Python error 'NoneType' object has no attribute '<....>'

Question

I'm having trouble with the following code:

import tweepy
from tweet import TweetBuilder
from libs.session import Session

class GameHandler:

    open_sessions = []


    def get_session(self, sessionname):
        for session in GameHandler.open_sessions:
            #FOLLOWING STATEMENT GOES WRONG
            if session.roomname == sessionname:
                return session
        return None

    def session_create(self, sessionname, owner_id, owner_name):
        new = Session(sessionname, owner_id, owner_name).add_player(owner_id, owner_name)
        GameHandler.open_sessions.append(new)
        return TweetBuilder.new_session(sessionname, owner_name)

    def session_join(self, sessionname, player_id, player_name):
        session = self.get_session(sessionname)
        if session != None:
            session.add_player(player_id, player_name)
            return TweetBuilder.join_session(session, player_name)
        return ""

Also part of the Session class:

class Session:

    def __init__(self, name, owner_id, owner_name):
         #keep track of tweets
         self.tweetid_start = None
         self.tweetid_current = None

         #game elements
         self.roomname = name
         #THIS LINE WORKS CORRECTLY
         print(self.roomname)
         self.players = []
         self.currentround = None
         self.roundnumber = 0        
         self.players.append(Player(owner_id, owner_name))

When I call session_create() everything works fine. The app runs Session.__init__(), the print statement prints self.roomname.

When I call session_join(), and session_join() calls get_session() problems arise. The for loop is supposed to iterate over the Session-array called open_sessions, but the moment it tries to access the Session-attribute called 'roomname' it gives me the following error:

'NoneType' object has no attribute 'roomname'

Why are my Session-objects suddenly NoneType?

Thanks in advance.

Answer 1

The problem is here:

    new = Session(sessionname, owner_id, owner_name).add_player(owner_id, owner_name)
    GameHandler.open_sessions.append(new)

By immediately calling add_player on the newly created Session, new is not the Session but the result of add_player, and whatever that does, it seems to return None. Thus you are adding a bunch of None objects to your open_sessions list. Use this instead:

    new = Session(sessionname, owner_id, owner_name)
    new.add_player(owner_id, owner_name)
    GameHandler.open_sessions.append(new)

Or if you want to keep it the way it was, you could change your Session class to provide kind of a "fluent interface" and have add_player (and other methods) return self:

class Session:
    ...
    def add_player(self, id_, name):
        ....
        return self