import csv
from collections import defaultdict

by_country = defaultdict(lambda: [])

with open("worldcities.csv") as infile:
  reader = csv.reader(infile, delimiter=",", quotechar='"')
  next(reader) # skip header
  for _, name, lat, lng, _, iso2, *_ in reader:
    by_country[iso2].append((iso2.lower(), name, lat, lng))

with open("urban-centers.csv", "w") as outfile:
  for k, v in by_country.items():
    csv.writer(outfile, delimiter=",", quotechar='"').writerows(v)