Adding attribute values from CSV to multiple shapefiles in PyQGIS

Question

I’m looking for a way to do the following in a Python script in QGIS. I have multiple shapefiles (shape_x.shp, shape_y.shp, shape_z.shp) and a CSV table which is structured like the following example:

Now I want the values value_a, value_b and value_c to be written to a new column in the respective shapefile. This is not particular a classical table join, because all the features from the shape should get the same value. So, there is no need for a common column.

The attribute table of the shape_x.shp might look like this before the data is added:

…and a knew column, based on the value of the CSV, should automatically be added, so that the attribute table of the shape_x.shp afterwards looks like this:

The different shapefiles have a varying number of features, so no matter how many features, the value from the CSV should be added as a new column to every feature.

I hope I made the problem clear. I searched for automatically adding data to shapefiles and any kind of join (like this example), but no success so far.

Answer 1

Here's a code that should achieve what you described in PyQGIS. Note that this is for QGIS 2.18:

import os
import csv
from PyQt4.QtCore import QVariant
source_dir = "C:\\GIS\\Shapefiles\\" #directory containing shapefiles and data csv

#Function to write line from CSV to layer
def addField(vl, fieldData):
    vl.startEditing()
    for i,j in enumerate(range(len(fieldData))):
        myField = QgsField( fieldData[i][0], QVariant.String,'',10,2) #Note here the field type!
        vl.dataProvider().addAttributes([myField])
        vl.updateFields()
        idx = vl.fieldNameIndex( fieldData[i][0] )
        for f in vl.getFeatures():
            f[idx] = fieldData[i][1]
            vl.updateFeature( f )
    vl.commitChanges()

#Read CSV and create Python dictionary of each entry
f = open(source_dir + "Data.csv", 'rb')
reader = csv.reader(f, delimiter = ";")
first=True
d1={}
for row in reader:
    if first==True:
        header=row
        first=False
    else:
        #print nrow
        d1[row[0]]=row
f.close()
print d1



for files in os.listdir(source_dir):
    if files.endswith(".shp"):
        # create vector layer object
        vlayer  = QgsVectorLayer(source_dir + "/" + files, files, "ogr")
        print(files)
        try:
            fieldData= zip(header,d1[files])
            addField(vlayer, fieldData)
            # add the layer to the registry
            QgsMapLayerRegistry.instance().addMapLayer(vlayer )
        except KeyError:
            print "Shapefile %s"  % (files) + " does not match CSV"

Just note that it creates all the fields as type 'String', as your some of your CSV entries contain text ('no data'). If you need that to be integer or float, you should modify your CSV and then the code to QVariant.Integer. Furthermore, the code assumes that the first line in the CSV is a header. Lastly, it doesn't matter if your there's redundant entries in the CSV (Shapefiles that don't exist in the folder), but other way around will throw the exception I created.