Resizing of QStackedWidget

Question

I want to embend an QStackedWidget in my application. Unfortunatly in my trials the resizing of the ApplicationWindow doesn't cause an resize of MyMplCanvas anymore.

If I would use an alternative way to include MyMplCanvas in the ApplicationWindow the resizing would work as expected. So I expect that the definition of MyMplCanvas is correct and something has to be ajusted at QStackedWidget instead.

from PyQt4 import QtGui, QtCore
import sys
from matplotlib.backends.backend_qt4agg import FigureCanvasQTAgg as FigureCanvas
from matplotlib.figure import Figure

class MyMplCanvas(FigureCanvas):
    def __init__(self, parent=None):
        self.fig = Figure()
        FigureCanvas.__init__(self, self.fig)

        self.setParent(parent)
        self.fig.add_subplot(111).plot((1, 2, 3), (4, 3, 4))      

        FigureCanvas.setSizePolicy(self,
                                   QtGui.QSizePolicy.Expanding,
                                   QtGui.QSizePolicy.Expanding)
        FigureCanvas.updateGeometry(self)

class ApplicationWindow(QtGui.QMainWindow):
    def __init__(self):
        QtGui.QMainWindow.__init__(self)
        self.setAttribute(QtCore.Qt.WA_DeleteOnClose)
        self.main_widget = QtGui.QWidget(self)

        # Plot object
        plot1=MyMplCanvas()

        # With this definition it would resize as expected
        # l = QtGui.QVBoxLayout(self.main_widget)
        # l.addWidget(plot1)

        # Unfortunatly it is not resizing if I use QStackedWidget
        self.viewsStack = QtGui.QStackedWidget(self.main_widget)
        self.viewsStack.setSizePolicy(QtGui.QSizePolicy.Expanding,
                                      QtGui.QSizePolicy.Expanding)        
        self.viewsStack.addWidget(plot1)

        # General code
        self.main_widget.setFocus()
        self.setCentralWidget(self.main_widget)

if __name__ == '__main__':
    qApp = QtGui.QApplication(sys.argv)
    aw = ApplicationWindow()
    aw.show()
    sys.exit(qApp.exec_())

Answer 1

This is the expected behaviour. Widgets will only resize to fit the available space if your hierarchy is widget -> layout -> widget -> layout -> ....

Currently you have ApplicationWindow (QMainWindow) -> QMainWindow layout (internally created by Qt) -> main_widget (QWidget) -> viewsStack (QStackedWidget)

The last layer has a QWidget within a QWidget with no layout, thus the QStackedWidget does not resize. When you uncomment the layout code, you get the expeted behaviour because your widget/layout hierarchy follow alternating widget and layout.

If you don't want to add an additional layout, just remove self.main_widget and set the stacked widget as the central widget of the main window directly.