.6/site-packages/PyQt5/Qt/lib/libQt5Core.so.5: version `Qt_5.9' not found (required by /opt/shared/anaconda/5.2.0-python3/lib/libQt5XcbQpa.so.5))"
+This application failed to start because it could not find or load the Qt platform plugin "xcb"
+in "".
+Available platform plugins are: eglfs, linuxfb, minimal, minimalegl, offscreen, xcb.
+Reinstalling the application may fix this problem.
+Aborted (core dumped)
+</code>
+While PyQt5 happily loaded the native Qt5 libraries, when it attempted to initialize the runtime environment to a minimum support level of version 5.9 the library provided by CentOS (5.6) couldn't satisfy that requirement.  In short, the PyPI wheel for PyQt5 was built against too new a Qt5 library.  The only solution to this problem is to build and install your own copy of PyQt5 specifically tailored to the system on which you are running.
+===== Setup a virtual environment =====
+All code examples were tested on the Caviness cluster.
+<code>
+$ vpkg_require anaconda/5.2.0:python3
+$ conda create -p ~/qt-test python=3 pip
+   :
+$ source activate ~/qt-test
+(/home/1001/qt-test) $ cd ~/qt-test
+</code>
+<WRAP center round tip 60%>
+If you already used ''pip'' to install PyQt5 into a virtualenv, remove it and the PyQt5-sip package prior to performing the rest of the actions in this recipe.  The ''pip uninstall PyQt5 PyQt5-sip sip'' commmand will ask you to confirm removal of the components.
+</WRAP>
+===== Dependencies =====
+==== SIP ====
+Building PyQt5 requires a tool called SIP that translates C/C++ APIs into low-level Python objects that can be compiled and linked into shared libraries.  The SIP tool is maintained by the authors of PyQt5.  For PyQt5 5.12.2, a minimum of SIP 4.9.14 is required.  As of the time of this writing, PyPI provides SIP 4.9.8.  So SIP must be built manually first.
+<WRAP center round important 60%>
+PyQt5 expects SIP to be present as a module within itself (PyQt5.sip) so it is important to specify the module name using the ''--sip-module'' parameter to the ''configure.py'' script.  Building and installing as a standalone module (sip) will not work.
+</WRAP>
+<code>
+(/home/1001/qt-test) $ mkdir attic src
+(/home/1001/qt-test) $ cd attic
+(/home/1001/qt-test) $ wget 'https://www.riverbankcomputing.com/static/Downloads/sip/4.19.17/sip-4.19.17.tar.gz'
+(/home/1001/qt-test) $ cd ../src
+(/home/1001/qt-test) $ tar -xf ../attic/sip-4.19.17.tar.gz
+(/home/1001/qt-test) $ cd sip-4.19.17
+(/home/1001/qt-test) $ python configure.py --sip-module=PyQt5.sip
+    :
+(/home/1001/qt-test) $ make
+    :
+(/home/1001/qt-test) $ make install
+    :
+(/home/1001/qt-test) $ pip show PyQt5-sip
+Name: PyQt5-sip
+Version: 4.19.17
+Summary: None
+Home-page: None
+Author: None
+Author-email: None
+License: None
+Location: /home/1001/qt-test/lib/python3.6/site-packages
+Requires:
+Required-by:
+</code>
+===== Build and install PyQt5 =====
+Once SIP is built and installed into the virtualenv, PyQt5 can be handled.  By default the configuration script will enable all PyQt5 sub-modules for which necessary pre-requisite Qt5 libraries and headers are available.  The ''--disable'' flag can be used to prevent any unnecessary sub-modules from being built.
+<code>
+(/home/1001/qt-test) $ cd ~/qt-test/attic
+(/home/1001/qt-test) $ wget 'https://www.riverbankcomputing.com/static/Downloads/PyQt5/5.12.2/PyQt5_gpl-5.12.2.tar.gz'
+(/home/1001/qt-test) $ cd ../src
+(/home/1001/qt-test) $ tar -xf ../attic/PyQt5_gpl-5.12.2.tar.gz
+(/home/1001/qt-test) $ cd PyQt5_gpl-5.12.2
+(/home/1001/qt-test) $ python configure.py
+    :
+(/home/1001/qt-test) $ make -j 10
+    :
+(/home/1001/qt-test) $ make install
+    :
+</code>
+===== Test PyQt5 =====
+At this point you can test PyQt5 in your virtualenv:
+<code>
+(/home/1001/qt-test) $ cd ~/qt-test
+(/home/1001/qt-test) $ cat >qt-test.py <<EOT
+from PyQt5.QtWidgets import QApplication, QLabel
+theApp = QApplication([])
+theLabel = QLabel('Hello, world!')
+theLabel.show()
+theApp.exec_()
+EOT
+(/home/1001/qt-test) $ python qt-test.py
+</code>
+If you have an X11 display present (the ''DISPLAY'' environment variable defined accordingly) you should see a small window with the text //Hello, world!// displayed.