How to edit shapefile name written to attribute table in QGIS?

Question

I have adapted some QGIS pythonscript code from a previous question to assign the filename of a shapefile to that shapefiles attribute table as a prelude to joining multiple shapefiles together (the SAGA plugin isn't working for me). It looks like this:

##Add Filename to Table=name
##Input_Folder=Folder
##nomodeler

import os
import glob
from os.path import expanduser
home = expanduser("~")

# Input Folder
#path_dir = home + "/Documents/GIS Data/TEMP/" +  Folder_Name 
path_dir = Input_Folder #This takes the input from the parameter

#Check that the folder "results" exists and if not create it
if not os.path.exists(path_dir + "/Results"):
    os.makedirs(path_dir + "/Results")
    progress.setInfo("Setting save location")
# Set the save location to the folder "results"
    path_res = path_dir + "/Results"

progress.setInfo("Starting file processing")

def run():

#   Set working directory and search for all shapefiles then run algorithm
    os.chdir(path_dir )
    for fname in glob.glob("*.shp"): 

        formula = "'" + fname + "'"
        processing.runalg("qgis:fieldcalculator", fname, 'Filename', 2, 50, 0, True, formula , path_res  + "/"+ fname)

#   Paths of the shapefiles in the Result folder with list comprehension
    output = [shp for shp in glob.glob(path_res + "*.shp")]

run()    

progress.setInfo("Finished")

Currently it adds the whole filename to the attribute table. How would I alter the formula to trim the filename, for example to extract the OS tile code of a shapefile called "HP_SomeText.shp"?

Answer 1

You can use the following line to get the first two characters of fname and have it inserted into your Attribute Table. Replace your current formula:

formula = "'" + fname + "'"

with this:

formula = "left('" + fname + "',2)"

Hope this helps!