python - Combining PyQt and Matplotlib

Question

I want to combine my Ui generated by QtDesigner. I want to merge it with myplotlib so that everything is in one window. When i run the following code i get an error:

"QWidget::setLayout: Attempting to set QLayout "" on Ui_MainWindow "", which already has a layout

I know that i should connect my canvas layout with something but i dont know with what. I hope you can help me out here:

from PyQt4 import QtCore, QtGui
from matplotlib.backends.backend_qt4agg import FigureCanvasQTAgg as FigureCanvas
from matplotlib.backends.backend_qt4agg import NavigationToolbar2QT as NavigationToolbar
import matplotlib.pyplot as plt


try:
    _fromUtf8 = QtCore.QString.fromUtf8
except AttributeError:
    def _fromUtf8(s):
        return s

try:
    _encoding = QtGui.QApplication.UnicodeUTF8
    def _translate(context, text, disambig):
        return QtGui.QApplication.translate(context, text, disambig, _encoding)
except AttributeError:
    def _translate(context, text, disambig):
        return QtGui.QApplication.translate(context, text, disambig)

class Ui_MainWindow(QtGui.QMainWindow):
    def __init__(self, parent = None):
        super(Ui_MainWindow, self).__init__(parent)

        # a figure instance to plot on
        self.figure = plt.figure()

        # this is the Canvas Widget that displays the `figure`
        # it takes the `figure` instance as a parameter to __init__
        self.canvas = FigureCanvas(self.figure)

        # this is the Navigation widget
        # it takes the Canvas widget and a parent
        self.toolbar = NavigationToolbar(self.canvas, self)

        # Just some button connected to `plot` method
        self.button = QtGui.QPushButton('Plot')

        #self.button.clicked.connect(lambda: self.plot(data['time'], data['value']))
        #self.button.clicked.connect(self.plot)

        self.lineEditMomentum1 = QtGui.QLineEdit()
        self.lineEditMomentum1.setMaximumSize(200, 30)


        # set the layout
        layout = QtGui.QVBoxLayout()
        layout.addWidget(self.toolbar)
        layout.addWidget(self.canvas)
        layout.addWidget(self.button)
        layout.addWidget(self.lineEditMomentum1)

        self.setLayout(layout)


