bpo-33175: dataclasses should look up __set_name__ on class, not instance (GH-6305)

Lib/dataclasses.py

@@ -248,11 +248,11 @@ class Field:
     #  the default value, so the end result is a descriptor that had
     #  __set_name__ called on it at the right time.
     def __set_name__(self, owner, name):
-        func = getattr(self.default, '__set_name__', None)
+        func = getattr(type(self.default), '__set_name__', None)
         if func:
             # There is a __set_name__ method on the descriptor,
             #  call it.
-            func(owner, name)
+            func(self.default, owner, name)
 
 
 class _DataclassParams:

Lib/test/test_dataclasses.py

@@ -2705,7 +2705,7 @@ class TestDescriptors(unittest.TestCase):
         # Create a descriptor.
         class D:
             def __set_name__(self, owner, name):
-                self.name = name
+                self.name = name + 'x'
             def __get__(self, instance, owner):
                 if instance is not None:
                     return 1
@@ -2716,7 +2716,7 @@ class TestDescriptors(unittest.TestCase):
         @dataclass
         class C:
             c: int=D()
-        self.assertEqual(C.c.name, 'c')
+        self.assertEqual(C.c.name, 'cx')
 
         # Now test with a default value and init=False, which is the
         #  only time this is really meaningful.  If not using
@@ -2724,7 +2724,7 @@ class TestDescriptors(unittest.TestCase):
         @dataclass
         class C:
             c: int=field(default=D(), init=False)
-        self.assertEqual(C.c.name, 'c')
+        self.assertEqual(C.c.name, 'cx')
         self.assertEqual(C().c, 1)
 
     def test_non_descriptor(self):
@@ -2733,12 +2733,41 @@ class TestDescriptors(unittest.TestCase):
 
         class D:
             def __set_name__(self, owner, name):
-                self.name = name
+                self.name = name + 'x'
 
         @dataclass
         class C:
             c: int=field(default=D(), init=False)
-        self.assertEqual(C.c.name, 'c')
+        self.assertEqual(C.c.name, 'cx')
+
+    def test_lookup_on_instance(self):
+        # See bpo-33175.
+        class D:
+            pass
+
+        d = D()
+        # Create an attribute on the instance, not type.
+        d.__set_name__ = Mock()
+
+        # Make sure d.__set_name__ is not called.
+        @dataclass
+        class C:
+            i: int=field(default=d, init=False)
+
+        self.assertEqual(d.__set_name__.call_count, 0)
+
+    def test_lookup_on_class(self):
+        # See bpo-33175.
+        class D:
+            pass
+        D.__set_name__ = Mock()
+
+        # Make sure D.__set_name__ is called.
+        @dataclass
+        class C:
+            i: int=field(default=D(), init=False)
+
+        self.assertEqual(D.__set_name__.call_count, 1)
 
 
 if __name__ == '__main__':

Misc/NEWS.d/next/Library/2018-03-29-04-32-25.bpo-33175._zs1yM.rst

+In dataclasses, Field.__set_name__ now looks up the __set_name__ special
+method on the class, not the instance, of the default value.