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)