Simplify online chat solution

solutions/object_oriented_design/online_chat/online_chat.ipynb

@@ -69,8 +69,6 @@
     "\n",
     "class UserService(object):\n",
     "\n",
-    "    __metaclass__ = Singleton\n",
-    "\n",
     "    def __init__(self):\n",
     "        self.users_by_id = {}  # key: user id, value: User\n",
     "\n",
@@ -145,16 +143,7 @@
     "    UNREAD = 0\n",
     "    READ = 1\n",
     "    ACCEPTED = 2\n",
-    "    REJECTED = 3\n",
-    "\n",
-    "\n",
-    "class Singleton(type):\n",
-    "\n",
-    "    _instances = {}\n",
-    "    def __call__(cls, *args, **kwargs):\n",
-    "        if cls not in cls._instances:\n",
-    "            cls._instances[cls] = super(Singleton, cls).__call__(*args, **kwargs)\n",
-    "        return cls._instances[cls]"
+    "    REJECTED = 3"
    ]
   }
  ],
 %% Cell type:markdown id: tags:
 
 This notebook was prepared by [Donne Martin](https://github.com/donnemartin). Source and license info is on [GitHub](https://github.com/donnemartin/system-design-primer-primer).
 
 %% Cell type:markdown id: tags:
 
 # Design an online chat
 
 %% Cell type:markdown id: tags:
 
 ## Constraints and assumptions
 
 * Assume we'll focus on the following workflows:
     * Text conversations only
     * Users
         * Add a user
         * Remove a user
         * Update a user
         * Add to a user's friends list
             * Add friend request
                 * Approve friend request
                 * Reject friend request
         * Remove from a user's friends list
     * Create a group chat
         * Invite friends to a group chat
         * Post a message to a group chat
     * Private 1-1 chat
         * Invite a friend to a private chat
         * Post a meesage to a private chat
 * No need to worry about scaling initially
 
 %% Cell type:markdown id: tags:
 
 ## Solution
 
 %% Cell type:code id: tags:
 
 ``` python
 %%writefile online_chat.py
 from abc import ABCMeta
 
 
 class UserService(object):
 
-    __metaclass__ = Singleton
-
     def __init__(self):
         self.users_by_id = {}  # key: user id, value: User
 
     def add_user(self, user_id, name, pass_hash):  # ...
     def remove_user(self, user_id):  # ...
     def add_friend_request(self, from_user_id, to_user_id):  # ...
     def approve_friend_request(self, from_user_id, to_user_id):  # ...
     def reject_friend_request(self, from_user_id, to_user_id):  # ...
 
 
 class User(object):
 
     def __init__(self, user_id, name, pass_hash):
         self.user_id = user_id
         self.name = name
         self.pass_hash = pass_hash
         self.friends_by_id = {}  # key: friend id, value: User
         self.friend_ids_to_private_chats = {}  # key: friend id, value: private chats
         self.group_chats_by_id = {}  # key: chat id, value: GroupChat
         self.received_friend_requests_by_friend_id = {}  # key: friend id, value: AddRequest
         self.sent_friend_requests_by_friend_id = {}  # key: friend id, value: AddRequest
 
     def message_user(self, friend_id, message):  # ...
     def message_group(self, group_id, message):  # ...
     def send_friend_request(self, friend_id):  # ...
     def receive_friend_request(self, friend_id):  # ...
     def approve_friend_request(self, friend_id):  # ...
     def reject_friend_request(self, friend_id):  # ...
 
 
 class Chat(metaclass=ABCMeta):
 
     def __init__(self, chat_id):
         self.chat_id = chat_id
         self.users = []
         self.messages = []
 
 
 class PrivateChat(Chat):
 
     def __init__(self, first_user, second_user):
         super(PrivateChat, self).__init__()
         self.users.append(first_user)
         self.users.append(second_user)
 
 
 class GroupChat(Chat):
 
     def add_user(self, user):  # ...
     def remove_user(self, user):  # ...
 
 
 class Message(object):
 
     def __init__(self, message_id, message, timestamp):
         self.message_id = message_id
         self.message = message
         self.timestamp = timestamp
 
 
 class AddRequest(object):
 
     def __init__(self, from_user_id, to_user_id, request_status, timestamp):
         self.from_user_id = from_user_id
         self.to_user_id = to_user_id
         self.request_status = request_status
         self.timestamp = timestamp
 
 
 class RequestStatus(Enum):
 
     UNREAD = 0
     READ = 1
     ACCEPTED = 2
     REJECTED = 3
-
-
-class Singleton(type):
-
-    _instances = {}
-    def __call__(cls, *args, **kwargs):
-        if cls not in cls._instances:
-            cls._instances[cls] = super(Singleton, cls).__call__(*args, **kwargs)
-        return cls._instances[cls]
 ```
 
 %%%% Output: stream
 
     Overwriting online_chat.py

solutions/object_oriented_design/online_chat/online_chat.py

@@ -3,8 +3,6 @@ from abc import ABCMeta
 
 class UserService(object):
 
-    __metaclass__ = Singleton
-
     def __init__(self):
         self.users_by_id = {}  # key: user id, value: User
 
@@ -38,8 +36,8 @@ class User(object):
 class Chat(metaclass=ABCMeta):
 
     def __init__(self, chat_id):
-        self.users = []
         self.chat_id = chat_id
+        self.users = []
         self.messages = []
 
 
@@ -79,13 +77,4 @@ class RequestStatus(Enum):
     UNREAD = 0
     READ = 1
     ACCEPTED = 2
-    REJECTED = 3
-
-
-class Singleton(type):
-
-    _instances = {}
-    def __call__(cls, *args, **kwargs):
-        if cls not in cls._instances:
-            cls._instances[cls] = super(Singleton, cls).__call__(*args, **kwargs)
-        return cls._instances[cls]
\ No newline at end of file
+    REJECTED = 3
\ No newline at end of file