identify an instance in drag-and-drop

Question

I try hard to understand how to identify an instance of a class within drag and drop, but i need some help.

Beneath I add a sample from " http://zetcode.com/gui/pyqt5/dragdrop/ " to explain.

#!/usr/bin/python3
# -*- coding: utf-8 -*-

"""
ZetCode PyQt5 tutorial

In this program, we can press on a button with a left mouse
click or drag and drop the button with  the right mouse click. 

Author: Jan Bodnar
Website: zetcode.com
Last edited: August 2017
"""

from PyQt5.QtWidgets import QPushButton, QWidget, QApplication
from PyQt5.QtCore import Qt, QMimeData
from PyQt5.QtGui import QDrag
import sys

class Button(QPushButton):

    def __init__(self, title, parent):
        super().__init__(title, parent)


    def mouseMoveEvent(self, e):

        if e.buttons() != Qt.RightButton:
            return

        mimeData = QMimeData()

        drag = QDrag(self)
        drag.setMimeData(mimeData)
        drag.setHotSpot(e.pos() - self.rect().topLeft())

        dropAction = drag.exec_(Qt.MoveAction)


    def mousePressEvent(self, e):

        super().mousePressEvent(e)

        if e.button() == Qt.LeftButton:
            print('press')


class Example(QWidget):

    def __init__(self):
        super().__init__()

        self.initUI()


    def initUI(self):

        self.setAcceptDrops(True)

        self.button = Button('Button', self)
        self.button.move(100, 65)

        self.setWindowTitle('Click or Move')
        self.setGeometry(300, 300, 280, 150)


    def dragEnterEvent(self, e):

        e.accept()


    def dropEvent(self, e):

        position = e.pos()
        self.button.move(position)

        e.setDropAction(Qt.MoveAction)
        e.accept()


if __name__ == '__main__':

    app = QApplication(sys.argv)
    ex = Example()
    ex.show()
    app.exec_() 

Here my questions:

if I instantiate a 2nd button

    self.button2 = Button('Button2', self)
    self.button2.move(100, 165)

i can also move the button2, but in the DropEvent the first button "button" is moved.

How is it possible to identify within DropEvent, that I move "button2" und place button2?

So: question is: how to identify, which button it is. (i found only drag n drop examples, how to replace a label or a button by another element of same type)

And additional question: to create a new instance works with this type of code. If I create the buttons within QT5 designer, it is not possible to have them as an instance of this type of class Button in this code. So, defining drag n drop in the QT5 designer, drag n drop of an item within a list to another list works, but i am not able to identify, which element I have dragged

Answer 1

If the object you are dragging from is a QWidget then you can get it through the QDropEvent source() method, and the QPushButton is a QWidget so it works in this case:

def dropEvent(self, e):
    if e.source() is not None:
        position = e.pos()
        e.source().move(position)
        e.setDropAction(Qt.MoveAction)
        e.accept()

_{Remember to verify that it is not None since the source may be another window that is not a QWidget so you would get an exception}

---

If you want the buttons added through Qt Designer you want them to have the same functionality then the easiest solution is to promote the buttons, there are many examples in SO where they show how to do it:

• Clear QLineEdit on click event

• How to insert video in ui file which made at qt designer?

• Painting in a QLabel with paintEvent , etc

Answer 2

You are always using self.button in your drop event, so it refers to the same button. Instead you can use QDropEvent.source() to get the correct widget.

def dropEvent(self, e):
    position = e.pos()
    e.source().move(position) # source widget

    e.setDropAction(Qt.MoveAction)
    e.accept()