PyQt5 add custom QWidget to QLayout

Question

Currently I am trying to add a custom QWidget class to a QVBoxLayout. The problem I'm getting is that the widget doesn't appear at all in the layout. I even tried setting the minimum size of the QWidget because I thought that the widget wasn't showing because it's default size was set to zero.

This is a simplification of what the class looks like:

class myWidget(QWidget):

    def __init__(self):
        super().__init__()

        # Slider
        self.mySlider = QSlider(Qt.Horizontal)
        self.mySlider.setRange(-360, 360)

        # LCD Screen
        self.lcd = QLCDNumber()
        self.lcd.setMinimumHeight(45)
        self.lcd.setMaximumHeight(75)

        # set Size
        self.setMinimumSize(QSize(400,300))

I removed the signals and slots made between the slider and the LCD screen because I am not worried here about the functionality. Only the fact that I get a gray area of QSize(400,300) directly between the two buttons in the following code:

class MainWindow(QMainWindow):

    def __init__(self):
        super().__init__()

        #Create Widgets to be Added to Central Widget
        self.w1 = QPushButton("First")
        self.w2 = myWidget()
        self.w3 = QPushButton("Third")

        #Set Central Widget and VBox
        self.central_widget = QWidget()
        self.setCentralWidget(self.central_widget)
        self.layout = QVBoxLayout()
        self.central_widget.setLayout(self.layout)

        #Add widgets
        self.layout.addWidget(self.w1)
        self.layout.addWidget(self.w2)
        self.layout.addWidget(self.w3)

So what I'm simply doing is creating the 3 widgets, and placing them into the QVBoxLayout within the central widget. The 2 button widgets w1 and w3 appear but my custom widget doesn't appear and increasing the size of the widget via setMinimumSize only adds grey spacing between w1 and w3 .

So the widget is there it just isn't visible for some reason. I am pretty new to PyQt so please explain why this has happened.

Answer 1

QWidgets are just containers for other widgets. A QWidget without any layout and subwidgets will just look like empty space unless you're doing some custom painting or styling.

In your example, you're not actually adding any sub-widgets to your custom widget . In order to add a sub-widget to another widget, you need to either set the parent of the subwidget, or add the subwidget to the layout of the parent widget (which automatically re-parents the subwidget)

class myWidget(QWidget):

    def __init__(self):
        super().__init__()

        # Slider
        self.mySlider = QSlider(Qt.Horizontal)

Here, you are creating a QSlider , but it's not actually owned by MyWidget , it's going to end up being owned by Qt , and I would expect it to be drawn in the upper left hand corner of your main window.

In order to make this a subwidget of MyWidget you need to set the parent and add it to a layout.

class myWidget(QWidget):

    def __init__(self):
        super().__init__()

        self.myLay = QVBoxLayout()
        self.setLayout(self.myLay)

        # Notice self is being passed in to set the parent
        self.mySlider = QSlider(Qt.Horizontal, self)

        # You need to add widgets to a layout to position them properly
        self.myLay.addWidget(self.mySlider)