    def setupUi(self, MainWindow):
        MainWindow.setObjectName(_fromUtf8("MainWindow"))
        MainWindow.resize(800, 600)
        self.centralwidget = QtGui.QWidget(MainWindow)
        self.centralwidget.setObjectName(_fromUtf8("centralwidget"))
        self.lineEdit = QtGui.QLineEdit(self.centralwidget)
        self.lineEdit.setGeometry(QtCore.QRect(160, 390, 113, 20))
        self.lineEdit.setObjectName(_fromUtf8("lineEdit"))
        self.lineEdit_2 = QtGui.QLineEdit(self.centralwidget)
        self.lineEdit_2.setGeometry(QtCore.QRect(160, 420, 113, 20))
        self.lineEdit_2.setObjectName(_fromUtf8("lineEdit_2"))
        self.lineEdit_3 = QtGui.QLineEdit(self.centralwidget)
        self.lineEdit_3.setGeometry(QtCore.QRect(160, 450, 113, 20))
        self.lineEdit_3.setObjectName(_fromUtf8("lineEdit_3"))
        self.lineEdit_4 = QtGui.QLineEdit(self.centralwidget)
        self.lineEdit_4.setGeometry(QtCore.QRect(160, 480, 113, 20))
        self.lineEdit_4.setObjectName(_fromUtf8("lineEdit_4"))
        self.lineEdit_5 = QtGui.QLineEdit(self.centralwidget)
        self.lineEdit_5.setGeometry(QtCore.QRect(160, 510, 113, 20))
        self.lineEdit_5.setObjectName(_fromUtf8("lineEdit_5"))
        self.lineEdit_6 = QtGui.QLineEdit(self.centralwidget)
        self.lineEdit_6.setGeometry(QtCore.QRect(160, 540, 113, 20))
        self.lineEdit_6.setObjectName(_fromUtf8("lineEdit_6"))
        self.lineEdit_7 = QtGui.QLineEdit(self.centralwidget)
        self.lineEdit_7.setGeometry(QtCore.QRect(650, 430, 113, 20))
        self.lineEdit_7.setObjectName(_fromUtf8("lineEdit_7"))
        self.lineEdit_8 = QtGui.QLineEdit(self.centralwidget)
        self.lineEdit_8.setGeometry(QtCore.QRect(650, 460, 113, 20))
        self.lineEdit_8.setObjectName(_fromUtf8("lineEdit_8"))
        self.lineEdit_9 = QtGui.QLineEdit(self.centralwidget)
        self.lineEdit_9.setGeometry(QtCore.QRect(650, 490, 113, 20))
        self.lineEdit_9.setObjectName(_fromUtf8("lineEdit_9"))
        self.lineEdit_10 = QtGui.QLineEdit(self.centralwidget)
        self.lineEdit_10.setGeometry(QtCore.QRect(650, 520, 113, 20))
        self.lineEdit_10.setObjectName(_fromUtf8("lineEdit_10"))
        self.label = QtGui.QLabel(self.centralwidget)
        self.label.setGeometry(QtCore.QRect(60, 390, 61, 16))
        self.label.setObjectName(_fromUtf8("label"))
        self.label_2 = QtGui.QLabel(self.centralwidget)
        self.label_2.setGeometry(QtCore.QRect(60, 420, 61, 16))
        self.label_2.setObjectName(_fromUtf8("label_2"))
        self.label_3 = QtGui.QLabel(self.centralwidget)
        self.label_3.setGeometry(QtCore.QRect(60, 450, 71, 16))
        self.label_3.setObjectName(_fromUtf8("label_3"))
        self.label_4 = QtGui.QLabel(self.centralwidget)
        self.label_4.setGeometry(QtCore.QRect(60, 480, 81, 16))
        self.label_4.setObjectName(_fromUtf8("label_4"))
        self.label_5 = QtGui.QLabel(self.centralwidget)
        self.label_5.setGeometry(QtCore.QRect(40, 510, 111, 20))
        self.label_5.setObjectName(_fromUtf8("label_5"))
        self.label_6 = QtGui.QLabel(self.centralwidget)
        self.label_6.setGeometry(QtCore.QRect(40, 540, 111, 16))
        self.label_6.setObjectName(_fromUtf8("label_6"))
        self.label_7 = QtGui.QLabel(self.centralwidget)
        self.label_7.setGeometry(QtCore.QRect(585, 430, 51, 20))
        self.label_7.setObjectName(_fromUtf8("label_7"))
        self.label_8 = QtGui.QLabel(self.centralwidget)
        self.label_8.setGeometry(QtCore.QRect(585, 460, 51, 20))
        self.label_8.setObjectName(_fromUtf8("label_8"))
        self.label_9 = QtGui.QLabel(self.centralwidget)
        self.label_9.setGeometry(QtCore.QRect(630, 400, 131, 16))
        self.label_9.setObjectName(_fromUtf8("label_9"))
        self.label_10 = QtGui.QLabel(self.centralwidget)
        self.label_10.setGeometry(QtCore.QRect(600, 490, 31, 16))
        self.label_10.setObjectName(_fromUtf8("label_10"))
        self.label_11 = QtGui.QLabel(self.centralwidget)
        self.label_11.setGeometry(QtCore.QRect(580, 520, 71, 16))
        self.label_11.setObjectName(_fromUtf8("label_11"))
        self.pushButton = QtGui.QPushButton(self.centralwidget)
        self.pushButton.setGeometry(QtCore.QRect(300, 370, 251, 91))
        palette = QtGui.QPalette()
        brush = QtGui.QBrush(QtGui.QColor(0, 0, 0))
        brush.setStyle(QtCore.Qt.SolidPattern)
        palette.setBrush(QtGui.QPalette.Active, QtGui.QPalette.WindowText, brush)
        brush = QtGui.QBrush(QtGui.QColor(0, 0, 255))
        brush.setStyle(QtCore.Qt.SolidPattern)
        palette.setBrush(QtGui.QPalette.Active, QtGui.QPalette.Button, brush)
        brush = QtGui.QBrush(QtGui.QColor(127, 127, 255))
        brush.setStyle(QtCore.Qt.SolidPattern)
