Categorized Renderer

Question

I am trying to set a categorised renderer for a point layer, according to its categories. I used this code:

vector = QgsVectorLayer("route to shape", "title", "ogr")      
mi_rend = QgsCategorizedSymbolRendererV2() 
mi_rend.setClassAttribute('TheAttribute') #Do I need one more attribute?
vector.setRendererV2(mi_rend)
QgsMapLayerRegistry.instance().addMapLayer(vector)

The problem is that the layer is loaded in the table of contents, nevertheless, it doesn't appear. I have been looking for solutions in this forum but I haven't found any solution at the moment.

Answer 1

I don't see you setting any categories. Use addCategory() to add categories.

Or even much more intuitive (modified from the example in Lawhead QGIS Python programming cookbook, pp 142):

# next line is pseudo code !! Replace and extent with your values !!
# my_classes = {class_value: (symbol_property, label_text), ...}
# for example:
my_classes = {1: ('yellow', 'First'),
              2: ('red', 'Second'),
              3: ('green', 'Third'),
              4: ('blue', 'Fourth')}
categories = []

for class_value, (symbol_property, label_text) in my_classes.items():

    # get default symbol for this geometry type
    symbol = QgsSymbolV2.defaultSymbol(vector.geometryType())

    # set symbol properties as you like
    # symbol.set*(symbol_property)
    symbol.setColor(QColor(symbol_property))

    # create a category with these properties
    category = QgsRendererCategoryV2(class_value, symbol, label_text)
    categories.append(category)

mi_rend = QgsCategorizedSymbolRendererV2('ESTRELLA', categories) 
vector.setRendererV2(mi_rend)

In this example category values, symbol properties (such as color, size, svg filename), and label strings are put into a dict. Then a loop picks each dict element one by one, creates a renderer category, and saves it in a list. Finally the renderer is initialized with this list of categories and assigned to the layer.