Drag and Drop in PyQt4
last modified October 18, 2023
In this part of the PyQt4 tutorial, we 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.
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 have a QtGui.QLineEdit
and a
QtGui.QPushButton
. We drag plain text from
the line edit widget and drop it onto the button widget. The button's
label will change.
#!/usr/bin/python """ ZetCode PyQt4 tutorial This is a simple drag and drop example. author: Jan Bodnar website: zetcode.com last edited: January 2015 """ 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) def main(): app = QtGui.QApplication(sys.argv) ex = Example() ex.show() 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 inherits 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 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.
Drag & drop a button widget
In the following example, we demonstrate how to drag & drop a button widget.
#!/usr/bin/python """ 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 the 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 operations.