PySide/PyQT5: How to emit signals from a QGraphicsItem?

Question

I want to emit a signal from a QGraphicsItem when it is doubled-clicked, in order to change a widget in the main window. The graphics-scene/-item does not provide an emit() method, but I was just wondering if there is an alternate way to do this. The code below has a function within a QGraphicsView class that will print to the terminal when an item is double-clicked. How can I make that into a slot/signal instead (if QGraphicsItem does not support signal/slots)?

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

class MyFrame(QGraphicsView):
    def __init__( self, parent = None ):
        super(MyFrame, self).__init__(parent)

        scene = QGraphicsScene()
        self.setScene(scene)
        self.setFixedSize(500, 500)

        pen = QPen(QColor(Qt.green))
        brush = QBrush(pen.color().darker(150))

        item = scene.addEllipse(0, 0, 45, 45, pen, brush)
        item.setPos(0,0)

    def mouseDoubleClickEvent(self, event):
        print("Circle Clicked!")
        # this double click event prints to terminal but how to setup
        # signal/slot to update the QWidget QLabel text instead?

class Example(QWidget):   
    def __init__(self):
        super(Example, self).__init__()
        self.initUI()

    def initUI(self):        
        hbox = QHBoxLayout(self)
        top = QLabel("Double Click Green Circle (Howto change this QWidget Label with signals?)")
        bottom = MyFrame()

        splitter = QSplitter(Qt.Vertical)
        splitter.addWidget(top)
        splitter.addWidget(bottom)

        hbox.addWidget(splitter)
        self.setLayout(hbox)
        self.setGeometry(0, 0, 500, 600)
        self.show()

def main():
    app = QApplication(sys.argv)
    ex = Example()
    sys.exit(app.exec_())

if __name__ == '__main__':
    main()

Answer 1

Below is a simple example showing one way to emit signals from a graphics-item. This defines a custom signal on a subclass of QGraphicsScene and then uses the scene() method of graphics-items to emit it:

import sys
from PySide import QtCore, QtGui

class GraphicsScene(QtGui.QGraphicsScene):
    itemDoubleClicked = QtCore.Signal(object)

class GraphicsRectangle(QtGui.QGraphicsRectItem):
    def mouseDoubleClickEvent(self, event):
        self.scene().itemDoubleClicked.emit(self)

class Window(QtGui.QWidget):
    def __init__(self):
        super(Window, self).__init__()
        self.view = QtGui.QGraphicsView()
        self.scene = GraphicsScene(self)
        self.view.setScene(self.scene)
        layout = QtGui.QVBoxLayout(self)
        layout.addWidget(self.view)
        for i in range(1, 4):
            self.scene.addItem(GraphicsRectangle(50 * i, 50 * i, 20, 20))
        self.scene.itemDoubleClicked.connect(self.handleItemDoubleClicked)

    def handleItemDoubleClicked(self, item):
        print(item.boundingRect())

if __name__ == '__main__':

    app = QtGui.QApplication(sys.argv)
    window = Window()
    window.setGeometry(600, 100, 300, 200)
    window.show()
    sys.exit(app.exec_())

UPDATE:

Below is a an example based on the code in your question. The basic idea is the same: define a custom signal on an available QObject (the graphics-view in this case), and use that to emit the double-click notification.

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

class MyFrame(QGraphicsView):
    itemDoubleClicked = Signal(object)

    def __init__(self, parent=None):
        super(MyFrame, self).__init__(parent)
        scene = QGraphicsScene()
        self.setScene(scene)
        self.setFixedSize(500, 500)
        for i, color in enumerate('red blue green'.split()):
            pen = QPen(QColor(color))
            brush = QBrush(pen.color().darker(150))
            item = scene.addEllipse(i * 50, i * 50, 45, 45, pen, brush)
            item.setData(0, color.upper())

    def mouseDoubleClickEvent(self, event):
        item = self.itemAt(event.pos())
        if item is not None:
            self.itemDoubleClicked.emit(item)

class Example(QWidget):
    def __init__(self):
        super(Example, self).__init__()
        self.initUI()

    def initUI(self):
        hbox = QHBoxLayout(self)
        top = QLabel('Double Click a Circle')
        bottom = MyFrame()
        bottom.itemDoubleClicked.connect(
            lambda item, top=top:
                top.setText('Double Clicked: %s' % item.data(0)))
        splitter = QSplitter(Qt.Vertical)
        splitter.addWidget(top)
        splitter.addWidget(bottom)
        hbox.addWidget(splitter)
        self.setLayout(hbox)
        self.setGeometry(0, 0, 500, 600)
        self.show()

def main():
    app = QApplication(sys.argv)
    ex = Example()
    sys.exit(app.exec_())

if __name__ == '__main__':
    main()