        palette.setBrush(QtGui.QPalette.Active, QtGui.QPalette.Light, brush)
        brush = QtGui.QBrush(QtGui.QColor(63, 63, 255))
        brush.setStyle(QtCore.Qt.SolidPattern)
        palette.setBrush(QtGui.QPalette.Active, QtGui.QPalette.Midlight, brush)
        brush = QtGui.QBrush(QtGui.QColor(0, 0, 127))
        brush.setStyle(QtCore.Qt.SolidPattern)
        palette.setBrush(QtGui.QPalette.Active, QtGui.QPalette.Dark, brush)
        brush = QtGui.QBrush(QtGui.QColor(0, 0, 170))
        brush.setStyle(QtCore.Qt.SolidPattern)
        palette.setBrush(QtGui.QPalette.Active, QtGui.QPalette.Mid, brush)
        brush = QtGui.QBrush(QtGui.QColor(0, 0, 0))
        brush.setStyle(QtCore.Qt.SolidPattern)
        palette.setBrush(QtGui.QPalette.Active, QtGui.QPalette.Text, brush)
        brush = QtGui.QBrush(QtGui.QColor(255, 255, 255))
        brush.setStyle(QtCore.Qt.SolidPattern)
        palette.setBrush(QtGui.QPalette.Active, QtGui.QPalette.BrightText, brush)
        brush = QtGui.QBrush(QtGui.QColor(0, 0, 0))
        brush.setStyle(QtCore.Qt.SolidPattern)
        palette.setBrush(QtGui.QPalette.Active, QtGui.QPalette.ButtonText, brush)
        brush = QtGui.QBrush(QtGui.QColor(37, 88, 255))
        brush.setStyle(QtCore.Qt.SolidPattern)
        palette.setBrush(QtGui.QPalette.Active, QtGui.QPalette.Base, brush)
        brush = QtGui.QBrush(QtGui.QColor(0, 0, 255))
        brush.setStyle(QtCore.Qt.SolidPattern)
        palette.setBrush(QtGui.QPalette.Active, QtGui.QPalette.Window, brush)
        brush = QtGui.QBrush(QtGui.QColor(0, 0, 0))
        brush.setStyle(QtCore.Qt.SolidPattern)
        palette.setBrush(QtGui.QPalette.Active, QtGui.QPalette.Shadow, brush)
        brush = QtGui.QBrush(QtGui.QColor(127, 127, 255))
        brush.setStyle(QtCore.Qt.SolidPattern)
        palette.setBrush(QtGui.QPalette.Active, QtGui.QPalette.AlternateBase, brush)
        brush = QtGui.QBrush(QtGui.QColor(255, 0, 255))
        brush.setStyle(QtCore.Qt.SolidPattern)
        palette.setBrush(QtGui.QPalette.Active, QtGui.QPalette.NoRole, brush)
        brush = QtGui.QBrush(QtGui.QColor(255, 255, 220))
        brush.setStyle(QtCore.Qt.SolidPattern)
        palette.setBrush(QtGui.QPalette.Active, QtGui.QPalette.ToolTipBase, brush)
        brush = QtGui.QBrush(QtGui.QColor(0, 0, 0))
        brush.setStyle(QtCore.Qt.SolidPattern)
        palette.setBrush(QtGui.QPalette.Active, QtGui.QPalette.ToolTipText, brush)
        brush = QtGui.QBrush(QtGui.QColor(0, 0, 0))
        brush.setStyle(QtCore.Qt.SolidPattern)
        palette.setBrush(QtGui.QPalette.Inactive, QtGui.QPalette.WindowText, brush)
        brush = QtGui.QBrush(QtGui.QColor(0, 0, 255))
        brush.setStyle(QtCore.Qt.SolidPattern)
        palette.setBrush(QtGui.QPalette.Inactive, QtGui.QPalette.Button, brush)
        brush = QtGui.QBrush(QtGui.QColor(127, 127, 255))
        brush.setStyle(QtCore.Qt.SolidPattern)
        palette.setBrush(QtGui.QPalette.Inactive, QtGui.QPalette.Light, brush)
        brush = QtGui.QBrush(QtGui.QColor(63, 63, 255))
        brush.setStyle(QtCore.Qt.SolidPattern)
        palette.setBrush(QtGui.QPalette.Inactive, QtGui.QPalette.Midlight, brush)
        brush = QtGui.QBrush(QtGui.QColor(0, 0, 127))
        brush.setStyle(QtCore.Qt.SolidPattern)
        palette.setBrush(QtGui.QPalette.Inactive, QtGui.QPalette.Dark, brush)
        brush = QtGui.QBrush(QtGui.QColor(0, 0, 170))
        brush.setStyle(QtCore.Qt.SolidPattern)
        palette.setBrush(QtGui.QPalette.Inactive, QtGui.QPalette.Mid, brush)
        brush = QtGui.QBrush(QtGui.QColor(0, 0, 0))
        brush.setStyle(QtCore.Qt.SolidPattern)
        palette.setBrush(QtGui.QPalette.Inactive, QtGui.QPalette.Text, brush)
        brush = QtGui.QBrush(QtGui.QColor(255, 255, 255))
        brush.setStyle(QtCore.Qt.SolidPattern)
        palette.setBrush(QtGui.QPalette.Inactive, QtGui.QPalette.BrightText, brush)
        brush = QtGui.QBrush(QtGui.QColor(0, 0, 0))
        brush.setStyle(QtCore.Qt.SolidPattern)
        palette.setBrush(QtGui.QPalette.Inactive, QtGui.QPalette.ButtonText, brush)
        brush = QtGui.QBrush(QtGui.QColor(37, 88, 255))
        brush.setStyle(QtCore.Qt.SolidPattern)
        palette.setBrush(QtGui.QPalette.Inactive, QtGui.QPalette.Base, brush)
        brush = QtGui.QBrush(QtGui.QColor(0, 0, 255))
        brush.setStyle(QtCore.Qt.SolidPattern)
        palette.setBrush(QtGui.QPalette.Inactive, QtGui.QPalette.Window, brush)
        brush = QtGui.QBrush(QtGui.QColor(0, 0, 0))
        brush.setStyle(QtCore.Qt.SolidPattern)
        palette.setBrush(QtGui.QPalette.Inactive, QtGui.QPalette.Shadow, brush)
        brush = QtGui.QBrush(QtGui.QColor(127, 127, 255))
        brush.setStyle(QtCore.Qt.SolidPattern)
        palette.setBrush(Qt

