PyQGIS 3: How can pie charts have a fixed size when automating the process?

Question

I'm unsure how to automate the process for PyQGIS to update the pie charts to a fixed size of 12. The approach I used is not working out and no pie charts are being created.

lyr = iface.activeLayer()

diagram = QgsPieDiagram()

ds = QgsDiagramSettings()

dColors = {'Field1': QColor('#5a09a6'),'Field2': QColor('#ff9200')}

ds.categoryColors = dColors.values()
ds.categoryAttributes = dColors.keys()
ds.sizeType = 0
ds.categoryLabels = ds.categoryAttributes

dr = QgsLinearlyInterpolatedDiagramRenderer()
#dr.setClassificationField('SAMPLE NUM')
dr.setDiagram(diagram)
dr.setDiagramSettings(ds)

lyr.setDiagramRenderer(dr)
dls = QgsDiagramLayerSettings()

lyr.setDiagramLayerSettings(dls)
lyr.triggerRepaint()

Answer 1

I spent a lot of time trying to find the method to check 'Fixed size' or/and uncheck 'Scaled size' options and it was not possible. However, I found out a totally equivalent approach by using following code. The key was in to use a high number (1000000) in 'setLowerValue' method of QgsLinearlyInterpolatedDiagramRenderer class to avoid 'Scaled size' in the field used as base of classification.

lyr = iface.activeLayer()

diagram = QgsPieDiagram()

ds = QgsDiagramSettings()

dColors = {'Field1': QColor('#5a09a6'),'Field2': QColor('#ff9200')}

ds.categoryColors = dColors.values()
ds.categoryAttributes = dColors.keys()
ds.categoryLabels = ds.categoryAttributes
ds.sizeType = 0 #0 = Millimeters, 1 = Map Units, 2 = Pixels, 4 = Points, 5 = Inches

# Set renderer:
dr = QgsLinearlyInterpolatedDiagramRenderer()
dr.setLowerValue(1000000)
dr.setUpperSize(QSizeF(12, 12))
dr.setClassificationField('field1')
dr.setDiagram(diagram)
dr.setDiagramSettings(ds)

lyr.setDiagramRenderer(dr)
dls = QgsDiagramLayerSettings()

lyr.setDiagramLayerSettings(dls)
lyr.triggerRepaint()

I ran above code with following vector layer; where it can be observed in its attributes table 'field1' and 'field2' fields with different arbitrary values.

Result with Pie Diagram was as follows (Observe that Size scale was placed in 12):

I also applied manually 'Fixed size' option and result remained unchanged; as it can be observed in following image. I ran the script again and result was identical. It worked.