[QGIS-Developer] Building QGIS with QT6 on Ubuntu: Python error at start

[Richard Duivenvoorde]

I'm on Trixie (next stable in a couple of weeks), and only build QWT myself from source:

  # https://qwt.sourceforge.io/qwtinstall.html
  # download the tar.bz2 from https://sourceforge.net/projects/qwt/files/qwt/6.3.0/
  # unpack and go into ~/git/qwt-6.3.0 dir
  # error: qmake: could not find a Qt installation of ''  -> https://askubuntu.com/questions/1460242/ubuntu-22-04-with-qt6-qmake-could-not-find-a-qt-installation-of?
  qtchooser -install qt6 $(which qmake6)
  export QT_SELECT=qt6
  # create Makefile
  /usr/bin/qmake qwt.pro
  # compile
  make
  # install (in /usr/local/qwt-6.3.0 )
  sudo make install

Then, leaving out a lot of stuff...:

mkdir buildqt6;cd buildqt6;ccmake -GNinja -DBUILD_TESTING=FALSE -DENABLE_TESTS=OFF -DWITH_SERVER=FALSE -DWITH_3D=FALSE -DCMAKE_INSTALL_PREFIX=/home/richard/bin/qgis_/qt6/debug -DWITH_QWTPOLAR=OFF -DCMAKE_BUILD_TYPE=Debug -DWITH_GRASS=FALSE -DBUILD_WITH_QT6=TRUE -DWITH_OAUTH2_PLUGIN=FALSE -DWITH_WITH_SERVER_PLUGINS=OFF -DWITH_STAGED_PLUGINS=OFF -DWITH_PDAL=OFF -DWITH_QTWEBKIT=OFF -DQWT_INCLUDE_DIR=/usr/local/qwt-6.3.0/include  -DQWT_LIBRARY=/usr/local/qwt-6.3.0/lib/libqwt.so  ..

in the same git repo as my qt5 build, QGIS works.

I'm on python 3.13.5 here

Not sure if this helps...

Richard




On 21-07-2025 17:52, Alessandro Pasotti via QGIS-Developer wrote:
> 
> Thanks, I'm using Python 3.13 (that comes with that Ubuntu version).
> 
> Now I'm trying with vcpkg ...
> 
> 
> On Mon, Jul 21, 2025 at 5:35 PM pathmapper <pathmapper at posteo.de <mailto:pathmapper at posteo.de>> wrote:
> 
>     Hi,
> 
>     I don't know what's wrong, but when I had similar problems, switching to
>     Python 3.12 solved them for me.
> 
>     Cheers,
>     Björn
> 
>     On 21.07.2025 16:58, Alessandro Pasotti via QGIS-Developer wrote:
>      > Hi,
>      >
>      > I am trying to build QGIS with QT6 on Ubuntu (tried 24.10 and 25.04),
>      > it builds without errors but when I run it, the following error
>      > appears:
>      >
>      > Couldn't load PyQGIS.
>      > Python support will be disabled.
>      >
>      > Traceback (most recent call last):
>      >   File "", line 1, in
>      >   File
>      > "/home/ale/dev/QGIS/build/Desktop_QT6-Debug/output/python/qgis/gui/__init__.py",
>      > line 25, in
>      >     from qgis._gui import *
>      > RuntimeError: qgis._gui cannot import type '���A� ' from
>      > PyQt6.QtCore
>      >
>      > Python version:
>      > 3.13.3 (main, Jun 16 2025, 18:15:32) [GCC 14.2.0]
>      >
>      > QGIS version:
>      > 3.99.0-Master 'Master', e717e3cd2b9
>      >
>      > Python path:
>      > ['/home/ale/dev/QGIS/build/Desktop_QT6-Debug/output/python',
>      > '/home/ale/.local/share/QGIS/QGIS3/profiles/default/python',
>      > '/home/ale/.local/share/QGIS/QGIS3/profiles/default/python/plugins',
>      > '/home/ale/dev/QGIS/build/Desktop_QT6-Debug/output/python/plugins',
>      > '/home/ale/dev/gdal/desktop-install/local/lib/python3.13/dist-packages',
>      > '/home/ale/dev/gdal/autotest/pymod', '/usr/lib/python313.zip',
>      > '/usr/lib/python3.13', '/usr/lib/python3.13/lib-dynload',
>      > '/usr/local/lib/python3.13/dist-packages',
>      > '/usr/lib/python3/dist-packages']
>      >
>      > I checked the QGIS/build/Desktop_QT6-Debug/output/python/qgis content
>      > and it looks legit.
>      >
>      > PyQt6 installation also looks ok, importing core works ok, only gui
>      > does not work.
>      >
>      > Any idea about what's going wrong?
>      >
>      > --
>      >
>      > Alessandro Pasotti
>      > QCooperative: www.qcooperative.net <http://www.qcooperative.net> [1]
>      > ItOpen: www.itopen.it <http://www.itopen.it> [2]
>      >
>      > Links:
>      > ------
>      > [1] https://www.qcooperative.net <https://www.qcooperative.net>
>      > [2] http://www.itopen.it <http://www.itopen.it>
>      >
>      > _______________________________________________
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> 
> 
> 
> -- 
> Alessandro Pasotti
> QCooperative: www.qcooperative.net <https://www.qcooperative.net>
> ItOpen: www.itopen.it <http://www.itopen.it>
> 
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