pyqt: why the loaded qss file do not work - pyqt

Question

I am loading qss file in my project. And I find it do not work.
My qss file is:
QMainWindow
{
font-size: 20px;
background: rgb(255, 0, 0);
}
My code is:
from PyQt5.QtWidgets import *
from PyQt5.QtCore import *
from PyQt5.QtGui import *
import sys
class Win(QMainWindow):
def __init__(self):
super().__init__()
with open('style.qss', 'r', encoding='utf-8') as file:
str = file.read()
self.setStyleSheet(str)
self.__widget = QWidget()
self.setCentralWidget(self.__widget)
if __name__ == '__main__':
app = QApplication(sys.argv)
win = Win()
win.show()
app.exec_()
However, the background color of showed window is not red.
-------------------- update --------------------------------------
According to the suggestion of #musicamante, I try that add a overwrited QWidget in the QMainWindow. However, it still do not work.

Answer 1

Try this:
style.qss
font-size: 20px;
background: rgb(255, 0, 0);
or like this:
style.qss
QMainWindow {
font-size: 20px;
background: rgb(255, 0, 0);
}

Answer 2

There are two issues with your code:
when using the curly brackets ({ property: value; }) syntax, the selector is mandatory (see the selector types), otherwise no brackets should be used at all (but that's usually frown upon);
when styling a plain QWidget, the paintEvent() must be overridden (see the related QWidget reference on the stylesheet documentation);
With the given code, the stylesheet syntax should be the following (note the asterisk):
* {
font-size: 20px;
background: rgb(255, 0, 0);
}
When styling a basic QWidget (or a QWidget subclass created in python), the following is also required:
class SomeCustomWidget(QWidget):
# ...
def paintEvent(self, event):
qp = QPainter(self)
opt = QStyleOption()
opt.initFrom(self)
self.style().drawPrimitive(QStyle.PE_Widget, opt, qp, self)
class Win(QMainWindow):
def __init__(self):
# ...
self.__widget = SomeCustomWidget()
self.setCentralWidget(self.__widget)