Answer 1

Please allow me to give a few tips on how to ask better questions before I answer your question. You're not getting many replies because your code example is too large and because we cannot just copy-paste-execute it and reproduce the problem ourselves.

Next time make a minimal but complete code snippet that still reproduces the issue (see also https://stackoverflow.com/help/mcve). If possible, make it so that we can run it without having to make any modifications ourselves. You will get a lot more help that way.

Start by making a copy of your program and remove all functions that do not contribute to the error until you have a minimal example left. By doing so there is a large chance that you will find the solution yourself. If not, a small example allows us to focus more on the issue without having to wade through hundreds of lines of code (such as the setupUi() function in your example, which isn't even executed).

A minimal stand-alone program that still produces your error is:

import sys
from PyQt4 import QtCore, QtGui

class Ui_MainWindow(QtGui.QMainWindow):
    def __init__(self, parent = None):
        super(Ui_MainWindow, self).__init__(parent)

        layout = QtGui.QVBoxLayout()
        self.setLayout(layout)        

if __name__ == "__main__":
    qApp = QtGui.QApplication(sys.argv)
    main_window = Ui_MainWindow()
    main_window.show()
    sys.exit(qApp.exec_())

As you can see it is unrelated to integrating matplotlib in PyQt. The problem is that QMainWindow must have a central widget which acts as a container for all your child widgets. This central widget has its own layout, the layout of the QMainWindowis for toolbars and dock widgets (see the illustration in http://doc.qt.io/qt-4.8/qmainwindow.html). Note that you should add toolbars with the addToolbar() method.

