Index out of bounds in custom QLayout subclass

This is really puzzling me:
I have a subclass of QLayout based on the flow example in the documentation. I've created a new widget that uses it, and for some reason, now Qt is trying to access itemAt with an index >= count() which is resulting in an exception. It is not calling count() before it asks for each item My code is not calling itemAt() anywhere. Here is the part of the widget class:

Code:

class DElementWidget(QWidget):
    def __init__(self, element, *args):
        super(DElementWidget, self).__init__(*args)
        self.element = element
        self.view_mode = True
        self.format = ['title', 'name', ('string', '='), 'value']
 
        layout = QGridLayout()
        layout.setSpacing(2)
        layout.setMargin(2)
 
Omitted, added 4 buttons here
 
        layout.setColumnMinimumWidth(4, 30)
        layout.setColumnStretch(4, 1)
 
        format_holder_w = QWidget()
 
        self.format_holder = DFlowLayout(2, 3, 3)
        format_holder_w.setLayout(self.format_holder)
        for index in range(len(self.format)):
            w = self.widget_for(index)
            self.format_holder.addWidget(w)
 
        if isinstance(self.element.value, Integer):
            self.format_holder.addWidget(DElementWidget(DObject(RealNumber(6)*RealNumber(7), parent=self.element)))
 
        scrollarea = QScrollArea()
        scrollarea.setWidget(format_holder_w)
        layout.addWidget(scrollarea, 1, 0, 1, 5)
 
        self.setLayout(layout)
 
    def widget_for(self, index):
        return QLabel(unicode(self.format[index]))

Here are parts of the layout:

Code:

class DFlowLayout(QLayout):
    '''Translated from http://doc.qt.nokia.com/latest/layouts-flowlayout.html'''
    def __init__(self, margins = -1, hspace=-1, vspace=-1, *args):
        super(DFlowLayout, self).__init__(*args)
        self.setContentsMargins(margins, margins, margins, margins)
        self._hspace = hspace
        self._vspace = vspace
        self.valign = Qt.AlignCenter
        self.halign = Qt.AlignLeft
        self.items = []
 
    def addItem(self, item):
        print 'added', item
        self.items.append(item)
 
    def replaceItem(self, old, item):
....
Omitted, not used
....
 
    def count(self):
        print len(self.items)
        return len(self.items)
 
    def horizontalSpacing(self):
        if self._hspace >= 0: return self._hspace
        else: return self.smartSpacing(QStyle.PM_LayoutHorizontalSpacing)
 
    def takeAt(self, index):
        v = self.items[index]
        self.items = self.items[:index] + self.items[index+1:]
        return v
 
    def itemAt(self, index):
        print 'get', index
        return self.items[index]
 
    def expandingDirections(self):
        return Qt.Orientation()
 
    def setGeometry(self, rect):
        QLayout.setGeometry(self, rect)
        self.doLayout(rect)
 
    def lines(self, width = -1):
        ...
Omitted, returns the wrapped lines and their dimensions
....
        return result, widths, heights
 
    def hasHeightForWidth(self):
        return True
 
    def heightForWidth(self, width):
        return sum(self.lines(width - 2*self.margin())[2]) + 2*self.margin()
 
    def sizeHint(self):
        return self.minimumSize()
 
    def minimumSize(self):
        res = self.lines()
        if len(res[0]) == 0:
            return QSize()
        width = max(res[1]) + 2*self.margin()
        height = sum(res[2]) + 2*self.margin()
        #print height
        return QSize(width, height)
 
    def doLayout(self, rect):
       .....
Omitted, places lines
       .....
 
    def verticalSpacing(self):
        if self._vspace >= 0: return self._vspace
        else: return self.smartSpacing(QStyle.PM_LayoutVerticalSpacing)
 
    def smartSpacing(self, thingy):
        parent = self.parent()
        if parent == None:
            return None
        elif parent.isWidgetType():
            return thingy.style().pixelMetric(thingy, 0, thingy)
        else:
            return parent.spacing()

Adding widgets to format_holder_w but not the layout does not change which items are accessed. It always tries to get one more item than exists in the array, except when the calls are preceeded by a call to count() in which case there is no exception. This class has worked before without errors. I'm really at a loss here, WTF is going on?