from PyQt4 import QtGui
 
 
class Board(QtGui.QWidget):
    def __init__(self):
        super(Board, self).__init__()
        self.all_notification = []
 
        self.scroll_layout = QtGui.QVBoxLayout()
 
        self.scroll_widget = QtGui.QWidget()
        self.scroll_widget.setLayout(self.scroll_layout)
 
        self.scroll_area = QtGui.QScrollArea()
        self.scroll_area.setWidgetResizable(True)
        self.scroll_area.setWidget(self.scroll_widget)
 
        self.main_layout = QtGui.QVBoxLayout()
        self.main_layout.addWidget(self.scroll_area)
        self.setLayout(self.main_layout)
 
        self.default_label = QtGui.QLabel(
            '{}Уведомлений нет{}'.format(' ' * 14, ' ' * 14)
        )
        self.scroll_layout.addWidget(self.default_label)
        self.scroll_layout.addStretch(0)
        self.all_notification.append(self.default_label)
 
    def add_notification(self, massage):
        self.all_notification[0].hide()
 
        notification = QtGui.QLabel(massage)
 
        self.scroll_layout.addWidget(notification)
        self.all_notification.append(notification)
        self._remove_stretch()
        self.scroll_layout.addStretch(0)
 
    def _remove_stretch(self):
        for i in reversed(range(self.scroll_layout.count())):
            item = self.scroll_layout.itemAt(i)
            if isinstance(item, QtGui.QSpacerItem):
                self.scroll_layout.removeItem(item)
                return
 
 
app = QtGui.QApplication([])
w = QtGui.QWidget()
l = QtGui.QHBoxLayout()
w.setLayout(l)
 
but = QtGui.QPushButton('Добавить сообщение')
l.addWidget(but)
 
b = Board()
l.addWidget(b)
 
but.clicked.connect(lambda: b.add_notification('wefew' * (b.scroll_layout.count() + 2)))
 
w.show()
app.exec_()