QGIS Python script for zonal statistics not running

Question

I got this Python script from this previous post. I've edited it for my project but nothing happens when I execute it. I downloaded the zonal statistics plugin, and usually when I run it, it adds a new field to the shapefile with the statistics I need, for example minimum. Since I need to do it multiple times, I was hoping to use the script. I tried the batch execute, but for GDAL it doesn't allow you to select one statistic, and Saga always gives an error somewhere.

Here's the script I use:

from PyQt4.QtGui import *
from PyQt4.QtCore import *
from qgis.core import *
from qgis.utils import iface
from qgis.analysis import QgsZonalStatistics


#specify polygon shapefile vector
polygonLayer = QgsVectorLayer('D:/all_structures_tez.shp', 'zonepolygons', "ogr") 

# specify raster filename
rasterFilePath = 'D:/slow.tif'

# usage - QgsZonalStatistics (QgsVectorLayer *polygonLayer, const QString &rasterFile, const QString &attributePrefix="", int rasterBand=1)
zoneStat = QgsZonalStatistics (polygonLayer, rasterFilePath, 'pre-', 1, QgsZonalStatistics.Min)
zoneStat.calculateStatistics(None)

QGIS ver 2.18.3

Answer 1

I performed a quick test and it seems that you need to turn on the edit mode for obtaining the desired result.

If you want to use Python for this, try to use the following code:

polygonLayer.startEditing() 
# usage - QgsZonalStatistics (QgsVectorLayer *polygonLayer, const QString &rasterFile, const QString &attributePrefix="", int rasterBand=1)
zoneStat = QgsZonalStatistics (polygonLayer, rasterFilePath, 'pre-', 1, QgsZonalStatistics.Min)
zoneStat.calculateStatistics(None)
polygonLayer.commitChanges()