I want to apply a QGraphicsDropShadowEffect and a QGraphicsOpacityEffect in my PyQt6 application. My goal is to have a widget, that has the QGraphicsDropShadowEffect and that I can animate the widget’s opacity to fade it in and out depending on the user’s action.

To enable the effects with PyQt6 I use the setGraphicsEffect method of the widget. Because PyQt6 does not allow to set multiple effects on the same widget, I put parts of the widget in a container. I apply the QGraphicsDropShadowEffect to a specific widget and the QGraphicsOpacityEffect to the whole container. Because the opacity is set to 0.7 by default (see this) I want to set it to 1.0 after the effect is created. Here, I get an error.

Here is a minimal example which produces the error:

from PyQt6.QtWidgets import (
    QApplication, QWidget, QLabel, QVBoxLayout, QGraphicsDropShadowEffect, QGraphicsOpacityEffect
)
from PyQt6.QtGui import QColor
from PyQt6.QtCore import Qt
import sys

class CircleWidget(QWidget):
    def __init__(self):
        super().__init__()

        # Set up the main layout
        layout = QVBoxLayout(self)
        layout.setAlignment(Qt.AlignmentFlag.AlignCenter)

        # Create a label to act as the circle with text
        self.circle_label = QLabel("Text")
        self.circle_label.setAlignment(Qt.AlignmentFlag.AlignCenter)
        self.circle_label.setFixedSize(100, 100)  # Set size to create a circle
        self.circle_label.setStyleSheet(
            """
            QLabel {
                background-color: black;
                color: white;
                border-radius: 50px;  /* Half of the width/height to create a circle */
                font-size: 20px;
            }
            """
        )

        # Create and set the drop shadow effect
        shadow = QGraphicsDropShadowEffect()
        shadow.setBlurRadius(15)
        shadow.setOffset(5, 5)
        shadow.setColor(QColor(0, 0, 0, 160))  # Set shadow color with some transparency
        self.circle_label.setGraphicsEffect(shadow)

        # Create a container widget to apply opacity effect
        container = QWidget()
        container_layout = QVBoxLayout(container)
        container_layout.setAlignment(Qt.AlignmentFlag.AlignCenter)
        container_layout.setContentsMargins(0, 0, 0, 0)
        container_layout.addWidget(self.circle_label)

        # Create and set the opacity effect on the container
        opacity_effect = QGraphicsOpacityEffect()
        opacity_effect.setOpacity(1.0)  # Set the desired opacity level
        container.setGraphicsEffect(opacity_effect)

        # Add the container with the label to the main layout
        layout.addWidget(container)

        # Set the layout to the main window
        self.setLayout(layout)
        self.setWindowTitle("Circle with Effects")
        self.setFixedSize(200, 200)

# Initialize the application
app = QApplication(sys.argv)
window = CircleWidget()
window.show()
sys.exit(app.exec())

The error I get:

QPainter::begin: A paint device can only be painted by one painter at a time.
QPainter::translate: Painter not active
QPainter::worldTransform: Painter not active
QWidgetEffectSourcePrivate::pixmap: Painter not active
QPainter::worldTransform: Painter not active
QPainter::setWorldTransform: Painter not active

The strange thing is, that I do not get the error, if I set the opacity to a slightly smaller value, e.g. 0.99.

I am running this app on Ubuntu 24.04.

So I don’t know whether this is a bug or if I am not applying (or stacking) the effects correctly.