Directly exporting rule-based count results in QGIS

Question

I have hundreds of rules specifying ranges of polygon areas. I would like to create a table of the ranges and their values outside of QGIS and since these results are exactly what I am after I would like to export them directly from the layer properties here. I know there are tools like statistics by category which are useful but it would be far easier and quicker if I could simply use these count results directly.

Answer 1

You can access the filter expression of each rule with this script. Copy-paste it to the script editor of the Python console and click Run

Make sure the layer is selected in the Layers Panel, and that it is not in edit mode, else the filter will not be applied.

# get layer
lyr = iface.activeLayer()
get the rule-based rendered from the layer
r = lyr.renderer()
get the root rule
root = r.rootRule()
iterate through rules
for rule in root.children():
    # get the rule filter expression
    exp = rule.filterExpression()
# set the rule filter on the layer
lyr.setSubsetString(exp)

# count filtered features
count = lyr.featureCount()

# reset the filter
lyr.setSubsetString('')

print(f'rule: {exp} | count: {count}')

## do something with rules (save to text file)

Output:

rule: "code" = 'A' | count: 12425
rule: "code" = 'B' | count: 12580
rule: "code" = 'C' | count: 12298
rule: "code" = 'D' | count: 12618
rule: "code" = 'E' | count: 12534
rule: "code" = 'F' | count: 12441

Answer 2

To elaborate on my other answer, and with the help of this excellent answer by Ben W, I came up with a script to save the rules, counts, and symbology to an Excel spreadsheet.

It relies on one non-native library, XlsxWriter. The easiest way to install it is to open the OsGeo4W shell (via the Start Menu on Windows) and use:

pip install XlsxWriter

Then run the script in the Script Editor in the QGIS Python console.

import os
import xlsxwriter
from io import BytesIO
path to save images
IMG_PATH = r'D:\OneDrive\geodata\symbols'
path to save the spreadsheet
EXPORT_PATH = r'D:\OneDrive\geodata\rules.xlsx'
workbook = xlsxwriter.Workbook(EXPORT_PATH)
worksheet = workbook.add_worksheet()
def convert_icon(icon, size=None, fmt='PNG'):
    '''
    This function modified from:
    https://stackoverflow.com/questions/40789856/python-convert-qt-qicon-into-jpeg
    Credit: stackoverflow member ekhumoro
    '''
    if size is None:
        size = 18
    pixmap = QPixmap.fromImage(icon)
    pixmap = pixmap.scaledToWidth(size)
array = QtCore.QByteArray()
buffer = QtCore.QBuffer(array)
buffer.open(QtCore.QIODevice.WriteOnly)
pixmap.save(buffer, fmt)
buffer.close()
return array.data()


write the spreadsheet headers
worksheet.write('A1', 'symbol')
worksheet.write('B1', 'label')
worksheet.write('C1', 'rule')
worksheet.write('D1', 'count')
get layer
lyr = iface.activeLayer()
get the rule-based rendered from the layer
r = lyr.renderer()
get the root rule
root = r.rootRule()
set the spreadsheet row to enter data
sheet_row = 2
iterate through rules
for rule in root.children():
# get various attributes from rule
sym = rule.symbol()
lab = rule.label()
exp = rule.filterExpression()

# set the rule filter on the layer
lyr.setSubsetString(exp)
# count filtered features
count = lyr.featureCount()
# reset the filter
lyr.setSubsetString('')

# get rule symbol as image
img = sym.asImage(QSize(40,40))

# optionally save symbols as png
img.save(
    os.path.join(
        IMG_PATH, 
        (exp.replace('<=', 'lte')
            .replace('>=', 'gte')
            .replace('<>', 'not')
            .replace('<', 'lt')
            .replace('>', 'gt')
            .replace('=', 'eq')                
            .replace('.', '_')
            .replace('"','')
            .replace("'", '') + '.png')
    )
)

# convert QImage to QByteArray
img_data = convert_icon(img)

# convert to BytesIO object
io_data = BytesIO(img_data) 

# assign cells for data
icon_cell  = f'A{sheet_row}'
label_cell = f'B{sheet_row}'
rule_cell  = f'C{sheet_row}'
count_cell = f'D{sheet_row}'

# insert symbol and corresponding data into cells
worksheet.insert_image(icon_cell, 'image.png', {'image_data': io_data, 'x_offset': 15, 'y_offset': 3})
worksheet.write(label_cell, lab)
worksheet.write(rule_cell, exp)
worksheet.write(count_cell, count)

# set the width of specific columns to accommodate data
worksheet.set_column('B:B', 20)  
worksheet.set_column('C:C', 40)  

# go to next row of spreadsheet
sheet_row+=1


commit the changes to the workbook
workbook.close()

The spreadsheet:

The saved image files: