Make stringdtype.NA a singleton so pickle doesn't create new instances

Author: ngoldbaum

stringdtype/stringdtype/missing.py

@@ -1,4 +1,13 @@
-class NAType:
+class Singleton:
+    _instance = None
+
+    def __new__(cls):
+        if cls._instance is None:
+            cls._instance = super().__new__(cls)
+        return cls._instance
+
+
+class NAType(Singleton):
     def __repr__(self):
         return "stringdtype.NA"