How to use Python main() function in GAE (Google App Engine)?

Question

I\'d like to use a main() function in my GAE code
(note: the code below is just a minimal demonstration for a much larger program, hence the need for a

Answer 1

https://webapp2.readthedocs.io/en/latest/tutorials/quickstart.nogae.html describes how to use GAE apps outside of the GAE environment:

from myapp import app

def main():
    from paste import httpserver
    httpserver.serve(app, host='localhost', port='8070')

if __name__ == '__main__':
    main()

I did that in order to debug my app using the Pycharm IDE instead of from the dev_appserver command window; it works fine. I compare results with dev_appserver running on port 8080 and the debugger running on 8070.

Answer 2

GAE apps are not designed to be standalone apps, a main() function doesn't make a lot of sense for them.

Basically GAE apps are really just collections of handler code and rules/configurations designed to extend and customize the behaviour of the generic GAE infra/sandbox code so that it behaves your app. You can see that from your backtrace - other code is invoking your handler code (and the stack before reaching your code can be a lot deeper than that).

In your particular case the app variable must be a global in main.py to match the script: main.app config line in the app.yaml config file. This is what the traceback is about.

As for organizing the code for huge apps, there are other ways of doing it:

• splitting the app in multiple modules/services, each with their own app.yaml config file. For example: Can a default service/module in a Google App Engine app be a sibling of a non-default one in terms of folder structure?

• splitting a service/module into multiple "scripts" - primary entry points into the app.yaml file similar to your main.py file, each with their own app config` - which really are just mappers between routes and handlers. For example: App Engine throws 404 Not Found for any path but root

• splitting the handlers for one app mapper into multiple files, using webapp2's lazy loaded handler technique. Examples:

  • App Engine: Few big scripts or many small ones?
  • What determines start up time of dynamic instance and can it vary between weeks if code is same

In an extreme case a main.py file could contain just the app variable - that is really the only requirement.

Answer 3

Seems that the solution was quite simple (it kept eluding me because it hid in plain sight): __name__ is main and not __main__!

In short, the following code utilises a main() as expected:

# with main()
import webapp2

class GetHandler(webapp2.RequestHandler):
    def get(self):
        self.response.headers['Content-Type'] = 'text/plain'
        self.response.write('in GET')

class SetHandler(webapp2.RequestHandler):
    def get(self):
        self.response.headers['Content-Type'] = 'text/plain'
        self.response.write('in SET')

def main():
    global app
    app = webapp2.WSGIApplication([
        ('/get',    GetHandler),
        ('/set',    SetHandler),
    ], debug=True)

# Note that it's 'main' and not '__main__'
if __name__ == 'main':
    main()