QGIS – How to Update Attribute Table in Multiple Layers Using Field Calculator

Question

I have dozens of layers in QGIS and all these layers have attribute field that I would like to update. I don't want to create new temporal layers with updated fields, rather just update the current layer attribute tables. And I wouldn't want to separately update each layer. I tried field calculator but it creates a new temporal layer. Is there any other solutions that could work? I'm using QGIS 3.22.

Answer 1

You can use pyqgis with re (to find the date) and datetime (to format the date) modules.

The layers must have a 8 digit date somewhere in the layername, formatted like 20211221 (YYYYMMDD). Try it on a subset of layers and backup your data first.

import re, datetime
fieldname = 'modified' #A string field that must exist in each table, change to match your fieldname

patt = r'\d{8}' #Search layername for 8 consecutive digits
for layer in QgsProject.instance().mapLayers().values(): #For each layer added to QGIS
    attrMap = {} #A dictionary to hold feature id: fieldindex:new value
    found = re.search(patt, layer.name()) #Find the 8 digits in a row
    fieldindex = layer.fields().indexFromName(fieldname) #Find the index of "modified" field
    if found:
        oldval = datetime.datetime.strptime(found.group(), '%Y%m%d') #datetime.datetime(2021, 11, 12, 0, 0) #Create a datetime object
        newval = datetime.datetime.strftime(oldval, '%m%d%Y') #Convert to a string in the format you want
        for f in layer.getFeatures():
            attrMap[f.id()] = {fieldindex:newval}
        layer.dataProvider().changeAttributeValues(attrMap)