auth.py: Make usage of exceptions instead of if-else-blocks

ki/auth.py

@@ -5,27 +5,40 @@ from app import app, db
 from ki.models import User, Token
 
 
+class UserWrongCredentialsException(Exception):
+    pass
+
+
+class UserAllreadyLoggedInException(Exception):
+    pass
+
+
 def auth(username, password):
     auth_file_path = app.config["KI_DATA_DIR"] + "/auth.yml"
 
     with open(auth_file_path, "r") as auth_file_stream:
+
+        try:
             users = yaml.safe_load(auth_file_stream)
 
-        if username not in users:
-            return None
+        except yaml.YAMLError:
+            print('Could not parse auth.yml.')
 
+        try:
             auth_user = users[username]
 
             if auth_user["password"] != password:
+                raise UserWrongCredentialsException
+
+        except (UserWrongCredentialsException, KeyError):
+            print('Wrong username/password combination')
             return None
 
+        else:
             user = User.query.filter(User.auth_id.__eq__(username)).first()
 
-        if user is None:
-            user = User(auth_id=username)
-            db.session.add(user)
-
             token = Token(token=str(uuid.uuid4()), user=user)
+
             db.session.add(token)
             db.session.commit()