hello.py

import os
import json
import StringIO
import shapely
from shapely.geometry import shape, mapping
from flask import Flask, request, send_file, jsonify, render_template
from werkzeug.utils import secure_filename

ALLOWED_EXTENSIONS = set(['js', 'json', 'geojson'])

app = Flask(__name__)

def allowed_file(filename):
  return '.' in filename and filename.rsplit('.', 1)[1] in ALLOWED_EXTENSIONS

def get_centroids(polygons_json):

  feature_collection = {
    'type': 'FeatureCollection',
    'features': []
  }

  for feature in json.loads(polygons_json)['features']:
    feature_geom = shape(feature['geometry'])
    feature_centroid = feature_geom.centroid
    centroid = mapping(feature_centroid)
    feature['geometry'] = centroid
    feature_collection['features'].append(feature)

  return feature_collection

@app.route('/', methods=['GET', 'POST'])
def operation():
  if request.method == 'POST':
    file = request.files['file']
    
    if file and allowed_file(file.filename):

      filename = secure_filename(file.filename)
      file_type = filename.rsplit('.', 1)[1]
      file_title = filename.rsplit('.', 1)[0]

      polygons_json = file.read()
      
      strIO = StringIO.StringIO()
      strIO.write(json.dumps(get_centroids(polygons_json)))
      strIO.seek(0)

      centroids_filename = file_title + "_centroids.geojson"

      return send_file(strIO, attachment_filename=centroids_filename, as_attachment=True)

  return render_template('index.html')

@app.route('/centroids', methods=['POST'])
def api_centroids():
  return jsonify(get_centroids(request.data))

if __name__ == '__main__':
    app.run(debug=True)