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bpo-36424: Add support for pickling and unpickling frozen dataclasses #17254

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12 changes: 12 additions & 0 deletions Lib/dataclasses.py
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Original file line number Diff line number Diff line change
Expand Up @@ -562,6 +562,18 @@ def _frozen_get_del_attr(cls, fields):
' raise FrozenInstanceError(f"cannot delete field {name!r}")',
f'super(cls, self).__delattr__(name)'),
globals=globals),
_create_fn('__getstate__',
('self',),
('fields = getattr(self, _fields)',
'valid_fields = tuple(f.name for f in fields.values() if f._field_type is _FIELD)',
'return [(attr, getattr(self, attr, None)) for attr in valid_fields]',
),
globals=dict(globals, _fields=_FIELDS, _FIELD=_FIELD)),
_create_fn('__setstate__',
('self', 'state'),
('for slot, value in state:',
' object.__setattr__(self, slot, value)'),
globals=globals),
)


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16 changes: 16 additions & 0 deletions Lib/test/test_dataclasses.py
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Expand Up @@ -1914,6 +1914,22 @@ class R:
self.assertEqual(new_sample.x, another_new_sample.x)
self.assertEqual(sample.y, another_new_sample.y)

def test_frozen_dataclasses_pickleable(self):
global S
@dataclass(frozen=True)
class S:
__slots__ = ('x', 'y')
x: int
y: int

sample = S(42, 24)
for proto in range(pickle.HIGHEST_PROTOCOL + 1):
with self.subTest(sample=sample, proto=proto):
new_sample = pickle.loads(pickle.dumps(sample, proto))
self.assertEqual(sample.x, new_sample.x)
self.assertEqual(sample.y, new_sample.y)
self.assertIsNot(sample, new_sample)


class TestFieldNoAnnotation(unittest.TestCase):
def test_field_without_annotation(self):
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6 changes: 6 additions & 0 deletions Misc/NEWS.d/next/Library/2019-11-19-10-20-44.bpo-36424.iF-VZL.rst
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@@ -0,0 +1,6 @@
Frozen dataclasses, in conjunction with with __slots__, cannot be unpickled,
as we were using `__setattr__` to set the new instance's attribute which
conflicts with the __slots__ definition. Adding `__getstate__` and
`__setstate__` for these classes should alleviate the issue, as we can use
`object.__setattr__` instead to set the frozen attributes on instance's
state assignment.