bpo-44649: Fix dataclasses(slots=True) with a field with a default, but init=False (GH-29692)
Special handling is needed, because for non-slots dataclasses the instance attributes are not set: reading from a field just references the class's attribute of the same name, which contains the default value. But this doesn't work for classes using __slots__: they don't read the class's attribute. So in that case (and that case only), initialize the instance attribute. Handle this for both normal defaults, and for fields using default_factory.
(cherry picked from commit d3062f672c92855b7e9e962ad4bf1a67abd4589b)
Co-authored-by: Eric V. Smith <ericvsmith@users.noreply.github.com>
diff --git a/Lib/test/test_dataclasses.py b/Lib/test/test_dataclasses.py
index b00d048..bcd004f 100644
--- a/Lib/test/test_dataclasses.py
+++ b/Lib/test/test_dataclasses.py
@@ -2880,6 +2880,28 @@ def test_frozen_pickle(self):
self.assertIsNot(obj, p)
self.assertEqual(obj, p)
+ def test_slots_with_default_no_init(self):
+ # Originally reported in bpo-44649.
+ @dataclass(slots=True)
+ class A:
+ a: str
+ b: str = field(default='b', init=False)
+
+ obj = A("a")
+ self.assertEqual(obj.a, 'a')
+ self.assertEqual(obj.b, 'b')
+
+ def test_slots_with_default_factory_no_init(self):
+ # Originally reported in bpo-44649.
+ @dataclass(slots=True)
+ class A:
+ a: str
+ b: str = field(default_factory=lambda:'b', init=False)
+
+ obj = A("a")
+ self.assertEqual(obj.a, 'a')
+ self.assertEqual(obj.b, 'b')
+
class TestDescriptors(unittest.TestCase):
def test_set_name(self):
# See bpo-33141.