bpo-43772: Fix TypeVar.__ror__ (GH-25339)

Author: JelleZijlstra

Lib/test/test_typing.py

@@ -186,6 +186,16 @@ def test_union_unique(self):
         self.assertEqual(Union[X, int].__parameters__, (X,))
         self.assertIs(Union[X, int].__origin__, Union)
 
+    def test_or(self):
+        X = TypeVar('X')
+        # use a string because str doesn't implement
+        # __or__/__ror__ itself
+        self.assertEqual(X | "x", Union[X, "x"])
+        self.assertEqual("x" | X, Union["x", X])
+        # make sure the order is correct
+        self.assertEqual(get_args(X | "x"), (X, ForwardRef("x")))
+        self.assertEqual(get_args("x" | X), (ForwardRef("x"), X))
+
     def test_union_constrained(self):
         A = TypeVar('A', str, bytes)
         self.assertNotEqual(Union[A, str], Union[A])

Lib/typing.py

@@ -648,8 +648,8 @@ def __init__(self, bound, covariant, contravariant):
     def __or__(self, right):
         return Union[self, right]
 
-    def __ror__(self, right):
-        return Union[self, right]
+    def __ror__(self, left):
+        return Union[left, self]
 
     def __repr__(self):
         if self.__covariant__:

Misc/NEWS.d/next/Library/2021-04-10-19-14-49.bpo-43772.Bxq0zQ.rst

@@ -0,0 +1 @@
+Fixed the return value of ``TypeVar.__ror__``. Patch by Jelle Zijlstra.