Categorizing by attribute using custom QGIS plugin via python programming?

Question

I'd like to ask on how can I categorize by attribute using my shapefile via python / custom plugin.

I have a Municipality and field name of "Land_Classification" with values "Residential, Commercial and Industrial". I would like to create a plugin that if i click it the layer will automatically categorize the shapefile by Land Classification and will be have colors by Residential, Commercial and Industrial.

Answer 1

Personally, I would avoid creating a function in your plugin which allows the user to click a layer and apply a certain style (unless you add some sort of checkbox to enable/disable this function).

What I would do is:

1. Create a combo box and populate this with the names of the loaded layers
2. Create a function which defines the parameters of the categorised style
3. Create a push button which applies the above function to the selected layer in the combo box

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So you could use something like the following in your main.py file. Add the imports at the top of the script:

from qgis.core import QgsMapLayerRegistry, QgsSymbolV2, QgsRendererCategoryV2, QgsCategorizedSymbolRendererV2
from PyQt4.QtGui import QColor

Then in the main run() function, you can use:

def run(self):
    # Define a list for layer names
    layer_list = []
    # Get all layer names loaded in QGIS and store them in the list   
    for layer in QgsMapLayerRegistry.instance().mapLayers().values():
        layerName = layer.name()
        layer_list.append(layerName)
    # Clear layer combo box to make sure if plugin is loaded again, the list is not duplicated
    self.dlg.comboBox_layer.clear()
    # Add layers from list to combobox
    self.dlg.comboBox_layer.addItems(layer_list)

    def apply_style():
        # Get currently selected layer in combo box
        current_layer = self.dlg.comboBox_layer.currentText()
        layer = QgsMapLayerRegistry.instance().mapLayersByName(str(current_layer))[0]

        # Define style parameters: value, colour, legend
        land_class = {
        'Residential': ('#f00', 'Residential'),
        'Commercial': ('#0f0', 'Commercial'),
        'Industrial': ('#fff', 'Industrial'),
        }

        # Define a list for categories
        categories = []
        # Define symbology depending on layer type, set the relevant style parameters
        for classes, (color, label) in land_class.items():
            symbol = QgsSymbolV2.defaultSymbol(layer.geometryType())
            symbol.setColor(QColor(color))
            category = QgsRendererCategoryV2(classes, symbol, label)
            categories.append(category)

        # Column/field name to be used to read values from
        column = 'Land_Class'
        # Apply the style rendering
        renderer = QgsCategorizedSymbolRendererV2(column, categories)
        layer.setRendererV2(renderer)
        # Refresh the layer
        layer.triggerRepaint()

    # Set up push button to connect to above function
    run_button = self.dlg.style_button
    run_button.clicked.connect(apply_style)