test3.py

import sys
from PyQt5.QtWidgets import *

class MainWindow(QWidget):
    def __init__(self):
        super(MainWindow, self).__init__()
        self.setFixedSize(480, 320)
        #self.setStyleSheet("background-color: rgb(255, 255, 255);")
        self.buttons()

        self.stack1 = QWidget()
        self.stack2 = QWidget()
        self.stack3 = QWidget()

        self.stack1UI()
        self.stack2UI()
        self.stack3UI()

        self.stack = QStackedWidget(self)
        self.stack.setGeometry(100, 50, 200, 200)
        self.stack.addWidget(self.stack1)
        self.stack.addWidget(self.stack2)
        self.stack.addWidget(self.stack3)
        self.stack.setCurrentIndex(0)

        self.show()

    def buttons(self):
        self.btn1 = QPushButton('page1', self)
        self.btn1.setGeometry(20, 20, 50, 25)
        self.btn1.clicked.connect(lambda: self.stack.setCurrentWidget(self.stack1))
        self.btn2 = QPushButton('page2', self)
        self.btn2.setGeometry(20, 45, 50, 25)
        self.btn2.clicked.connect(lambda: self.stack.setCurrentWidget(self.stack2))
        self.btn3 = QPushButton('page3', self)
        self.btn3.setGeometry(20, 70, 50, 25)
        self.btn3.clicked.connect(lambda: self.stack.setCurrentWidget(self.stack3))

    def stack1UI(self):
        layout = QGridLayout(self)
        label1 = QLabel('huhu', self)
        label1.setGeometry(120, 100, 100, 50)
        layout.addWidget(label1)
        self.stack1.setLayout(layout)

    def stack2UI(self):
        layout = QVBoxLayout(self)
        label1 = QLabel('huhu222', self)
        label1.setGeometry(120, 100, 100, 50)
        label2 = QLabel('BLAH', self)
        label2.setGeometry(80, 80, 100, 50)
        layout.addWidget(label1)
        layout.addWidget(label2)
        self.stack2.setLayout(layout)

    def stack3UI(self):
        #layout = QGridLayout(self)
        label1 = QLabel('huhu', self.stack3)
        label1.setGeometry(120, 100, 100, 50)
        #layout.addWidget(label1)
        #self.stack3.setLayout(layout)


if __name__ == '__main__':
    app = QApplication(sys.argv)
    w = MainWindow()

    sys.exit(app.exec_())