Simple Fill and Line Pattern Fill in Categorized Symbols in the same layer

Question

I have a unique layer with different polygon features. The symbology for each one is a simple fill with a random color, except if the name of the feature starts with "UR". In this case, the color is still random but the filling must be a Line Pattern.

I have to do it for QGIS using Python but I have to admit that I am a starter in programming. I know the simple fill can be done by QgsFillSymbol.createSimple(), but how can I get the line pattern filling? I have used the QgsLinePatternFillSymbolLayer.create(), but I am sure I am missing something...

Is there a way to do it using Categorized Symbols?

Here is a little example of what I have. I know it has mistakes, but as I told you, I am just a starter and this is only the "chassis" of what I am trying to do.

if name.startswith("UR"):

    symb = QgsLinePatternFillSymbolLayer.create( { 'angle' : 30, 'distance' : 2.0, 'width' : 0.5, 'color' : color } )
    symb.setlineAngle(angle)symb.setDistance(distance)
    symb.setLineWidth(width)
    symb.setcolor(color)
else:
    symb = QgsFillSymbol.createSimple( { 'color' : color,  'outline_color' :'gray'  } )

Answer 1

One way to achieve your goal could be to use the QgsRuleBasedRenderer and to set the relevant rules. An example that combines a default plain fill symbol and a line pattern symbol is given in the following code snippet:

# Define a default symbol with a single symbol layer. 
symbol_lyr_default = QgsSimpleFillSymbolLayer()
symbol_lyr_default.setFillColor(QColor("blue"))
symbol_default = QgsFillSymbol()
symbol_default.appendSymbolLayer(symbol_lyr_default)

# Define a line pattern symbol. 
symbol_lyr_line = QgsLinePatternFillSymbolLayer()
symbol_lyr_line.setLineAngle(30)
symbol_lyr_line.setDistance(2)
symbol_lyr_line.setLineWidth(0.5)
symbol_lyr_line.setColor(QColor("orange"))
symbol_line = QgsFillSymbol()
symbol_line.deleteSymbolLayer(0)
symbol_line.appendSymbolLayer(symbol_lyr_line)

# Define a default rule that associates all features to the default symbol.
root_rule = QgsRuleBasedRenderer.Rule(symbol_default)

# Add another rule that sets the line symbol for features whose "name" attribute starts with 'UR'. Add the rule as a child of the root_rule.
s_filter = 'substr("name", 0, 2) = \'UR\''
line_rule = QgsRuleBasedRenderer.Rule(symbol_line, 0, 0, s_filter)
root_rule.appendChild(line_rule)

# Define a rule based renderer and use it to render the layer. 
rule_renderer = QgsRuleBasedRenderer(root_rule)
lyr.setRenderer(rule_renderer)
lyr.triggerRepaint()

The code above works on some sample data downloaded from the natural earth database. I hope it does also for your case.