Solve issues with pickled schemas (graphql-python#173)

src/graphql/pyutils/undefined.py

@@ -1,11 +1,24 @@
-from typing import Any
+import warnings
+from typing import Any, Optional
 
 __all__ = ["Undefined", "UndefinedType"]
 
 
 class UndefinedType(ValueError):
     """Auxiliary class for creating the Undefined singleton."""
 
+    _instance: Optional["UndefinedType"] = None
+
+    def __new__(cls) -> "UndefinedType":
+        if cls._instance is None:
+            cls._instance = super().__new__(cls)
+        else:
+            warnings.warn("Redefinition of 'Undefined'", RuntimeWarning, stacklevel=2)
+        return cls._instance
+
+    def __reduce__(self) -> str:
+        return "Undefined"
+
     def __repr__(self) -> str:
         return "Undefined"
 

src/graphql/type/definition.py

@@ -228,6 +228,23 @@ class GraphQLNamedType(GraphQLType):
     ast_node: Optional[TypeDefinitionNode]
     extension_ast_nodes: Tuple[TypeExtensionNode, ...]
 
+    reserved_types: Dict[str, "GraphQLNamedType"] = {}
+
+    def __new__(cls, name: str, *_args: Any, **_kwargs: Any) -> "GraphQLNamedType":
+        if name in cls.reserved_types:
+            raise TypeError(f"Redefinition of reserved type {name!r}")
+        return super().__new__(cls)
+
+    def __reduce__(self) -> Tuple[Callable, Tuple]:
+        return self._get_instance, (self.name, tuple(self.to_kwargs().items()))
+
+    @classmethod
+    def _get_instance(cls, name: str, args: Tuple) -> "GraphQLNamedType":
+        try:
+            return cls.reserved_types[name]
+        except KeyError:
+            return cls(**dict(args))
+
     def __init__(
         self,
         name: str,