PySide

Introduction
The PySide library gives access to the cross-platform graphical user interface (GUI) toolkit Qt from Python. Qt is a collection of C++ libraries, but with the help of PySide, the same components can be used from Python. Every graphical interface that can be created in C++, can also be created and modified in Python. An advantage of using Python is that Qt interfaces can be developed and tested live, as we don't need to compile the source files.

When you install FreeCAD, you should get both Qt and PySide as part of the package. If you are compiling yourself then you must verify that these two libraries are installed in order for FreeCAD to run correctly. Of course, PySide will only work if Qt is present.

In the past, FreeCAD used PyQt, another Qt binding for Python, but in 2013 (1dc122dc9a) the project migrated to PySide because it has a more permissible license.

For more information see:
 * Wikipedia:PySide
 * Differences Between PySide and PyQt



PySide in FreeCAD with Qt5
FreeCAD was developed to be used with Python 2 and Qt4. As these two libraries became obsolete, FreeCAD transitioned to Python 3 and Qt5. In most cases this transition was done without needing to break backwards compatibility.

Normally, the module provides support for Qt4, while  provides support for Qt5. However, in FreeCAD there is no need to use directly, as a special  module is included to handle Qt5.

This module is located in the  directory of an installation of FreeCAD compiled for Qt5.

This module just imports the necessary classes from, and places them in the namespace. This means that in most cases the same code can be used with both Qt4 and Qt5, as long as we use the single module.

The only unusual aspect is that the classes are placed in the  namespace.

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Examples of PySide use

 * PySide Beginner Examples, hello world, announcements, enter text, enter number.
 * PySide Intermediate Examples, window sizing, hiding widgets, popup menus, mouse position, mouse events.
 * PySide Advanced Examples, many widgets.

The examples of PySide are divided into 3 parts, differentiated by level of exposure to PySide, Python and the FreeCAD internals. The first page has an overview on PySide; the second and third pages are mostly code examples at different levels.

It is expected that these examples are useful to get started, and afterwards the user can consult other resources online, or the official documentation.

Documentation
There are some differences in handling of widgets in Qt4 (PySide) and Qt5 (PySide2). The programmer should be aware of these incompatibilities, and should consult the official documentation if something doesn't seem to work as expected on a given platform. Nevertheless, Qt4 is considered obsolete, so most development should target Qt5 and Python 3.

The PySide documentation refers to the Python-style classes; however, since Qt is originally a C++ library, the same information should be available in the corresponding C++ reference.
 * Qt Modules available from PySide2 (Qt5).
 * All Qt classes by module in Qt5 for C++.
 * Qt Modules available from PySide (Qt4).