Add utility __pos__ and __neg__ operators to TypeVar

This makes it easier to explicitly just use +/-T to make generics
co/contra-variant.

Author: mariusvniekerk

src/test_typing.py

@@ -236,6 +236,17 @@ def test_no_bivariant(self):
         with self.assertRaises(ValueError):
             TypeVar('T', covariant=True, contravariant=True)
 
+    def test_typevar_unary_operators(self):
+        A = TypeVar('A')
+        self.assertFalse(A.__covariant__)
+        self.assertFalse(A.__contravariant__)
+        A_co = +A
+        self.assertTrue(A.__covariant__)
+        self.assertFalse(A.__contravariant__)
+        A_contra = -A
+        self.assertFalse(A.__covariant__)
+        self.assertTrue(A.__contravariant__)
+
 
 class UnionTests(BaseTestCase):
 

src/typing.py

@@ -534,6 +534,12 @@ def __instancecheck__(self, instance):
     def __subclasscheck__(self, cls):
         raise TypeError("Type variables cannot be used with issubclass().")
 
+    def __neg__(self):
+        return TypeVar(self.__name__, *self.__constraints__, bound=self.__bound__, covariant=False, contravariant=True)
+
+    def __pos__(self):
+        return TypeVar(self.__name__, *self.__constraints__, bound=self.__bound__, covariant=True, contravariant=False)
+
 
 # Some unconstrained type variables.  These are used by the container types.
 # (These are not for export.)