How to stretch QLabel in PyQt5 - pyqt5

Question

How to change the following code to get the QLabel stretch to width of the window ?
from PyQt5.QtGui import *
from PyQt5.QtWidgets import *
from PyQt5.QtCore import *
class Window(QMainWindow):
def __init__(self):
super().__init__()
self.setGeometry(100, 100, 200, 100)
self.label = QLabel('Hello World!', self)
self.label.setAlignment(Qt.AlignCenter)
self.label.setStyleSheet('font-size: 12pt; background-color: red')
self.show()
app = QApplication([])
win = Window()
app.exec()

Answer 1

As the documentation about QMainWindow says, you must set a central widget for it:
Creating a main window without a central widget is not supported. You must have a central widget even if it is just a placeholder.
The problem is that you need a layout manager in order to properly adapt widget sizes inside a parent, and just manually setting widget geometries is generally discouraged.
You created the label as a direct child, so it will have no knowledge about its parents size changes.
Just set the label as central widget.
self.setCentralWidget(self.label)
Otherwise, you can use a container widget, set a layout and add the label to it, but you still must set the central widget.
central = QWidget()
layout = QVBoxLayout(central)
layout.addWidget(self.label)
self.setCentralWidget(central)
The alternative is to directly use a QWidget instead of QMainWindow as you did in your answer.

Answer 2

you can use sizePolicy
self.label.setSizePolicy(QSizePolicy.Expanding, QSizePolicy.Preferred)

Answer 3

Thank you very much for your answers. The problem is now solved by the following code changes
from PyQt5.QtGui import *
from PyQt5.QtWidgets import *
from PyQt5.QtCore import *
class Window(QWidget):
def __init__(self):
super().__init__()
self.setGeometry(100, 100, 200, 100)
self.label = QLabel('Hello World!', self)
self.label.setAlignment(Qt.AlignCenter)
self.label.setStyleSheet('font-size: 12pt; background-color: red')
self.box_layout = QHBoxLayout()
self.box_layout.addWidget(self.label)
self.setLayout(self.box_layout)
self.show()
app = QApplication([])
win = Window()
app.exec()
Edit: laytout -> box_layout