App Engine is a good home for web applications, and web applications are often built using a web framework.
The first public versions of App Engine supported a framework called webapp. Webapp was deprecated along with Python 2.5 in 2013, but the open source webapp2 framework is still popular for Python 2.7 apps. While users have gotten that framework to work on Python 3 App Engine](https://github.com/fili/webapp2-gae-python37), it is no longer bundled nor supported by Google Cloud, so you must migrate to frameworks that are WSGI-compliant and handle routing for Python 3.
The popular frameworks Flask and Django work well with Python 2.7 and Python 3, so a good migration strategy for existing webapp2 apps is:
-
Replace the use of webapp2 in your Python 2.7 app with Flask or Django, while remaining on App Engine for Python 2.7.
-
When the updated app is stable, migrate the code to Python 3 and deploy and test using App Engine for Python 3.
Flask is a micro-framework that is quite similar in scope to webapp2, and is a good choice for this migration task. Some of the key changes between them are shown in this table:
| Framework | webapp2 | Flask |
|---|---|---|
| import statement | import webapp2 |
from flask import Flask |
| initialization | app = webapp2.WSGIApplication([]) |
app = Flask(__name__) |
| routing | defined at initialization, as (route, Class Name) pairs, e.g., app = webapp2.WSGIapplication([ ('/', HandlerClass)]) |
decorators, e.g., @app.route('/', methods=['GET']) |
| handlers | class methods, named for HTTP verb, e.g.,class MainPage(webapp2.RequestHandler):def get(self):self.response.write('Hello, World!') |
decorated methods, any name, e.g.,@app.route('/', methods=['GET'])def hello():return 'Hello, World!' |
| "Hello, world" examples | GitHub | GitHub |
Due to the minimal nature of the webapp2 framework, migration to Flask is generally straightforward. See the "Hello, world" examples linked above to survey the process, or, if you feel like you need to practice this migration on a sample app, check out the Flask migration codelab and corresponding code samples.