Allow overriding some methods in NamedTuple (#370)

* Allow owerriding some methods in NamededTuple

* Fix lint

* Response to review comments (performance)

Author: ilevkivskyi

src/test_typing.py

@@ -1484,14 +1484,23 @@ class XMeth(NamedTuple):
     x: int
     def double(self):
         return 2 * self.x
+
+class XRepr(NamedTuple):
+    x: int
+    y: int = 1
+    def __str__(self):
+        return f'{self.x} -> {self.y}'
+    def __add__(self, other):
+        return 0
 """
 
 if PY36:
     exec(PY36_TESTS)
 else:
     # fake names for the sake of static analysis
     ann_module = ann_module2 = ann_module3 = None
-    A = B = CSub = G = CoolEmployee = CoolEmployeeWithDefault = XMeth = object
+    A = B = CSub = G = CoolEmployee = CoolEmployeeWithDefault = object
+    XMeth = XRepr = object
 
 gth = get_type_hints
 
@@ -2128,6 +2137,8 @@ class NonDefaultAfterDefault(NamedTuple):
     def test_annotation_usage_with_methods(self):
         self.assertEqual(XMeth(1).double(), 2)
         self.assertEqual(XMeth(42).x, XMeth(42)[0])
+        self.assertEqual(str(XRepr(42)), '42 -> 1')
+        self.assertEqual(XRepr(1, 2) + XRepr(3), 0)
 
         with self.assertRaises(AttributeError):
             exec("""

src/typing.py

@@ -1982,6 +1982,13 @@ def _make_nmtuple(name, types):
 
 _PY36 = sys.version_info[:2] >= (3, 6)
 
+# attributes prohibited to set in NamedTuple class syntax
+_prohibited = ('__new__', '__init__', '__slots__', '__getnewargs__',
+               '_fields', '_field_defaults', '_field_types',
+               '_make', '_replace', '_asdict')
+
+_special = ('__module__', '__name__', '__qualname__', '__annotations__')
+
 
 class NamedTupleMeta(type):
 
@@ -2009,13 +2016,10 @@ def __new__(cls, typename, bases, ns):
         nm_tpl._field_defaults = defaults_dict
         # update from user namespace without overriding special namedtuple attributes
         for key in ns:
-            if not hasattr(nm_tpl, key):
-                setattr(nm_tpl, key, ns[key])
-            elif (
-                key not in ['__module__', '__qualname__', '__annotations__'] and
-                key not in nm_tpl._field_defaults
-            ):
+            if key in _prohibited:
                 raise AttributeError("Cannot overwrite NamedTuple attribute " + key)
+            elif key not in _special and key not in nm_tpl._fields:
+                setattr(nm_tpl, key, ns[key])
         return nm_tpl