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

ki/auth.py

@@ -5,28 +5,41 @@ 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:
-        users = yaml.safe_load(auth_file_stream)
 
-        if username not in users:
+        try:
+            users = yaml.safe_load(auth_file_stream)
+
+        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
 
-        auth_user = users[username]
+        else:
+            user = User.query.filter(User.auth_id.__eq__(username)).first()
 
-        if auth_user["password"] != password:
-            return None
+            token = Token(token=str(uuid.uuid4()), user=user)
 
-        user = User.query.filter(User.auth_id.__eq__(username)).first()
+            db.session.add(token)
+            db.session.commit()
 
-        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()
-
-    return token
+            return token