PyQGIS – Updating Attribute Field with Values from List Calculation using PyQGIS

editingfields-attributesiteratorpyqgisupdate

I have a shapefile and I need to update a field with calculation results. In the calculation I'm iterating through two list with equal items as features of the shapefile. The sum of the list items gets multiplied with an integer value (constant). The field name which needs to be updated is "SH".

Here is how I tried solving it with some example data:

a = [134.24, 134.24, 134.24, 134.99]
b = [58.848430067837946, 70.46615125391358, 63.02756211443306, 60.662300968221174]
c = 0.6673779977746334

newlayer = iface.addVectorLayer(outfile_rel_revised,"", "ogr")
newlayer.setCrs(crsSrc)

with edit(newlayer):
    for rel_feature in newlayer.getFeatures():
        for i in a:
            for j in b:
                rel_feature['SH'] = ((i + j)* c)
                newlayer.updateFeature(rel_feature)
                break

If I run this, I get no error but the field is not updated. For example the calculation for feature 1 should look like this:
(134.24 + 58.848430067837946)*0.6673779977746334

The finished result should look like this:

Best Answer

Try this logic:

a = [134.24, 134.24, 134.24, 134.99]
b = [58.848430067837946, 70.46615125391358, 63.02756211443306, 60.662300968221174]
c = 0.6673779977746334

newlayer = iface.activeLayer()

with edit(newlayer):
    for i,rel_feature in enumerate(newlayer.getFeatures()):
        rel_feature['SH'] = round((a[i] + b[i])*c,2)
        newlayer.updateFeature(rel_feature)

Make sure your file is not write protected and the field (SH) is of type real.

This should actually work. If for some reason it does not update the file, you can give changeAttributeValue() method a try:

a = [134.24, 134.24, 134.24, 134.99]
b = [58.848430067837946, 70.46615125391358, 63.02756211443306, 60.662300968221174]
c = 0.6673779977746334

newlayer = iface.activeLayer()

with edit(newlayer):
    for i,rel_feature in enumerate(newlayer.getFeatures()):
        result = round((a[i] + b[i])*c,2)
        newlayer.changeAttributeValue(rel_feature.id(), newlayer.fields().indexOf('SH'), result)

With this kind of logic an order by request might make sense:

a = [134.24, 134.24, 134.24, 134.99]
b = [58.848430067837946, 70.46615125391358, 63.02756211443306, 60.662300968221174]
c = 0.6673779977746334

newlayer = iface.activeLayer()

request = QgsFeatureRequest()
clause = QgsFeatureRequest.OrderByClause('id', ascending=True)
orderby = QgsFeatureRequest.OrderBy([clause])
request.setOrderBy(orderby)

with edit(newlayer):
    for i,rel_feature in enumerate(newlayer.getFeatures(request)):
        result = round((a[i] + b[i])*c,2)
        newlayer.changeAttributeValue(rel_feature.id(), newlayer.fields().indexOf('SH'), result)

Related Solutions

QGIS Shapefile – Getting List of Distinct Values from Field

Update answer (QGIS Version >= 2.14)

Since QGIS 2.14, you can use run SQL statements on any loaded vector layer using Virtual layers.

Having the layer loaded in QGIS, go to Layer > Add Layer > Add/Edit Virtual Layer;
In the Create virtual layer dialog, enter you SQL statement in the Query field. Something like:

SELECT DISTINCT city_name FROM layer_name
For geometry set No Geometry
Click Ok and a table will load in QGIS with the desired unique values.

Note: this table will be updated if new values are added to the city_name column.

Legacy answer (QGIS Version < 2.14)

You have a few choices to do what you ask.

Import your shapefile in a Spatialite or Postgis database, and then you can query your table using complete SQL statements;
Use the Dissolve tool (Vector > Geoprocessing Tools > Dissolve), to dissolve your shapefiles using the field "city_name". Although is an strange method, the dbf file of the resulting shapefile will provide the list you need;
Take a look at group stats plugin (1.6), you can use "city_name" as classification field, and press calculate. It will calculate some stats about each city, you can then copy the result and extract the city list.

I have just noticed that, in the Vector > Analysis Tools, there is a List unique Values tool that is precisely what one needs for this task. So easy... no workarounds and no need for Plugins.

ArcPy – How to Update Attributes from List of Values in Excel or CSV Table

If you need to write a lot of if statements you should consider using a dictionary instead. Code below will read a csv file and create a dictionary of names and codes and then use the dictionary in the Updatecursor for all shapefiles in a folder. Backup your data Before you try it:

import arcpy, csv

shapefile_folder = r'C:\folder'
arcpy.env.workspace = shapefile_folder
shapefile_fields_to_update = ["name","code"]

codefile = r"C:\Program123\codes.csv"

with open(codefile, 'r') as f:
    reader = csv.reader(f)
    dict = {k:v for (k,v) in reader} #Dictionary of names and codes is created here

for shapefile in arcpy.ListFeatureClasses():
    with arcpy.da.UpdateCursor(shapefile, shapefile_fields_to_update) as cursor:
        for row in cursor:
            if row[0] in dict:
                row[1]=int(dict[row[0]]) #And used here
                cursor.updateRow(row)
            else:
                print "Name {0} not found in the dictionary, no update made for this name".format(row[0])

Best Answer

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QGIS Shapefile – Getting List of Distinct Values from Field

ArcPy – How to Update Attributes from List of Values in Excel or CSV Table

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