import sys
from PyQt5.QtCore import pyqtSignal, QObject
from PyQt5.QtWidgets import QMainWindow, QApplication
 
class Communicate(QObject):
    closeApp = pyqtSignal() 
 
class Example(QMainWindow):
    
    def __init__(self):
        super().__init__()
        self.initUI()
        
    def initUI(self):      
        self.c = Communicate()
        self.c.closeApp.connect(self.close)       
        
        self.setGeometry(300, 300, 290, 150)
        self.setWindowTitle('Emit signal')
        self.show()
        
    def mousePressEvent(self, event):
        self.c.closeApp.emit()
        
if __name__ == '__main__':
    app = QApplication(sys.argv)
    ex = Example()
    sys.exit(app.exec_())

 class Communicate(QObject):
    closeApp = pyqtSignal()    # наш сигнал

 self.c = Communicate()                 # инстанс класса Communicate
self.c.closeApp.connect(self.close)   # соединяем сигнал self.c.closeApp с слотом self.close

 def mousePressEvent(self, event):
    self.c.closeApp.emit()  # генерируем сигнал

  
>>> class A:
...     def __init__(self, name):
...         self.name = name
... 
>>> class B:
...     a = A('name1')
... 
>>> class C:
...     def m(self):
...         b = B()
...         print(b.a.name, id(b.a))
... 
>>> class D:
...     def m(self):
...         b = B()
...         print(b.a.name, id(b.a))
... 
>>> c = C()
>>> d = D()
>>> c.m()
name1 140118798922864
>>> d.m()
name1 140118798922864
>>>

 import sys
import PySide.QtGui as QtGui
import PySide.QtCore as QtCore
 
class X(QtGui.QMainWindow):
	def __init__(self):
		QtGui.QMainWindow.__init__(self)
		self.label = QtGui.QLabel(self)
		self.setCentralWidget(self.label)
		dialog = Dialog(self)
		dialog.signal.connect(self.slot)
		dialog.show()
	 
	def slot(self):
		self.label.setText(self.label.text()+u"Обломись баклан!\n")
 
class Dialog(QtGui.QDialog):
	signal = QtCore.Signal()
	def __init__(self, parent):
		QtGui.QDialog.__init__(self, parent)
		self.knopka = QtGui.QPushButton(self)
		self.knopka.setText(u"Нажми меня")
		self.knopka.clicked.connect(self.signal)
 
if __name__ == '__main__':
	app = QtGui.QApplication(sys.argv)
	win = X()
	win.show()
	sys.exit(app.exec_())