The fix to your error is thus:

import sys
from PyQt4 import QtCore, QtGui

class Ui_MainWindow(QtGui.QMainWindow):
    def __init__(self, parent = None):
        super(Ui_MainWindow, self).__init__(parent)

        self.main_widget = QtGui.QWidget(self)
        self.setCentralWidget(self.main_widget)

        layout = QtGui.QVBoxLayout()
        self.main_widget.setLayout(layout)        
        # The main_widget is a container to which you
        # can add your child widgets here...

if __name__ == "__main__":
    qApp = QtGui.QApplication(sys.argv)
    main_window = Ui_MainWindow()
    main_window.show()
    sys.exit(qApp.exec_())

Now, your code example contains another error, which is the one that @tcaswell provided an answer for. If you build the fix into your program you will see that the figure canvas will end up in its own, separate window.

Again, a minimal-ish example is:

import sys
from PyQt4 import QtCore, QtGui
from matplotlib.backends.backend_qt4agg import 
    FigureCanvasQTAgg as FigureCanvas
from matplotlib.backends.backend_qt4agg import 
    NavigationToolbar2QT as NavigationToolbar
import matplotlib.pyplot as plt

class Ui_MainWindow(QtGui.QMainWindow):
    def __init__(self, parent = None):
        super(Ui_MainWindow, self).__init__(parent)

        self.figure = plt.figure()
        self.canvas = FigureCanvas(self.figure)

        # use addToolbar to add toolbars to the main window directly!
        self.toolbar = NavigationToolbar(self.canvas, self)
        self.addToolBar(self.toolbar)

        self.button = QtGui.QPushButton('Plot')
        self.lineEditMomentum1 = QtGui.QLineEdit()
        self.lineEditMomentum1.setMaximumSize(200, 30)

        self.main_widget = QtGui.QWidget(self)
        self.setCentralWidget(self.main_widget)

        layout = QtGui.QVBoxLayout()
        layout.addWidget(self.canvas)
        layout.addWidget(self.button)
        layout.addWidget(self.lineEditMomentum1)

        self.main_widget.setLayout(layout)

if __name__ == "__main__":
    qApp = QtGui.QApplication(sys.argv)
    main_window = Ui_MainWindow()
    main_window.show()
    sys.exit(qApp.exec_())

When combining matplotlib in PyQt, don't use the matlab-like interface of the pyplot module at all. Use the matplotlib classes directly.

Change plt.figure() in Figure() like so:

import sys
from PyQt4 import QtCore, QtGui
from matplotlib.backends.backend_qt4agg 
    import FigureCanvasQTAgg as FigureCanvas
from matplotlib.backends.backend_qt4agg 
    import NavigationToolbar2QT as NavigationToolbar
from matplotlib.figure import Figure

class Ui_MainWindow(QtGui.QMainWindow):
    def __init__(self, parent = None):
        super(Ui_MainWindow, self).__init__(parent)

        self.figure = Figure() # don't use matplotlib.pyplot at all!
        self.canvas = FigureCanvas(self.figure)

        # use addToolbar to add toolbars to the main window directly!
        self.toolbar = NavigationToolbar(self.canvas, self)
        self.addToolBar(self.toolbar)

        self.button = QtGui.QPushButton('Plot')
        self.lineEditMomentum1 = QtGui.QLineEdit()
        self.lineEditMomentum1.setMaximumSize(200, 30)

        self.main_widget = QtGui.QWidget(self)
        self.setCentralWidget(self.main_widget)

        layout = QtGui.QVBoxLayout()
        layout.addWidget(self.canvas)
        layout.addWidget(self.button)
        layout.addWidget(self.lineEditMomentum1)

        self.main_widget.setLayout(layout)

if __name__ == "__main__":
    qApp = QtGui.QApplication(sys.argv)
    main_window = Ui_MainWindow()
    main_window.show()
    sys.exit(qApp.exec_())

Finally, you might consider to create your programs without using the Qt Designer. You will gain a lot of insight by building them from scratch, your code will be more readable and it will be easier to make minimal working examples of your problems. I think the extra time it takes to type the widget statements will pay itself back.