Python+Qt, QScrollArea problem: what's wrong with this code?

Question

I have the following code:

#!/usr/bin/env python

import sys
from PyQt4 import QtGui, QtCore

class SimfilePanel(QtGui.QWidget):
  '''This class provides the simfile panel shown on the right side of the main window.'''
  def __init__(self, parent=None):
    '''Load song info here.'''
    QtGui.QWidget.__init__(self, parent)

    ## Make widgets.
    # Pane with simfile information.
    simfileInfoPane = QtGui.QWidget()
    simfileInfoPane.setSizePolicy(QtGui.QSizePolicy.Expanding, QtGui.QSizePolicy.Expanding)
    simfileInfoGrid = QtGui.QGridLayout()
    simfileInfoPane.setLayout(simfileInfoGrid)

    simfileInfoScrollArea = QtGui.QScrollArea()
    simfileInfoScrollArea.setWidget(simfileInfoPane)
    #if DEBUG: simfileInfoScrollArea.setBackgroundRole(QtGui.QPalette.Dark);

    # This will change
    labels = []
    textfields = []
    for i in range(0,20):
      labels.append( QtGui.QLabel("Label "+str(i)) )
      textfields.append( QtGui.QLineEdit() )
      labels[i].setBuddy(textfields[i])
      simfileInfoGrid.addWidget(labels[i], i, 0)
      simfileInfoGrid.addWidget(textfields[i], i, 1)

    ## Put widgets in a grid layout.
    mainvbox = QtGui.QVBoxLayout()
    mainvbox.addWidget(simfileInfoScrollArea)
    self.setLayout(mainvbox)


# Standalone testing
if __name__ == "__main__":
  app = QtGui.QApplication(sys.argv)
  panel = SimfilePanel()
  panel.show()
  sys.exit(app.exec_())

I can't get anything that I'm putting into the simfileInfoGrid to display! They'll display if I leave out the scroll area, but I need the scroll area as I will have a lot of fields to edit in the final version and I don't want to stretch the entire window over the screen.

As you see I've tried to add a size policy to simfileInfoPane, but it doesn't seem to affect anything. The area that's supposed to contain my pane stays empty!

Answer 1

Add the pane to the scroll area after you've added all the grid's contents. In particular you need to call QScrollArea.setWidget after you have finished creating the widget you add.

I don't know exactly why this is the problem, but I do know that I tend to initialize widgets "bottom-up": I finish adding all the contents of a sub-layout before I ever add it to a parent layout. I believe this is Qt optimizing order of rendering but I could be wrong about that.

The code below is a patch, mostly so you can see where the one-line change is.

    diff -u 1848547.py  tmp2.py
--- 1848547.py  2009-12-04 11:19:09.000000000 -0800
+++ tmp2.py 2009-12-04 11:34:58.000000000 -0800
@@ -19,7 +19,6 @@
     simfileInfoPane.setLayout(simfileInfoGrid)

     simfileInfoScrollArea = QtGui.QScrollArea()
-    simfileInfoScrollArea.setWidget(simfileInfoPane)
     #if DEBUG: 
     simfileInfoScrollArea.setBackgroundRole(QtGui.QPalette.Dark)

@@ -33,6 +32,8 @@
       simfileInfoGrid.addWidget(labels[i], i, 0)
       simfileInfoGrid.addWidget(textfields[i], i, 1)

+    simfileInfoScrollArea.setWidget(simfileInfoPane)
+
     ## Put widgets in a grid layout.
     mainvbox = QtGui.QVBoxLayout()
     mainvbox.addWidget(simfileInfoScrollArea)