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"
    ]
   }
  ],

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]
+    REJECTED = 3