Creating categorization by symbols using PyQGIS

Question

I am trying to create a categorization by symbols I have reviewed this address but I failed to adapt it Apply symbol to each feature (Categorized symbol)

I need each point to be different according to the field. For example:

VALVULA 
0..............circle   
Aire...........triangle
0..............circle
Mariposa.......star
0..............circle
Alivio.........square
Aire...........triangle
0..............circle
Mariposa.......star
0..............circle
Alivio.........square

# Modifying simple symbol
layer6.renderer().symbol().setSize(2.5)
layer6.renderer().symbol().setColor(QColor("blue"))
layer6.renderer().symbol().symbolLayer(0).setShape(QgsSimpleMarkerSymbolLayerBase.Star)
layer6.triggerRepaint()
iface.layerTreeView().refreshLayerSymbology(layer6.id())

I only have this and it only works for me to change all the nodes of a layer.

Can you help me?

Ben W · Accepted Answer · 2019-11-01T00:17:34.243

You could use a rule based renderer.

I have partially based this answer on another answer here by @Joseph). I have tried this solution on a test layer and it works well:

Note: The second item in each nested list of rules is the filter expression for each rule. From your question I assume that your field which contains these values is called "VALVULA". If this is not the case you will have to change the expressions accordingly. Otherwise this should work fine on your layer.

# Get layer object
layer6 = QgsProject().instance().mapLayersByName('Layer_Name')[0] #Change to your layer name

symbol = QgsSymbol.defaultSymbol(layer6.geometryType())
renderer = QgsRuleBasedRenderer(symbol)


rules = [['0', """"VALVULA" LIKE '0'""", QgsSimpleMarkerSymbolLayerBase.Circle, 'Yellow'],
        ['Aire', """"VALVULA" LIKE 'Aire'""", QgsSimpleMarkerSymbolLayerBase.Triangle, 'Blue'],
        ['Mariposa', """"VALVULA" LIKE 'Mariposa'""", QgsSimpleMarkerSymbolLayerBase.Star, 'Red'],
        ['Alivio', """"VALVULA" LIKE 'Alivio'""", QgsSimpleMarkerSymbolLayerBase.Square, 'Green']]


def rule_based_symbology(layer, renderer, label, expression, symbol, shape, color):
    root_rule = renderer.rootRule()
    rule = root_rule.children()[0].clone()
    rule.setLabel(label)
    rule.setFilterExpression(expression)
    rule.symbol().symbolLayer(0).setShape(shape)
    rule.symbol().setColor(QColor(color))
    root_rule.appendChild(rule)
    layer.setRenderer(renderer)
    layer.triggerRepaint()

for rule in rules:
    rule_based_symbology(layer6, renderer, rule[0], rule[1], symbol, rule[2], rule[3])

renderer.rootRule().removeChildAt(0)
iface.layerTreeView().refreshLayerSymbology(layer6.id())

Result:

Edit: If you wish to collapse the legend nodes for the layer symbology categories:

layer_node = QgsProject().instance().layerTreeRoot().findLayer(layer6.id())
layer_node.setExpanded(False) #or True to expand again

Hi Ben W, can you help me ? – Jose Manolo Barahona Nov 07 '19 at 19:33 — Jose Manolo Barahona, Nov 07 '19 at 19:33

score 2 · Answer 2 · answered Oct 31 '19 at 10:48

They key part is to create a dictionary which holds all the symbology properties that you are interested in such as the field value, colour, label and shape. Once this is done, you create a list to store these properties which is then fed into the QgsCategorizedSymbolRenderer and then set your expression and apply the renderer.

So you could try something like the following:

# Get active layer
layer = iface.activeLayer()
# Define shape class
shape = QgsSimpleMarkerSymbolLayerBase
# Create dictionary to store field value, colour, label and shape
land_class = {
    '0': (QColor("yellow"), '0', shape.Circle),
    'Aire': (QColor("blue"), 'Aire', shape.Triangle),
    'Alivio': (QColor("green"), 'Alivio', shape.Diamond),
    'Mariposa': (QColor("red"), 'Mariposa', shape.Star),
}
# Create list to store symbology properties
categories = []
# Iterate through the dictionary
for classes, (colour, label, shape) in land_class.items():
    # Define symbol based on layer's geometry
    symbol = QgsSymbol.defaultSymbol(layer.geometryType())
    # Set shape
    symbol.symbolLayer(0).setShape(shape)
    # Set colour
    symbol.setColor(QColor(colour))
    # Set the renderer properties
    category = QgsRendererCategory(classes, symbol, label, shape)
    categories.append(category)

# Field name
expression = 'VALVULA'
# Set the categorized renderer
renderer = QgsCategorizedSymbolRenderer(expression, categories)
layer.setRenderer(renderer)
# Refresh layer
layer.triggerRepaint()

Thank you very much friend works perfect. A small query When categorizing a layer it shows the categories (it expands). Which expression can I use to get it? Thank you — Jose Manolo Barahona, Oct 31 '19 at 19:57
@JoseManoloBarahona - I believe Ben W has answered that in his solution :) — Joseph, Nov 01 '19 at 11:33

Creating categorization by symbols using PyQGIS

2 Answers2