Add forward compatible alias 'Undefined' for 'INVALID' (#77)

Author: Cito

README.md

@@ -15,8 +15,7 @@ a query language for APIs created by Facebook.
 The current version 3.0.2 of GraphQL-core is up-to-date
 with GraphQL.js version 14.6.0.
 
-All parts of the API are covered by an extensive test suite
-of currently 1991 unit tests.
+All parts of the API are covered by an extensive test suite of nearly 2000 unit tests.
 
 
 ## Documentation

src/graphql/__init__.py

@@ -328,7 +328,7 @@
     located_error,
     format_error,
     print_error,
-    INVALID,
+    INVALID,  # deprecated in the next minor release
 )
 
 # Utilities for operating on GraphQL type schema and parsed sources.
@@ -399,6 +399,8 @@
     find_dangerous_changes,
 )
 
+Undefined = INVALID  # forward-compatible alias
+
 # The GraphQL-core version info.
 __version__ = version
 __version_info__ = version_info
@@ -688,4 +690,5 @@
     "BreakingChangeType",
     "DangerousChange",
     "DangerousChangeType",
+    "Undefined",
 ]

src/graphql/error/invalid.py

@@ -30,4 +30,7 @@ def __ne__(self, other):
 
 This singleton object is used to describe invalid or undefined values.
 It corresponds to the ``undefined`` value in GraphQL.js.
+
+Note: This will be considered deprecated in the next minor release.
+Please use ``graphql.Undefined`` instead to create forward compatible code.
 """

tests/error/test_invalid.py

@@ -26,3 +26,8 @@ def only_equal_to_itself():
         false_object = False
         assert INVALID != false_object
         assert not INVALID == false_object
+
+    def forward_compatibility():
+        from graphql import Undefined
+
+        assert INVALID is Undefined