enhance code

main.py

@@ -4,18 +4,50 @@
 
 from flask import Flask
 from flask import request
+from flask import make_response
 
 from google.appengine.ext import blobstore
 from google.appengine.api import images
 
+JSON_MIME_TYPE = 'application/json'
+
 app = Flask(__name__)
 
 @app.route('/image-url', methods=['GET'])
 def image_url():
 	bucket = request.args.get('bucket')
 	image = request.args.get('image')
 
-	filename = (bucket + "/" + image)
-	servingImage = images.get_serving_url(None, filename='/gs/' + filename)
+	if not all([bucket, image]):
+		error = json.dumps({'error': 'Missing `bucket` or `image` parameter.'})
+		return json_response(error, 422)
+
+	filepath = (bucket + "/" + image)
+
+	try:
+		servingImage = images.get_serving_url(None, filename='/gs/' + filepath)
+	except images.AccessDeniedError:
+		error = json.dumps({'error': 'Ensure the GAE service account has access to the object in Google Cloud Storage.'})
+		return json_response(error, 401)
+	except images.ObjectNotFoundError:
+		error = json.dumps({'error': 'The object was not found.'})
+		return json_response(error, 404)
+	except images.TransformationError:
+		# A TransformationError may happen in several scenarios - if
+		# the file is simply too large for the images service to
+		# handle, if the image service doesn't have access to the file,
+		# or if the file was already uploaded to the image service by
+		# another App Engine app. For the latter case, we can try to
+		# work around that by copying the file and re-uploading it to
+		# the image service.
+		error = json.dumps({'error': 'There was a problem transforming the image. Ensure the GAE service account has access to the object in Google Cloud Storage.'})
+		return json_response(error, 400)
+
+	return json_response(json.dumps({'image_url': servingImage}))
+
+def json_response(data='', status=200, headers=None):
+	headers = headers or {}
+	if 'Content-Type' not in headers:
+		headers['Content-Type'] = JSON_MIME_TYPE
 
-	return(servingImage)
+	return make_response(data, status, headers)