self.buttonBox.rejected.connect(stack.setPage1)

# Result: Traceback (most recent call last):
File "<string>", line 28, in paintEvent
AttributeError: 'FaderWidget' object has no attribute 'pixmap_opacity'

import sys
from PySide.QtCore import QTimeLine
from PySide.QtGui import *

class FaderWidget(QWidget):

    def __init__(self, old_widget, new_widget):

        QWidget.__init__(self, new_widget)

        self.old_pixmap = QPixmap(new_widget.size())
        old_widget.render(self.old_pixmap)
        self.pixmap_opacity = 1.0

        self.timeline = QTimeLine()
        self.timeline.valueChanged.connect(self.animate)
        self.timeline.finished.connect(self.close)
        self.timeline.setDuration(333)
        self.timeline.start()

        self.resize(new_widget.size())
        self.show()

    def paintEvent(self, event):

        painter = QPainter()
        painter.begin(self)
        painter.setOpacity(self.pixmap_opacity)
        painter.drawPixmap(0, 0, self.old_pixmap)
        painter.end()

    def animate(self, value):

        self.pixmap_opacity = 1.0 - value
        self.repaint()

class StackedWidget(QStackedWidget):

    def __init__(self, parent = None):
        QStackedWidget.__init__(self, parent)

    def setCurrentIndex(self, index):
        self.fader_widget = FaderWidget(self.currentWidget(), self.widget(index))
        QStackedWidget.setCurrentIndex(self, index)

    def setPage1(self):
        self.setCurrentIndex(0)

    def setPage2(self):
        self.setCurrentIndex(1)

    def setPage3(self):
        self.setCurrentIndex(2)

    def setPage4(self):
        self.setCurrentIndex(3)


class editor_(QWidget):
    """build a text editor"""
    def __init__(self):
        super(editor_, self).__init__()

        editor = QTextEdit()
        editor.setPlainText("Kawabungaaaa! "*100)

        self.buttonBox = QDialogButtonBox(QDialogButtonBox.Ok | QDialogButtonBox.Cancel)

        self.buttonBox.accepted.connect(self.close)
        self.buttonBox.rejected.connect(stack.setPage1)

        layout = QVBoxLayout()
        layout.addWidget(editor)
        layout.addWidget(self.buttonBox)

        self.setLayout(layout)


class mainWidget(QWidget):
    def __init__(self):
        super(mainWidget, self).__init__()

        self.setMinimumSize(500,300)

        widget1 = QWidget()
        widget2 = QCalendarWidget()
        widget3 = QListView()
        widget4 = editor_()

        grid = QGridLayout()

        allPlates = QPushButton("Calendar")
        singlePlate = QPushButton("ListView")
        allRoto = QPushButton("Editor")
        allLighting = QPushButton("Buttons")

        grid.addWidget(allPlates,0,0)
        grid.addWidget(singlePlate,0,1)
        grid.addWidget(allRoto,1,0)
        grid.addWidget(allLighting,1,1)

        widget1.setLayout(grid)

        stack = StackedWidget()
        stack.addWidget(widget1)
        stack.addWidget(widget2)
        stack.addWidget(widget3)
        stack.addWidget(widget4)

        layout = QHBoxLayout()
        layout.addWidget(stack)
        self.setLayout(layout)

        allPlates.clicked.connect(stack.setPage2)
        singlePlate.clicked.connect(stack.setPage3)
        allRoto.clicked.connect(stack.setPage4)
        allLighting.clicked.connect(stack.setPage1)


a = mainWidget()
a.show()