I am creating a QGIS plugin a part of which is like the SAGA Rasters sum tool. The user interface will have a widget to which the user will load the raster layers. I am trying to get the pixel-wise sum of values of the loaded rasters.

Like if the user loads 3 rasters R1, R2, R3 then for a pixel Pij, having values 5, 7 and 6 in R1, R2 and R3 respectively, in the new raster Pij = R1ij+R2ij+R3ij = 5+7+6 = 18

I am stuck up with both - creating the UI and the calculations.

So the first part of my question is about creating a QListWIdget and dragging and dropping the layers into the widget and using these layers for additions. The best I could get to was to add the QListWidget into the UI using QtCreator. Couldn't get to add and call layers from the QListWidget.

Here is the code I used with a push button to add layers in the QListWidget, but the layers add as a single row and I don't know how to use these for the actual calculations.

def add_rasters(self):
    name=QFileDialog.getOpenFileNames(self.dlg, 'Select Rasters', '','*.tif')
    name_str = str(name)
    list = []


Using the above code the ListWidget gets filled up like: (['C:/QGIS_plugin/rstr1.tif', 'C:/QGIS_plugin/rstr2.tif', 'C:/QGIS_plugin/rstr3.tif'], '*.tif')

Next I am querying about the calculations:

I can do the addition calculation by manually adding up all the layers in the list; but I could not figure out a way to do the calculation automatically for n-number of the layers loaded in the widget.

Below is my code for addition of layers already loaded in the QGIS map project.

import processing
from qgis.core import *
from PyQt5.QtCore import *
from qgis.analysis import QgsRasterCalculator, QgsRasterCalculatorEntry

entries = []

myList=[layer for layer in QgsProject.instance().mapLayers().values()]

for i in range(0, len(myList)):
     raster = myList[i]
     #readRst = QgsProcessingUtils.mapLayerFromString(raster)
     ras1 = QgsRasterCalculatorEntry()
     ras1.raster = raster
     ras1.ref = "ext_lyr" + str(i+1) +"@1"
     ras1.bandNumber = 1

formula = '(' + entries[0].ref + ' + ' + entries[1].ref +  ')'

output = r"C:/QGIS_plugin/rstrs_sum.tif"
calc = QgsRasterCalculator(formula, output, 'GTiff', raster.extent(), raster.width(), raster.height(), entries)

I am afraid that I am unable to frame this question properly (mixing two questions), but I am myself not sure how this would work best. I have gone through several posts on this site and now I am completely lost!

I am quite new to python programming and QGIS, hence need help with the simplest things also. Hoping to get some help here with this.

1 Answer 1


Drag & Drop

It is fairly easy to implement a QListWidget which accepts drops from the system file explorer, or the QGIS Browser.

from pathlib import Path
from PyQt5.QtWidgets import QListWidget

class DropList(QListWidget):

    def __init__(self, parent=None):
        self.items = set()  # Keep a set of filepaths

    def addItem(self, item):  # Ensure that files are only added once
        if item in self.items:

    def dragEnterEvent(self, event):

    def dragMoveEvent(self, event):

    def dragLeaveEvent(self, event):

    def dropEvent(self, event):
        mimeData = event.mimeData()

        # Drop from QGIS Browser, or from system file explorer
        if mimeData.hasUrls():
            for url in mimeData.urls():
                filepath = Path(url.toLocalFile())
                if filepath.suffix.lower() in (".tif", ".tiff"):

It is a bit more complicated to handle drops from the QGIS Layer panel. You have to get the qgis-specific mimeData, and recreate a QgsLayerTreeNode from it. Once you got the node, check if it matches a raster layer, and add its source to your list. dropEvent becomes:

def dropEvent(self, event):
    mimeData = event.mimeData()
    # Drop from QGIS Browser, or from system file explorer
    if mimeData.hasUrls():
        for url in mimeData.urls():
            filepath = Path(url.toLocalFile())
            if filepath.suffix.lower() in (".tif", ".tiff"):

    # Drop from QGIS Layers panel
        if not mimeData.hasFormat(  "application/qgis.layertreemodeldata"):
        encodedData = mimeData.data( "application/qgis.layertreemodeldata" )
        doc = QDomDocument()
        if not doc.setContent( encodedData ):
        rootElem = doc.documentElement();
        if rootElem.tagName() != "layer_tree_model_data":

        elem = rootElem.firstChildElement()
        while not elem.isNull():
            node = QgsLayerTreeNode.readXml( elem, QgsProject.instance() )
            if (node and QgsLayerTree.isLayer(node) and isinstance(node.layer(), QgsRasterLayer) and node.layer().source().lower().endswith(".tif")):
            elem = elem.nextSiblingElement()

Sum n rasters

Here is a simple function which takes a list of filepath, generates QRasterLayers from them, and sum them.

def sum_rasters(paths, outputname):

    # Compute entries from filenames
    layers =  [QgsRasterLayer(str(path)) for path in paths]
    entries = []
    for i, layer in enumerate(layers, 1):
        entry = QgsRasterCalculatorEntry()
        entry.ref = f"layer_{i}@1"
        entry.raster = layer
        entry.bandNumber = 1

    # Create operation string
    operation = "+".join(entry.ref for entry in entries)

    # Actual computation
    calc = QgsRasterCalculator(
        layers[0].extent(),  # Use first layer as ref for extent & size
    return calc.processCalculation()

You may want to load the generated raster into your project afterward.

output_raster = QgsRasterLayer(outputname)
QgsProject.instance().addMapLayer(output_raster )

Binding it together

Here is a simple dialog which uses the previously defined widgets and functions.

class ComputationDialog(QDialog):

    def __init__(self, parent=None):
        layout = QVBoxLayout(self)
        self.dropList = DropList(self)

        # Optional Toolbar: add raster from file explorer, and remove raster from dialog
        self.toolbar = QToolBar(self)
        addFromFileAction = self.toolbar.addAction(
            QIcon(":/images/themes/default/mActionAdd.svg"),  # qgis icon
            "Add new raster"
        removeSelectedAction = self.toolbar.addAction(
            QIcon(":/images/themes/default/mActionRemove.svg"), # qgis icon
            "Remove selected raster"

        self.sumButton = QPushButton("Sum")


    def onSum(self):
        sum_rasters(self.dropList.items, "C:/Temp/Output.tif")

In order to use the toolbar, you have to add the following method to yout DropList Class

def addFromFile(self):
    names, _ =  QFileDialog.getOpenFileNames(self, 'Select Rasters', '','*.tif')
    for name in names:

def removeSelected(self):
    for item in self.selectedItems():
  • This is amazing!!! Thank you so very much! I could load the layers in the widget using the addFromFile() method and do the calculations using sum_rasters(). I am still trying the drag and drop option. All this while I was trying to understand and implement this code. Hence the delay in accepting this answer. But really thanks a lot!
    – D_lind
    Commented Dec 8, 2019 at 19:04

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