How to programmatically set a rule-based renderer in QGIS?

Question

I'm in the process of developing a plugin which highlights/changes the color of a set of features in a vector layer, depending on some calculation.

I found a nice article about QGIS – Rule based styling and i need to do the same thing but using python code.

What is the best way of doing this? Are there any examples out there?

Answer 1

This page has a good example of doing rule based renderer via python:

Rule-based renderer

The rule-based renderer styles features using a list of rules, defined by expressions. The rules are processed sequentially, with the first matching rule used to render a feature.

The example below defines a new rule-based renderer for different kinds of road features. Major roads are shown in orange, minor roads are shown in black at scales larger than 1:2,500 and residential roads are shown in grey at scales between 1:100 and 1:1,000.

# define some rules: label, expression, color name, (min scale, max scale)
road_rules = (
    ('Major road', '"type" LIKE \'major\'', 'orange', None),
    ('Minor road', '"type" LIKE \'minor\'', 'black', (0.0, 2500.0,)),
    ('Residential road', '"type" LIKE \'residential\'', 'grey', (100.0, 1000.0,)),
)

# create a new rule-based renderer
symbol = QgsSymbolV2.defaultSymbol(layer.geometryType())
renderer = QgsRuleBasedRendererV2(symbol)

# get the "root" rule
root_rule = renderer.rootRule()

for label, expression, color_name, scale in road_rules:
    # create a clone (i.e. a copy) of the default rule
    rule = root_rule.children()[0].clone()
    # set the label, expression and color
    rule.setLabel(label)
    rule.setFilterExpression(expression)
    rule.symbol().setColor(QColor(color_name))
    # set the scale limits if they have been specified
    if scale is not None:
        rule.setScaleMinDenom(scale[0])
        rule.setScaleMaxDenom(scale[1])
    # append the rule to the list of rules
    root_rule.appendChild(rule)

# delete the default rule
root_rule.removeChildAt(0)

# apply the renderer to the layer
layer.setRendererV2(renderer)