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Drag and Drop in PyQt4

In this part of the PyQt4 tutorial, we will talk about drag & drop operations.

In computer graphical user interfaces, drag-and-drop is the action of (or support for the action of) clicking on a virtual object and dragging it to a different location or onto another virtual object. In general, it can be used to invoke many kinds of actions, or create various types of associations between two abstract objects. (Wikipedia)

Drag and drop is part of the graphical user interface. Drag and drop operations enable users to do complex things intuitively.

Usually, we can drag and drop two things: data or some graphical objects. If we drag an image from one application to another, we drag and drop binary data. If we drag a tab in Firefox and move it to another place, we drag and drop a graphical component.

Simple Drag and Drop

In the first example, we will have a QtGui.QLineEdit and a QtGui.QPushButton. We will drag plain text from the line edit widget and drop it onto the button widget.

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

"""
ZetCode PyQt4 tutorial

This is a simple drag and drop example. 

author: Jan Bodnar
website: zetcode.com
last edited: October 2013
"""

import sys
from PyQt4 import QtGui


class Button(QtGui.QPushButton):
  
    def __init__(self, title, parent):
        super(Button, self).__init__(title, parent)
        
        self.setAcceptDrops(True)

    def dragEnterEvent(self, e):
      
        if e.mimeData().hasFormat('text/plain'):
            e.accept()
            
        else:
            e.ignore() 

    def dropEvent(self, e):
        
        self.setText(e.mimeData().text()) 


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

        edit = QtGui.QLineEdit('', self)
        edit.setDragEnabled(True)
        edit.move(30, 65)

        button = Button("Button", self)
        button.move(190, 65)
        
        self.setWindowTitle('Simple Drag & Drop')
        self.setGeometry(300, 300, 300, 150)
        self.show()


def main():
  
    app = QtGui.QApplication([])
    ex = Example()
    app.exec_()  
  

if __name__ == '__main__':
    main()  

The example presents a simple drag & drop operation.

class Button(QtGui.QPushButton):
  
    def __init__(self, title, parent):
        super(Button, self).__init__(title, parent)

In order to drop text on the QtGui.QPushButton widget, we must reimplement some methods. Therefore, we create our own Button class which will inherit from the QtGui.QPushButton class.

self.setAcceptDrops(True)

We enable drop events for the widget.

def dragEnterEvent(self, e):
  
    if e.mimeData().hasFormat('text/plain'):
        e.accept()
        
    else:
        e.ignore() 

First, we reimplement the dragEnterEvent() method. We inform about the data type that we accept. In our case it is plain text.

def dropEvent(self, e):

    self.setText(e.mimeData().text()) 

By reimplementing the dropEvent() method we define what we will do upon the drop event. Here we change the text of the button widget.

edit = QtGui.QLineEdit('', self)
edit.setDragEnabled(True)

The QtGui.QLineEdit widget has a built-in support for drag operations. All we need to do is to call setDragEnabled() method to activate it.

Simple Drag & Drop
Figure: Simple Drag & Drop

Drag & drop a button widget

In the following example, we will demonstrate how to drag & drop a button widget.

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

"""
ZetCode PyQt4 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: October 2013
"""

import sys
from PyQt4 import QtCore, QtGui


class Button(QtGui.QPushButton):
  
    def __init__(self, title, parent):
        super(Button, self).__init__(title, parent)

    def mouseMoveEvent(self, e):

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

        mimeData = QtCore.QMimeData()

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

        dropAction = drag.start(QtCore.Qt.MoveAction)

    def mousePressEvent(self, e):
      
        super(Button, self).mousePressEvent(e)
        
        if e.button() == QtCore.Qt.LeftButton:
            print 'press'


class Example(QtGui.QWidget):
  
    def __init__(self):
        super(Example, self).__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)
        self.show()

    def dragEnterEvent(self, e):
      
        e.accept()

    def dropEvent(self, e):

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

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

def main():
  
    app = QtGui.QApplication([])
    ex = Example()
    sys.exit(app.exec_())


if __name__ == '__main__':
    main()

In our code example, we have a QtGui.QPushButton on the window. If we click on the button with a left mouse button, the 'press' message is printed to the console. By right clicking and moving the button, we perform a drag & drop operation on the button widget.

class Button(QtGui.QPushButton):
  
    def __init__(self, title, parent):
        super(Button, self).__init__(title, parent)

We create a Button class which will derive from the QtGui.QPushButton. We also reimplement two methods of the QtGui.QPushButton: the mouseMoveEvent() and the mousePressEvent(). The mouseMoveEvent() method is the place where the drag & drop operation begins.

if event.buttons() != QtCore.Qt.RightButton:
    return

Here we decide that we can perform drag & drop only with a right mouse button. The left mouse button is reserved for clicking on the button.

mimeData = QtCore.QMimeData()

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

The QDrag object is created. The class provides support for MIME-based drag and drop data transfer.

dropAction = drag.start(QtCore.Qt.MoveAction)

The start() method of the drag object starts the drag & drop operation.

def mousePressEvent(self, e):
  
    super(Button, self).mousePressEvent(e)
    
    if e.button() == QtCore.Qt.LeftButton:
        print 'press'

We print 'press' to the console if we left click on the button with the mouse. Notice that we call mousePressEvent() method on the parent as well. Otherwise, we would not see the button being pushed.

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

In the dropEvent() method we code what happens after we release the mouse button and finish the drop operation. We find out the current mouse pointer position and move a button accordingly.

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

We specify the type of the drop action. In our case it is a move action.

This part of the PyQt4 tutorial was dedicated to drag and drop.