This is a simple example: my 'low battery' warning on Kubuntu
```python
#!/usr/bin/env python
from PyQt5.QtCore import *
from PyQt5.QtGui import *
from PyQt5.QtWidgets import *
from PyQt5.QtNetwork import *
import sys

app = QApplication(sys.argv)

font = QFont("Optima",100)

class Label(QLabel):
  clicked = pyqtSignal()
  def __init__(self,*xs,**kw):
    super().__init__(*xs,**kw)
    self.mouseDown = False

  def mousePressEvent(self,e):
    self.mouseDown = True

  def mouseReleaseEvent(self,e):
    if self.mouseDown:
      app.exit(0)
      self.clicked.emit()
      self.mouseDown = False
    super().mouseReleaseEvent(e)

w = Label("Low battery!")
w.setFont(font)
w.setStyleSheet("padding: 0.3em")
w.show()

exit(app.exec())

```