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gh-91126: Docs and tests for slotted dataclasses with __init_subclass__ #123342

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sobolevn merged 2 commits into from
Sep 1, 2024
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gh-91126: Docs and tests for slotted dataclasses with __init_subclass__ #123342

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gh-91126: Docs and tests for slotted dataclasses with `__init_subclas…
sobolevn Aug 26, 2024
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sobolevn Aug 26, 2024
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18 changes: 14 additions & 4 deletions Doc/library/dataclasses.rst
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Expand Up @@ -185,10 +185,20 @@ Module contents
- *slots*: If true (the default is ``False``), :attr:`~object.__slots__` attribute
will be generated and new class will be returned instead of the original one.
If :attr:`!__slots__` is already defined in the class, then :exc:`TypeError`
is raised. Calling no-arg :func:`super` in dataclasses using ``slots=True`` will result in
the following exception being raised:
``TypeError: super(type, obj): obj must be an instance or subtype of type``.
The two-arg :func:`super` is a valid workaround. See :gh:`90562` for full details.
is raised.

.. warning::
Calling no-arg :func:`super` in dataclasses using ``slots=True``
will result in the following exception being raised:
``TypeError: super(type, obj): obj must be an instance or subtype of type``.
The two-arg :func:`super` is a valid workaround.
See :gh:`90562` for full details.

.. warning::
Having a base class with :meth:`~object.__init_subclass__` with parameters
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when using ``slots=True`` will result in a :exc:`TypeError`.
Either use ``__init_subclass__`` with no parameters or use default values.
See :gh:`91126` for full details.

.. versionadded:: 3.10

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28 changes: 28 additions & 0 deletions Lib/test/test_dataclasses/__init__.py
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Expand Up @@ -3664,6 +3664,34 @@ class A(WithDictSlot): ...
self.assertEqual(A().__dict__, {})
A()

def test_slots_with_wrong_init_subclass(self):
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class WrongSuper:
def __init_subclass__(cls, arg):
pass

with self.assertRaisesRegex(
TypeError,
"missing 1 required positional argument: 'arg'",
):
@dataclass(slots=True)
class WithWrongSuper(WrongSuper, arg=1):
pass

def test_slots_with_correct_init_subclass(self):
class CorrectSuper:
args = []
def __init_subclass__(cls, arg="default"):
cls.args.append(arg)

@dataclass(slots=True)
class WithCorrectSuper(CorrectSuper):
pass

# __init_subclass__ is called twice: once for `WithCorrectSuper`
# and once for `WithCorrectSuper__slots__` new class
# that we create internally.
self.assertEqual(CorrectSuper.args, ["default", "default"])


class TestDescriptors(unittest.TestCase):
def test_set_name(self):
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