bpo-32892: Support subclasses of base types in isinstance checks for AST constants. (GH-9934)

Some projects (e.g. Chameleon) create ast.Str containing an instance
of the str subclass.

Lib/ast.py

@@ -346,7 +346,7 @@ class _ABC(type):
             except AttributeError:
                 return False
             else:
-                return type(value) in _const_types[cls]
+                return isinstance(value, _const_types[cls])
         return type.__instancecheck__(cls, inst)
 
 def _new(cls, *args, **kwargs):

Lib/test/test_ast.py

@@ -399,6 +399,10 @@ class AST_Tests(unittest.TestCase):
         self.assertFalse(isinstance(ast.Constant(), ast.NameConstant))
         self.assertFalse(isinstance(ast.Constant(), ast.Ellipsis))
 
+        class S(str): pass
+        self.assertTrue(isinstance(ast.Constant(S('42')), ast.Str))
+        self.assertFalse(isinstance(ast.Constant(S('42')), ast.Num))
+
     def test_subclasses(self):
         class N(ast.Num):
             def __init__(self, *args, **kwargs):