Question

I’m attempting my first google app engine project – a simple player stats database for a sports team I’m involved with. Given this model:

class Player(db.Model):
    """ Represents a player in the club. """

    first_name = db.StringProperty()
    surname = db.StringProperty()
    gender = db.StringProperty()

I want to make a basic web interface for creating and modifying players. My code structure looks something like this:

class PlayersPage(webapp.RequestHandler):
    def get(self):
        # Get all the current players, and store the list.
        # We need to store the list so that we can update
        # if necessary in post().
        self.shown_players = list(Player.all())

        # omitted: html-building using django template

This code produces a very basic HTML page consisting of a form and a table. The table has one row for each Player, looking like something like this:

  <tr>
    <td><input type=text name="first_name0" value="Test"></td>
    <td><input type=text name="surname0" value="Guy"></td>
    <td><select name="gender0">
        <option value="Male" selected>Male</option>
        <option value="Female" >Female</option>
      </select></td>
  </tr>
  <!-- next row: first_name1, etc. -->

My idea is that I would store the Player instances that I used in self.shown_players, so that I could later update Players if necessary in my post() method (of the same class) by doing:

def post(self):
    # some code skipped

    for i, player in enumerate(self.shown_players):
        fn = self.request.get('first_name'+str(i)).strip()
        sn = self.request.get('surname'+str(i)).strip()
        gd = self.request.get('gender'+str(i)).strip()
        if any([fn != player.first_name,
                sn != player.surname,
                gd != player.gender]):
            player.first_name = fn
            player.surname = sn
            player.gender = gd
            player.put()

However, this doesn’t work because self.shown_players does not exist when the post() method is called. I guess the app engine creates a new instance of the class every time the page is accessed.

I experimented with the same idea but putting shown_players at the class or module level (and calling it global) but this didn’t work for reasons that I cannot divine.
For example:

shown_players = []
class PlayersPage(webapp.RequestHandler):
    def get(self):
        # Get all the current players, and store the list.
        # We need to store the list so that we can update
        # if necessary in post().
        global shown_players
        shown_players[:] = list(Player.all())

shown_players appears to have the right value within get() because the HTML generates correctly, but it is empty within post().

What should I do?

EDIT: Thanks, all. The answer (“Just retrieve the players again!”) should have been obvious 🙂 Maybe I’ll look at memcache one day, but I’m not expecting the player list to get beyond 30 in the near future..

Answer 1

On each request you are working on a new instance of the same class. That’s why you can’t create a varable in get() and use its value in post(). What you could do is either retrieve the values again in your post()-method or store the data in the memcache.

Refer to the documentation of memcache here:

http://code.google.com/intl/de-DE/appengine/docs/python/memcache/