Scripting the mapping of style columns attributes in QGIS?

Question

In a vector layer is it possible to tune up/scripting the style columns attributes to map "Label" column to a specific attribute field/column instead of being populated with same content as the "Value" column? I'm working on QGIS 2.14.

We have symbology by categories and symbols in a XML created based on a key field ("ID_UCF" [type integer]) which is used to classify the style attributes - match to symbols from file - but this way we have the legend of the map taking the "ID_UCF" codes as symbol labels and we need those labels as a description of those codes. This description/text is given by a column present in the attribute table of the label ("DSCR_UCF" [type text]).

How can we change the content of style column "Label" - in this case "Legend" to assume the descriptions?

We have been looking at qgis.core such as rendererV2 class but could not find how to deal with the issue.

Answer 1

Hoping I have understood well your issue, you may set the text from a specified field as Legend by directly creating the categorized style.

Assuming to start from this self-explanatory Attribute Table:

you may assign the "DSCR_UCF" field as Legend for the "ID_UCF" values using the following code from the Python Console:

from PyQt4.QtGui import *
import random

layer = iface.activeLayer()

my_dict = {}
for feat in layer.getFeatures(): #iterate over the features for defining the lookup: value -> (color, label) 
    if feat["ID_UCF"] not in my_dict.keys():
        color = QColor.fromRgb(random.randint(0,255),random.randint(0,255),random.randint(0,255)) #assign a random color
        my_dict[feat["ID_UCF"]] = (color.name(), feat["DSCR_UCF"]) #lookup: value -> (color, label) 
    else: # go to the next feature since the label is already stored
        continue

# Create a category for each item in my_dict
categories = []
for value, (color_name, label) in my_dict.items():
    symbol = QgsSymbolV2.defaultSymbol(layer.geometryType())
    symbol.setColor(QColor(color_name))
    category = QgsRendererCategoryV2(value, symbol, label)
    categories.append(category)

# Create the render and assign it to the layer
renderer = QgsCategorizedSymbolRendererV2("ID_UCF", categories)
layer.setRendererV2(renderer)
layer.triggerRepaint()

You will get this result:

EDIT

If you want to keep the original colors unchanged, you may run the following code (assuming you previously assigned a custom categorized renderer with custom colors):

from PyQt4.QtGui import *

layer = iface.activeLayer()
f_index = layer.dataProvider().fieldNameIndex('ID_UCF')
unique_values = layer.uniqueValues(f_index)
rend = layer.rendererV2()
colors = [symb.color().name() for symb in rend.symbols()]  
diz = dict(zip(unique_values, colors))

my_dict = {}
for feat in layer.getFeatures(): #iterate over the features for defining the lookup: value -> (color, label) 
    if feat["ID_UCF"] not in my_dict.keys():
        val = [diz[feat["ID_UCF"]]]
        val.append(feat["DSCR_UCF"])
        my_dict[feat["ID_UCF"]] = tuple(val) #lookup: value -> (color, label) 
    else: # go to the next feature since the label is already stored
        continue

# Create a category for each item in my_dict
categories = []
for value, (color_name, label) in sorted(my_dict.items()):
    symbol = QgsSymbolV2.defaultSymbol(layer.geometryType())
    symbol.setColor(QColor(color_name))
    category = QgsRendererCategoryV2(value, symbol, label)
    categories.append(category)

# Create the render and assign it to the layer
renderer = QgsCategorizedSymbolRendererV2("ID_UCF", categories)
layer.setRendererV2(renderer)
layer.triggerRepaint()