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[3.10] gh-91330: Tests and docs for dataclass descriptor-typed fields (GH-94424) (GH-94577)
Co-authored-by: Erik De Bonte <erikd@microsoft.com> Co-authored-by: Łukasz Langa <lukasz@langa.pl> (cherry picked from commit 5f31930)
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‎Doc/library/dataclasses.rst

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@@ -726,3 +726,54 @@ Mutable default values
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x: list = field(default_factory=list)
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assert D().x is not D().x
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Descriptor-typed fields
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-----------------------
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Fields that are assigned :ref:`descriptor objects <descriptors>` as their
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default value have the following special behaviors:
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* The value for the field passed to the dataclass's ``__init__`` method is
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passed to the descriptor's ``__set__`` method rather than overwriting the
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descriptor object.
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* Similarly, when getting or setting the field, the descriptor's
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``__get__`` or ``__set__`` method is called rather than returning or
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overwriting the descriptor object.
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* To determine whether a field contains a default value, ``dataclasses``
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will call the descriptor's ``__get__`` method using its class access
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form (i.e. ``descriptor.__get__(obj=None, type=cls)``. If the
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descriptor returns a value in this case, it will be used as the
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field's default. On the other hand, if the descriptor raises
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:exc:`AttributeError` i 8000 n this situation, no default value will be
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provided for the field.
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::
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class IntConversionDescriptor:
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def __init__(self, *, default):
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self._default = default
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def __set_name__(self, owner, name):
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self._name = "_" + name
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def __get__(self, obj, type):
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if obj is None:
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return self._default
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return getattr(obj, self._name, self._default)
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def __set__(self, obj, value):
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setattr(obj, self._name, int(value))
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@dataclass
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class InventoryItem:
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quantity_on_hand: IntConversionDescriptor = IntConversionDescriptor(default=100)
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i = InventoryItem()
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print(i.quantity_on_hand) # 100
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i.quantity_on_hand = 2.5 # calls __set__ with 2.5
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print(i.quantity_on_hand) # 2
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Note that if a field is annotated with a descriptor type, but is not assigned
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a descriptor object as its default value, the field will act like a normal
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field.

‎Lib/test/test_dataclasses.py

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self.assertEqual(D.__set_name__.call_count, 1)
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def test_init_calls_set(self):
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class D:
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pass
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D.__set__ = Mock()
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@dataclass
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class C:
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i: D = D()
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# Make sure D.__set__ is called.
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D.__set__.reset_mock()
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c = C(5)
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self.assertEqual(D.__set__.call_count, 1)
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def test_getting_field_calls_get(self):
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class D:
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pass
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D.__set__ = Mock()
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D.__get__ = Mock()
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@dataclass
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class C:
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i: D = D()
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c = C(5)
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# Make sure D.__get__ is called.
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D.__get__.reset_mock()
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value = c.i
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self.assertEqual(D.__get__.call_count, 1)
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def test_setting_field_calls_set(self):
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class D:
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pass
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D.__set__ = Mock()
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@dataclass
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class C:
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i: D = D()
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c = C(5)
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# Make sure D.__set__ is called.
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D.__set__.reset_mock()
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c.i = 10
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self.assertEqual(D.__set__.call_count, 1)
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def test_setting_uninitialized_descriptor_field(self):
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class D:
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pass
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D.__set__ = Mock()
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@dataclass
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class C:
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i: D
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# D.__set__ is not called because there's no D instance to call it on
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D.__set__.reset_mock()
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c = C(5)
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self.assertEqual(D.__set__.call_count, 0)
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# D.__set__ still isn't called after setting i to an instance of D
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# because descriptors don't behave like that when stored as instance vars
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c.i = D()
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c.i = 5
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self.assertEqual(D.__set__.call_count, 0)
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def test_default_value(self):
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class D:
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def __get__(self, instance: Any, owner: object) -> int:
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if instance is None:
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return 100
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return instance._x
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def __set__(self, instance: Any, value: int) -> None:
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instance._x = value
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@dataclass
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class C:
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i: D = D()
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c = C()
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self.assertEqual(c.i, 100)
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c = C(5)
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self.assertEqual(c.i, 5)
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def test_no_default_value(self):
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class D:
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def __get__(self, instance: Any, owner: object) -> int:
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if instance is None:
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raise AttributeError()
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return instance._x
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def __set__(self, instance: Any, value: int) -> None:
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instance._x = value
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@dataclass
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class C:
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i: D = D()
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with self.assertRaisesRegex(TypeError, 'missing 1 required positional argument'):
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c = C()
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class TestStringAnnotations(unittest.TestCase):
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def test_classvar(self):

‎Misc/NEWS.d/next/Tests/2022-07-05-17-53-13.gh-issue-91330.Qys5IL.rst

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Added more tests for :mod:`dataclasses` to cover behavior with data
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descriptor-based fields.
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# Write your Misc/NEWS entry below. It should be a simple ReST paragraph. #
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Don't start with "- Issue #<n>: " or "- gh-issue-<n>: " or that sort of
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stuff.
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###########################################################################

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