💡 style: add some comment

Author: InnoFang

code_counter/tools/singleton.py

@@ -1,10 +1,48 @@
 #!/usr/bin/env python3
 # -*- coding: utf-8  -*-
 
+from typing import Type, Any, Dict
+
+
 class SingletonMeta(type):
-    _instance = {}
+    """
+    A metaclass for implementing the Singleton design pattern.
+
+    This metaclass ensures that a class has only one instance and provides a global point of
+    access to that instance.
+
+    Usage
+    -----
+    ```
+    class MyClass(metaclass=SingletonMeta):
+        pass
+    ```
+
+    Now, `MyClass` will have only one instance, and subsequent calls to `MyClass()` will return
+    the existing instance.
+    """
+    _instance: Dict[Type, Any] = {}
 
     def __call__(cls, *args, **kwargs):
+        """
+        Call method for creating an instance of the class.
+
+        Parameter
+        ---------
+        cls: Type
+            The class to create an instance of.
+
+        *args
+            Positional arguments to be passed to the class constructor.
+
+        **kwargs
+            Keyword arguments to be passed to the class constructor.
+
+        Returns
+        -------
+        Any
+            The instance of the class.
+        """
         if cls not in cls._instance:
             cls._instance[cls] = super(SingletonMeta, cls).__call__(*args, **kwargs)
         return cls._instance[cls]