How do I make QDockWidget expand to fill all the available space? And why can't I put more than two docks side by side?

Question

I am creating an application in pyqt5 to plot multiple figures side by side and above/below each other, and I want to make the plots dockable. The result is not behaving the way I want/would expect and after a long time of searching the internet for answers/solutions I still don't know why.

So far this is my code:

from matplotlib.backends.backend_qt5agg import FigureCanvasQTAgg as FigureCanvas
from matplotlib.figure import Figure
from PyQt5 import QtWidgets, QtCore
import traceback
import logging


def exception_hook(exc_type, exc_value, exc_tb):
    tb = "".join(traceback.format_exception(exc_type, exc_value, exc_tb))
    logging.error('\n' + tb)
    QtWidgets.QApplication.quit()


class PlotWindow(QtWidgets.QMainWindow):
    def __init__(self, parent=None):
        super(PlotWindow, self).__init__(parent=parent)
        self.setWindowTitle('plot window')
        self.setCentralWidget(QtWidgets.QWidget(self))

    def add_figure(self, figure):
        mplCanvas = FigureCanvas(figure)
        mplCanvas.setSizePolicy(QtWidgets.QSizePolicy.Expanding, QtWidgets.QSizePolicy.Expanding)
        dockableMplCanvas = QtWidgets.QDockWidget(self)
        dockableMplCanvas.setWidget(mplCanvas)
        self.addDockWidget(QtCore.Qt.RightDockWidgetArea, dockableMplCanvas)


if __name__ == '__main__':
    import sys

    logging.basicConfig(level=logging.DEBUG)
    logging.getLogger('matplotlib').setLevel(logging.ERROR)
    sys.excepthook = exception_hook

    app = QtWidgets.QApplication(sys.argv)

    pw = PlotWindow()

    for _ in range(4):
        fig = Figure(tight_layout=True)
        ax = fig.add_subplot()
        pw.add_figure(fig)

    pw.show()

    exit(app.exec_())

It results in the following:

If I now rearrange the plots by first putting one of them on the left of the other three:

..And then putting one underneath the one on the left:

Now if I rescale the window then the space between the two columns of plots expands but the plots don't expand to fill the space:

Why is that? and how can I fix it?

Answer 1

The central widget is resizing to fill the space between the two columns of plots, the solution is to add:

self.centralWidget().setFixedSize(0, 0)

to prevent the central widget from expanding