bpo-33141: Have dataclasses.Field pass through __set_name__ to any default argument. (GH-6260)
This is part of PEP 487 and the descriptor protocol.
diff --git a/Lib/dataclasses.py b/Lib/dataclasses.py
index 8ccc4c8..8c197fe 100644
--- a/Lib/dataclasses.py
+++ b/Lib/dataclasses.py
@@ -240,6 +240,20 @@
f'metadata={self.metadata}'
')')
+ # This is used to support the PEP 487 __set_name__ protocol in the
+ # case where we're using a field that contains a descriptor as a
+ # defaul value. For details on __set_name__, see
+ # https://www.python.org/dev/peps/pep-0487/#implementation-details.
+ # Note that in _process_class, this Field object is overwritten with
+ # 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)
+ if func:
+ # There is a __set_name__ method on the descriptor,
+ # call it.
+ func(owner, name)
+
class _DataclassParams:
__slots__ = ('init',
diff --git a/Lib/test/test_dataclasses.py b/Lib/test/test_dataclasses.py
index f7f132c..2745eaf 100755
--- a/Lib/test/test_dataclasses.py
+++ b/Lib/test/test_dataclasses.py
@@ -2698,6 +2698,48 @@
# We can add a new field to the derived instance.
d.z = 10
+class TestDescriptors(unittest.TestCase):
+ def test_set_name(self):
+ # See bpo-33141.
+
+ # Create a descriptor.
+ class D:
+ def __set_name__(self, owner, name):
+ self.name = name
+ def __get__(self, instance, owner):
+ if instance is not None:
+ return 1
+ return self
+
+ # This is the case of just normal descriptor behavior, no
+ # dataclass code is involved in initializing the descriptor.
+ @dataclass
+ class C:
+ c: int=D()
+ self.assertEqual(C.c.name, 'c')
+
+ # Now test with a default value and init=False, which is the
+ # only time this is really meaningful. If not using
+ # init=False, then the descriptor will be overwritten, anyway.
+ @dataclass
+ class C:
+ c: int=field(default=D(), init=False)
+ self.assertEqual(C.c.name, 'c')
+ self.assertEqual(C().c, 1)
+
+ def test_non_descriptor(self):
+ # PEP 487 says __set_name__ should work on non-descriptors.
+ # Create a descriptor.
+
+ class D:
+ def __set_name__(self, owner, name):
+ self.name = name
+
+ @dataclass
+ class C:
+ c: int=field(default=D(), init=False)
+ self.assertEqual(C.c.name, 'c')
+
if __name__ == '__main__':
unittest.main()