Use a static secret key when running on App Engine, otherwise generate a random key.

main.py

@@ -12,18 +12,22 @@ from opencensus.trace.tracer import Tracer
 from opencensus.ext.flask.flask_middleware import FlaskMiddleware
 from opencensus.trace.samplers import ProbabilitySampler
 
+app = Flask(__name__)
 
-depl_env = os.getenv("DEPLOYMENT_ENVIRONMENT")
-
-if depl_env == 'production' or depl_env == 'staging':
+if os.getenv('GAE_ENV', '').startswith('standard'):
     client = google.cloud.logging.Client()
     handler = CloudLoggingHandler(client)
     setup_logging(handler)
 
 logging.getLogger().setLevel(logging.DEBUG)
 
-app = Flask(__name__)
-app.config.from_pyfile('config/config.py')
+if os.getenv('GAE_ENV', '').startswith('standard'):
+    logging.info("Loading app configuration from file")
+    app.config.from_pyfile('config/config.py')
+
+if not app.secret_key:
+    logging.info("Generating a random secret key")
+    app.secret_key = os.urandom(16)
 
 def main():
     sde = StackdriverExporter(