How to Autoresize QLabel pixmap keeping ratio without using classes?

pyqt, python, qlabel, qt-designer, resize

Solution

There are a couple of ways to do this.

Firstly, you can promote your QLabel in Qt Designer to a custom subclass that is written in python. Right-click the QLabel and select "Promote to...", then give the class a name (e.g. "ScaledLabel") and set the header file to the python module that the custom subclass class will be imported from (e.g. 'mylib.classes').

The custom subclass would then re-implement the `resizeEvent` like this:

class ScaledLabel(QtGui.QLabel):
    def __init__(self, *args, **kwargs):
        QtGui.QLabel.__init__(self)
        self._pixmap = QtGui.QPixmap(self.pixmap())

    def resizeEvent(self, event):
        self.setPixmap(self._pixmap.scaled(
            self.width(), self.height(),
            QtCore.Qt.KeepAspectRatio))

For this to work properly, the QLabel should have its size policy set to expanding or minimumExpanding, and the minimum size should be set to a small, non-zero value (so the image can be scaled down).

The second method avoids using a subclass and uses an event-filter to handle the resize events:

class MainWindow(QtGui.QMainWindow):
    def __init__(self):
        ...
        self._pixmap = QtGui.QPixmap(self.label.pixmap())
        self.label.installEventFilter(self)

    def eventFilter(self, widget, event):
        if (event.type() == QtCore.QEvent.Resize and
            widget is self.label):
            self.label.setPixmap(self._pixmap.scaled(
                self.label.width(), self.label.height(),
                QtCore.Qt.KeepAspectRatio))
            return True
        return QtGui.QMainWindow.eventFilter(self, widget, event)

Problem

We are making a GUI using PyQt and Qt Designer. Now we need that an image(pixmap) placed in a QLabel rescales nicely keeping ratio when the window is resized. I've been reading other questions/answers but all of them use extended classes. As we are making constant changes in our UI, and it's created with Qt Creator, the .ui and (corresponding).py files are automatically generated so, if I'm not wrong, using a class-solution is not a good option for us because we should manually change the name of the class each time we update the ui. Is there any option to autoresize the pixmap in a QLAbel keeping the ratio and avoiding using extended clases? Thanks.

Original source