PyQt on Mac can be installed via homebrew or port. You'll need to install the Qt and SIP libraries before installing PySide (to be documented). The necessary dependencies (Qt and SIP) will be taken care of automatically. On Debian/Ubuntu systems, the package name is generally python-PySide, while on RPM-based systems it is named pyside.

The simplest way to install PySide is through your distribution's package manager. Note: FreeCAD progressively moved away from PyQt after version 0.13, in favour of PySide, which does exactly the same job but has a license (LGPL) more compatible with FreeCAD. If those modules (Draft, Arch, etc) are enabled after FreeCAD is installed, it means PySide (previously PyQt) is already there, and you don't need to do anything more. It is already bundled in the windows verison of FreeCAD, and is usually installed automatically by FreeCAD on Linux, when installing from official repositories. PySide (previously PyQt) is required by several modules of FreeCAD to access FreeCAD's Qt interface. optional, but needed by several modules: Draft, Arch, Ship, Plot, OpenSCAD, Spreadsheet.

This page lists several additional python modules or other pieces of software that can be downloaded freely from the internet, and add functionality to your FreeCAD installation.

2.3 Example of transition from PyQt4 and PySide.

Install python module windows