How QApplication() and QWidget() objects are connected in PySide/PyQt?

pyqt, pyside, python, qt

Solution

`QApplication` is a singleton so it would be pretty easy, for `QWidget` to do: `QApplication.instance()` and interact with the `QApplication` instance.

In fact trying to instantiate `QWidget` before the `QApplication` leads to an error:

>>> QtGui.QWidget()
QWidget: Must construct a QApplication before a QPaintDevice

Which probably means this is what happens.

Edit: I've downloaded the qt sources and in fact, in `src/gui/kernel/qwidget.cpp`, line 328, there is:

if (!qApp) {
    qFatal("QWidget: Must construct a QApplication before a QPaintDevice");
    return;
}

Where `qApp` is a pointer to the `QApplication` instance(i.e. it is equivalent to calling `QApplication.instance()`).

So, in the end, the `QWidget` interacts with the `QApplication` via a global variable, even though it isn't necessary. They probably use `qApp` instead of `QApplication.instance()` to avoid unnecessary overhead that might happen when creating/destroying many `QWidget`s.

Problem

How QApplication() and QWidget() are connected? This is an example code that I copied, it creates QApplication object and QWidget object, but there is no link between the two objects. I expected something like `app.setWidget(did)` to teach PySide/PyQt controller about the widget that was created. ``` # http://zetcode.com/gui/pysidetutorial/firstprograms/ # 1. PySide.QtGui is the class import sys from PySide import QtGui # 2. setup the application app = QtGui.QApplication(sys.argv) # 3. create the widget and setup wid = QtGui.QWidget() wid.resize(250, 150) wid.setWindowTitle('Simple') # 4. Show the widget wid.show() # 5. execute the app sys.exit(app.exec_()) ``` What's the magic behind this?

Original source