How To Debug a C++ Extension of a PySide6 Application?

When debugging PySide code, very often you would also like to debug the corresponding C++ extension of the PySide module. This is done by attaching your debugger to the Python interpreter. In this tutorial, we are going to take you through a comprehensive guide in building Qt 6, using the built Qt 6 to build PySide6, and then starting a debugging process in either Qt Creator or VSCode.

With VSCode, you should be able to see the combined call stacks for both C++ and Python together. With Qt Creator, unfortunately you would only be able to debug the native C++ code of the PySide module; that is you won’t be able to set breakpoints inside the Python code.

Note

This tutorial is created on Ubuntu 20.04 LTS with the debugger as GDB. As such, this tutorial is mainly focused on Linux users. Nevertheless, links to setup everything in other platforms are mentioned along with each sub-section.

Let’s get started.

Install All The Library Dependencies Based on Your Platform

sudo apt install libfontconfig1-dev libfreetype6-dev \
libx11-dev libx11-xcb-dev libxext-dev libxfixes-dev \
libxi-dev libxrender-dev libxcb1-dev libxcb-glx0-dev \
libxcb-keysyms1-dev libxcb-image0-dev libxcb-shm0-dev \
libxcb-icccm4-dev libxcb-sync-dev libxcb-xfixes0-dev \
libxcb-shape0-dev libxcb-randr0-dev libxcb-render-util0-dev \
libxcb-util-dev libxcb-xinerama0-dev libxcb-xkb-dev \
libxkbcommon-dev libxkbcommon-x11-dev libatspi2.0-dev \
libopengl0 -y

If you have to use the Qt Multimedia module, you have to install gstreamer also.

sudo apt-get install libgstreamer1.0-dev libgstreamer-plugins-base1.0-dev \
libgstreamer-plugins-bad1.0-dev gstreamer1.0-plugins-base \
gstreamer1.0-plugins-good gstreamer1.0-plugins-bad \
gstreamer1.0-plugins-ugly gstreamer1.0-libav gstreamer1.0-doc \
gstreamer1.0-tools gstreamer1.0-x gstreamer1.0-alsa gstreamer1.0-gl \
gstreamer1.0-gtk3 gstreamer1.0-qt5 gstreamer1.0-pulseaudio

Generally, any information on missing packages can be found by inspecting the config.summary in you CMake build folder.

Note

For other platforms install the same packages using the instructions mentioned here Qt Install on Linux

Build Qt

This is an optional step in case you only want to debug the CPython bindings or if you have DEBUG_SYMBOLS for Qt 6 already.

There are multiple ways to build Qt - configure script or manually with CMake. Find the build system information Qt 6 Build System.

  1. Get the source code.

    git clone git://code.qt.io/qt/qt5.git
# Get submodules
cd qt5
perl init-repository

  2. Once you have the source code, the next step is to generate the build using CMake, then building and installing it.

    cmake -GNinja -DMCAKE_BUILD_TYPE=Debug \
-DCMAKE_INSTALL_PREFIX=/path/to/install/Qt -DBUILD_qtwebengine=OFF ..
cmake --build . --parallel
cmake --install .

    As you notice here, we are skipping the Qt WebEngine module because this greatly decreases the build time. However, PySide6 supports Qt WebEngine features. So feel free to add it, if you need it.

More instructions on building Qt 6 can be found in the following links:

Add the bin and lib path to the environment variables

export PATH="/path/to/custom/qt/bin:$PATH"
export LD_LIBRARY_PATH="/path/to/custom/qt/lib:$LD_LIBRARY_PATH"

Build PySide6 using the Qt 6 that you built earlier

Follow the steps mentioned Getting Started - Qt for Python

You may manually select the modules to install using the --module-subset cli argument for setup.py. This was my installation script

python setup.py install --qpaths=/path/to/qpaths --debug \
--ignore-git --reuse-build

It is recommended to use a Python virtual environment rather than installing in the global Python.

Debugging the process using your preferred IDE

The following sections guide you through the setup for Qt Creator or VSCode.