Как закруглить края взаимосвязанных виджетов QListWidget и QStackedWidget или второй вариант с QTabWidget?

Я конечно не уверен, что мои фантазии оптимальны, но оно как бы работает. За основу взят ВТОРОЙ ВАРИАНТ С QTabWidget, т.к. в таблице стилей QTabBar есть - :first и :last.

Обратите внимание, как вставляются изображения:

border-image: url(btn72.png) repeat;

Картинки, которые я использовал, надо нарисовать поаккуратнее.

from PyQt5 import QtWidgets, QtGui, QtCore
from PyQt5.QtGui import QIcon
from PyQt5.QtCore import Qt


class Color(QtWidgets.QWidget):
    def __init__(self, color, *args, **kwargs):
        super(Color, self).__init__(*args, **kwargs)
        self.setAutoFillBackground(True)
        palette = self.palette()
        palette.setColor(QtGui.QPalette.Window, QtGui.QColor(color))
        self.setPalette(palette)


class TabBar(QtWidgets.QTabBar):
    def tabSizeHint(self, index):
        s = QtWidgets.QTabBar.tabSizeHint(self, index)
        s.transpose()
        return s

    def paintEvent(self, event):
        painter = QtWidgets.QStylePainter(self)
        opt = QtWidgets.QStyleOptionTab()
        for i in range(self.count()):
            self.initStyleOption(opt, i)
            painter.drawControl(QtWidgets.QStyle.CE_TabBarTabShape, opt)
            painter.save()
            s = opt.rect.size()
            s.transpose()
            r = QtCore.QRect(QtCore.QPoint(), s)
            r.moveCenter(opt.rect.center())
            opt.rect = r
            c = self.tabRect(i).center()
            painter.translate(c)
            painter.rotate(90)
            painter.translate(-c)
            painter.drawControl(QtWidgets.QStyle.CE_TabBarTabLabel, opt);
            painter.restore()


class TabWidget(QtWidgets.QTabWidget):
    def __init__(self, *args, **kwargs):
        QtWidgets.QTabWidget.__init__(self, *args, **kwargs)
        self.setTabBar(TabBar(self))
        self.setTabPosition(QtWidgets.QTabWidget.West)


class MyWindow(QtWidgets.QWidget):
    def __init__(self, parent = None):
        super().__init__(parent)
        self.setFixedSize(800, 400)

        self.current_index = 1                                                   # +++

        self.tabs = TabWidget()                                  
        self.tabs.currentChanged.connect(self.tabsCurrentChanged)                # +++->(int index)

        self.tabs.setTabPosition(QtWidgets.QTabWidget.West)                      
        self.tabs.setDocumentMode(True)
        #self.tabs.setMovable(True)

        self.tabs.addTab(QtWidgets.QLabel('1', alignment=QtCore.Qt.AlignCenter), '') # 'Вкладка first'
        self.tabs.addTab(QtWidgets.QLabel('1', alignment=QtCore.Qt.AlignCenter), '') # 'Вкладка 1'
        self.tabs.addTab(QtWidgets.QLabel('2', alignment=QtCore.Qt.AlignCenter), '') # 'Вкладка 2'
        self.tabs.addTab(QtWidgets.QLabel('2', alignment=QtCore.Qt.AlignCenter), '') # 'Вкладка last'                               #  'Вкладка last'

        self.tabs.setCurrentIndex(self.current_index)                            # + self.current_index

        box = QtWidgets.QVBoxLayout(self)
        box.addWidget(self.tabs)
        box.setContentsMargins(0, 0, 0, 0)

# ++ vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv        
    def tabsCurrentChanged(self, index): 
        #print(f'def tabsCurrentChanged(self, index): {index} - {self.current_index}')
        if index == 0 or index == 3:
            self.tabs.setCurrentIndex(self.current_index)

        if index == 1:  
            self.tabs.setStyleSheet('''
                QTabBar::tab:first:!selected  {
                    border-image: url(btn72.png) repeat;         
                } 
                QTabBar::tab:!selected {
                    border-image: url(btn6.png) repeat;          
                } 
                QTabBar::tab:last:!selected  {
                    border-image: url(btn82.png) repeat;
                } 
            ''')
            self.current_index = index  

        if index == 2:                         
            self.tabs.setStyleSheet('''
                QTabBar::tab:first:!selected  {
                    border-image: url(btn71.png) repeat;
                }  
                QTabBar::tab:last:!selected  {
                    border-image: url(btn81.png) repeat;
                }  
                QTabBar::tab:!selected {
                    border-image: url(btn41.png) repeat; 
                }
            ''')
            self.current_index = index
# ++ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^       


qss = '''
QTabBar {
    background-color: #fff;    
}

QTabBar::tab {
    background: rgb(34, 137, 163);
    color: white;
}

QTabBar::tab:selected {
    color: #ff0000;
    background-color: green;
    border-image: url(btn2.png);
    border-top-left-radius: 25px; 
    border-bottom-left-radius: 25px;  
    width: 80px;
    height: 80px;    
}

QTabBar::tab:!selected {
    background-color: yellow;    
    border-image: url(btn5.png) repeat;
    border-top-left-radius: 25px; 
    border-bottom-left-radius: 25px; 
    width: 80px;
    height: 80px;
}

/* ********* first ********************** */
QTabBar::tab:first:!selected  {
    border-image: url(btn72.png) repeat;
    margin-top: 60px;
    width: 80px;
    height: 20px;
} 
QTabBar::tab:first:selected  {
    border-image: url(btn71.png) repeat;       
    margin-top: 60px;
    width: 80px;
    height: 20px;
} 

