import sys, os from PyQt5.QtWidgets import QTabWidget, QLabel, QWidget, QVBoxLayout, QApplication, QMessageBox class BaseWindow(QWidget): def __init__(self): super().__init__() self.initUI() def initUI(self): self.tabs = QTabWidget() self.tabs.blockSignals(True) #just for not showing the initial message self.tabs.currentChanged.connect(self.onChange) #changed! self.lbl1 = QLabel("Label 1") self...