bpo-36517: Raise error on multiple inheritance with NamedTuple (GH-19363)

Lib/test/test_typing.py

@@ -3626,6 +3626,13 @@ class XMethBad2(NamedTuple):
         return 'no chance for this as well'
 """)
 
+    def test_multiple_inheritance(self):
+        class A:
+            pass
+        with self.assertRaises(TypeError):
+            class X(NamedTuple, A):
+                x: int
+
     def test_namedtuple_keyword_usage(self):
         LocalEmployee = NamedTuple("LocalEmployee", name=str, age=int)
         nick = LocalEmployee('Nick', 25)

Lib/typing.py

@@ -1728,6 +1728,9 @@ class NamedTupleMeta(type):
     def __new__(cls, typename, bases, ns):
         if ns.get('_root', False):
             return super().__new__(cls, typename, bases, ns)
+        if len(bases) > 1:
+            raise TypeError("Multiple inheritance with NamedTuple is not supported")
+        assert bases[0] is NamedTuple
         types = ns.get('__annotations__', {})
         nm_tpl = _make_nmtuple(typename, types.items())
         defaults = []

Misc/NEWS.d/next/Library/2020-04-04-17-49-39.bpo-36517.Ilj1IJ.rst

@@ -0,0 +1,2 @@
+Multiple inheritance with :class:`typing.NamedTuple` now raises an error
+instead of silently ignoring other types.