/* ********* last *********************** */
QTabBar::tab:last:!selected  {
    border-image: url(btn82.png) repeat;
    margin-bottom: 60px;
    width: 80px;
    height: 20px;
} 
QTabBar::tab:last:selected  {
    border-image: url(btn81.png) repeat; 
    margin-bottom: 60px;
    width: 80px;
    height: 20px;
} 

QWidget>QLabel {                        /* ! QWidget>QLabel */
    margin: 0px;
    color: white;
    font: 21pt;
    background: #7189DB;
    border-top-left-radius: 25px;
    border-top-right-radius: 25px;
    border-bottom-left-radius: 25px; 
    border-bottom-right-radius: 25px;
}

QWidget {
    background-color: #fff;

}
'''


if __name__ == '__main__':
    import sys
    app = QtWidgets.QApplication(sys.argv)
    app.setStyleSheet(qss)                             # +
    window = MyWindow()
    window.setWindowTitle('QTabWidget')
    window.show()
    sys.exit(app.exec_())

btn2.png

btn41.png

btn5.png

btn6.png

btn71.png

btn72.png

btn81.png

btn82.png

Еще один способ.

Тут я использую две кнопки и контейнер со свойством background-color.

1.png

1sleep.png

2sleep.png

corner.png

corner1.png

corner2.png

nothing.png - пустая картинка 10px*10px. Нужна, чтобы один из контейнеров не менял размер

вот она><

from PyQt5 import QtCore, QtGui, QtWidgets

class MyWindow(QtWidgets.QWidget):
    def __init__(self, *args, **kwargs):
        super(MyWindow, self).__init__(*args, **kwargs)

        self.corner = QtWidgets.QLabel()
        self.corner.setPixmap(QtGui.QPixmap('corner.png'))
        self.corner.setFixedHeight(10)

        self.nothing = QtWidgets.QLabel()
        self.nothing.setPixmap(QtGui.QPixmap('nothing.png'))
        self.nothing.setFixedSize(10, 10)

        self.corner1 = QtWidgets.QLabel()
        self.corner1.setPixmap(QtGui.QPixmap('corner1.png'))
        self.corner1.setFixedSize(50, 10)

        self.corner2 = QtWidgets.QLabel()
        self.corner2.setPixmap(QtGui.QPixmap('corner2.png'))
        self.corner2.setFixedSize(50, 10)

        self.corner_box = QtWidgets.QHBoxLayout()
        self.corner_box.addWidget(self.corner, alignment = QtCore.Qt.AlignLeft)
        self.corner_box.addWidget(self.nothing, alignment = QtCore.Qt.AlignRight)
        self.corner.hide()

        self.current_index = 1

        main_box = QtWidgets.QHBoxLayout(self)
        btn = QtWidgets.QPushButton('Main Box')
        main_box.addWidget(btn)

        box1 = QtWidgets.QHBoxLayout(spacing = 0)
        main_box.addLayout(box1)

        self.box2 = QtWidgets.QVBoxLayout()

        self.label = QtWidgets.QLabel('Page 1', alignment = QtCore.Qt.AlignCenter)

        self.btn1 = self.make_btn('1.png')
        self.btn1.clicked.connect(lambda: self.remake_btn(1))
        self.btn2 = self.make_btn('2sleep.png')
        self.btn2.clicked.connect(lambda: self.remake_btn(2))
        self.box2.addWidget(self.btn1)
        self.box2.addWidget(self.btn2)
        self.box2.addWidget(self.corner1)
        self.corner1.hide()
        self.box2.addWidget(self.corner2)

        self.box2.addStretch(1)

        box1.addLayout(self.box2)

        self.container = QtWidgets.QWidget()
        self.container.setStyleSheet(qss_box)
        self.container.setFixedWidth(200)
        self.container.setMinimumHeight(150)
        box1.addWidget(self.container)
        self.box3 = QtWidgets.QVBoxLayout(self.container)
        self.box3.setContentsMargins(0, 0, 0, 20)

        self.box3.addLayout(self.corner_box)
        self.box3.addWidget(self.label)

    def make_btn(self, picture):
        btn = QtWidgets.QPushButton()
        btn.setFixedSize(50, 50)
        btn.setIcon(QtGui.QIcon(picture))
        btn.setIconSize(QtCore.QSize(50, 50))
        btn.setStyleSheet(qss_btn)
        return btn

    def remake_btn(self, index):
        if (index == 1) and (self.current_index != 1):
            self.corner.hide()
            self.corner1.hide()
            self.corner2.show()
            self.btn1.setIcon(QtGui.QIcon('1'))
            self.btn2.setIcon(QtGui.QIcon('2sleep'))
            self.label.setText('Page 1')
            self.current_index = 1

        if (index == 2) and (self.current_index != 2):
            self.corner.show()
            self.corner2.hide()
            self.corner1.show()
            self.btn2.setIcon(QtGui.QIcon('1'))
            self.btn1.setIcon(QtGui.QIcon('1sleep'))
            self.label.setText('Page 2')
            self.current_index = 2


qss_btn = '''QPushButton {
                 border: none;
                 margin: 0px;
                 padding: 0px;
             }'''

qss_box = '''QWidget {
                 background-color: #7189DB;
                 border-top-right-radius: 10px;
                 border-bottom-left-radius: 10px;
                 border-bottom-right-radius: 10px;
             }'''


if __name__ == '__main__':
    import sys
    app = QtWidgets.QApplication(sys.argv)
    w = MyWindow()
    w.show()
    sys.exit(app.exec_())