python - matplotlib in second window pyqt5

Question

I wish to plot a matplotlib graph in a second window when a button is clicked in the main window. Using https://matplotlib.org/examples/user_interfaces/embedding_in_qt4.html , which plots a graph in the main window, I wrote the code below. The matplotlib graph is indeed plotted in a second window, but it has a very small size and I don't understand how to adjust the size of the graph to the window. May I have some help?

from PyQt5.QtWidgets import QMainWindow, QPushButton, QApplication, QLabel, QWidget
from matplotlib.backends.backend_qt5agg import FigureCanvasQTAgg as FigureCanvas
from matplotlib.figure import Figure
from PyQt5 import QtCore, QtWidgets
import numpy as np

class MainWindow(QMainWindow):
     def __init__(self):
          super(MainWindow, self).__init__()
          btn = QPushButton('Click me!', self)
          btn.clicked.connect(self.onClick)

     def onClick(self):
          self.SW = SecondWindow()
          self.SW.resize(300,300)
          self.SW.show()

class SecondWindow(QMainWindow):
    def __init__(self):
         super(SecondWindow, self).__init__()
         self.main_widget = QtWidgets.QWidget(self)

         layout = QtWidgets.QVBoxLayout(self.main_widget)
         sc = MyMplCanvas(self.main_widget, width = 300, height = 300)
         layout.addWidget(sc)

class MyMplCanvas(FigureCanvas):

    def __init__(self, parent=None, width= 300, height= 300):
        fig = Figure(figsize=(width, height))
        self.axes = fig.add_subplot(111)

        self.compute_figure()

        FigureCanvas.__init__(self, fig)
        self.setParent(parent)

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

    def compute_figure(self):
        t = np.arange(0.0, 3.0, 0.01)
        s = np.sin(2*np.pi*t)
        self.axes.plot(t, s)

if __name__ == '__main__':
    import sys
    app = QApplication(sys.argv)
    MW = MainWindow()
    MW.resize(500, 500)
    MW.show()
    sys.exit(app.exec_())

Answer 1

In your case the main_widget does not have the same size as the QMainWindow so if you change the size with self.SW.resize(300,300) it will not change the size of self.main_widget, the solution would be to use a layout but in the case of QMainWindow it is not correct to do so since this has a custom layout that should not be modified:

The solution in the case of QMainWindow is always to establish a centralwidget, in your case it could be using the self.main_widget:

class SecondWindow(QMainWindow):
    def __init__(self):
         super(SecondWindow, self).__init__()
         self.main_widget = QtWidgets.QWidget()
         self.setCentralWidget(self.main_widget)

         layout = QtWidgets.QVBoxLayout(self.main_widget)
         sc = MyMplCanvas(self.main_widget, width = 300, height = 300)
         layout.addWidget(sc)