Undefined should not be an exception (#187)

Author: Cito

README.md

@@ -10,7 +10,7 @@ a query language for APIs created by Facebook.
 ![Lint Status](https://github.com/graphql-python/graphql-core/actions/workflows/lint.yml/badge.svg)
 [![Code Style](https://img.shields.io/badge/code%20style-black-000000.svg)](https://github.com/ambv/black)
 
-An extensive test suite with over 2300 unit tests and 100% coverage comprises a
+An extensive test suite with over 2400 unit tests and 100% coverage comprises a
 replication of the complete test suite of GraphQL.js, making sure this port is
 reliable and compatible with GraphQL.js.
 

src/graphql/pyutils/undefined.py

@@ -7,7 +7,7 @@
 __all__ = ["Undefined", "UndefinedType"]
 
 
-class UndefinedType(ValueError):
+class UndefinedType:
     """Auxiliary class for creating the Undefined singleton."""
 
     _instance: Optional[UndefinedType] = None
@@ -34,7 +34,7 @@ def __bool__(self) -> bool:
         return False
 
     def __eq__(self, other: Any) -> bool:
-        return other is Undefined
+        return other is Undefined or other is None
 
     def __ne__(self, other: Any) -> bool:
         return not self == other

tests/pyutils/test_undefined.py

@@ -21,16 +21,23 @@ def is_hashable():
     def as_bool_is_false():
         assert bool(Undefined) is False
 
-    def only_equal_to_itself():
+    def only_equal_to_itself_and_none():
+        # because we want it to behave similarly to JavaScript
         assert Undefined == Undefined
         assert not Undefined != Undefined
         none_object = None
-        assert Undefined != none_object
-        assert not Undefined == none_object
+        assert Undefined == none_object
+        assert not Undefined != none_object
         false_object = False
         assert Undefined != false_object
         assert not Undefined == false_object
 
+    def should_not_be_an_exception():
+        # because we want to create similar code to JavaScript where
+        # undefined return values are different from exceptions
+        # (for instance, this is used in the completeValue function)
+        assert not isinstance(Undefined, Exception)
+
     def cannot_be_redefined():
         with warns(RuntimeWarning, match="Redefinition of 'Undefined'"):
             redefined_undefined = UndefinedType()