Use locking when creating singleton

app/classes.py

@@ -1,9 +1,10 @@
 # Standard library imports
 from __future__ import annotations
 
-from dataclasses import dataclass, field
+from dataclasses import dataclass
 from enum import auto, Enum
 import functools
+import threading
 from typing import NamedTuple
 
 # Third party imports
@@ -34,12 +35,18 @@ def singleton(cls):
     """
     Make a class a Singleton class (see
     https://realpython.com/primer-on-python-decorators/#creating-singletons)
+
+    Added locking.
     """
 
+    lock = threading.Lock()
+
     @functools.wraps(cls)
     def wrapper_singleton(*args, **kwargs):
-        if not wrapper_singleton.instance:
-            wrapper_singleton.instance = cls(*args, **kwargs)
+        if wrapper_singleton.instance is None:
+            with lock:
+                if wrapper_singleton.instance is None:  # Check still None
+                    wrapper_singleton.instance = cls(*args, **kwargs)
         return wrapper_singleton.instance
 
     wrapper_singleton.instance = None