geosplit.py

import sys
import json
import collections

FILENAME = sys.argv[1]
KEY = sys.argv[2]

with open(FILENAME) as f:
  data = json.load(f)

split = collections.defaultdict(list)
for feature in data['features']:
  split[feature['properties'][KEY]].append(feature)

for key, features in split.items():
  with open('split-%s.geojson'%key, 'w') as f:
    json.dump({
      'type': 'FeatureCollection',
      'features': features
    }, f)