Looping GRASS "v.extract" in python

Question

I have a multipolygon shapefile representing different cities. I am trying to loop the "v.extract" GRASS command through the different polygons via the Python interface. This is the code I am using:

    for index, row in df.iterrows():
        city_code = (row['city_ID'])
        grass.run_command('v.extract', overwrite=True, input='six_cities', output= 'out' + city_code,
                      where= 'city_ID' + "=" + city_code)

Where df is the df version of the attribute table. However, I always get the module error. I believe this is due to the "where" condition, as I tried to do it without the loop and it works (code below):

grass.run_command('v.extract', overwrite=True, input='six_cities', output= 'out' + city_code,
                      where= " 'city_ID' = 'CH_CH6421' ")

It seems that in order to run, the "where" condition needs to be within quotes. However, I cannot run the loop within quotes, because in this way python recognizes the city_code keyword needed for the loop as a string.

Answer 1

If you prepare your where clause first, it should work (I use f-strings below):

Edited Sorry for the mistake. If you enclose the where clause in double quotes, it should work. (tested here, this time).

  for index, row in df.iterrows():
        city_code = (row['city_ID'])
        expr = f'"city_ID={city_code}"'
        out = f"out_{city_code}"
        grass.run_command('v.extract', overwrite=True, input='six_cities', output=out, where=expr)

Give this a try, and post back.