8000 gh-119260: Clarify is_dataclass Behavior for Subclasses in Documentation and Tests by adiaholic · Pull Request #119480 · python/cpython · GitHub
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gh-119260: Clarify is_dataclass Behavior for Subclasses in Documentation and Tests #119480

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Merged
merged 4 commits into from
May 29, 2024
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gh-119260: Clarify is_dataclass Behavior for Subclasses in Documentation and Tests #119480

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cdb0e49
Clarify is_dataclass Behavior for Subclasses in Documentation and Tests
adiaholic May 28, 2024
05d17d9
Apply suggestions from code review
carljm May 28, 2024
012be20
Update Doc/library/dataclasses.rst
carljm May 28, 2024
a1201ba
Merge branch 'main' into fix-issue-119260
carljm May 29, 2024
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4 changes: 2 additions & 2 deletions Doc/library/dataclasses.rst
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Expand Up @@ -461,8 +461,8 @@ Module contents

.. function:: is_dataclass(obj)

Return ``True`` if its parameter is a dataclass or an instance of one,
otherwise return ``False``.
Return ``True`` if its parameter is a dataclass (including subclasses of a
dataclass) or an instance of one, otherwise return ``False``.

If you need to know if a class is an instance of a dataclass (and
not a dataclass itself), then add a further check for ``not
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18 changes: 18 additions & 0 deletions Lib/test/test_dataclasses/__init__.py
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Expand Up @@ -1547,6 +1547,24 @@ class A(types.GenericAlias):
self.assertTrue(is_dataclass(type(a)))
self.assertTrue(is_dataclass(a))

def test_is_dataclass_inheritance(self):
@dataclass
class X:
y: int

class Z(X):
pass

self.assertTrue(is_dataclass(X), "X should be a dataclass")
self.assertTrue(
is_dataclass(Z),
"Z should be a dataclass because it inherits from X",
)
z_instance = Z(y=5)
self.assertTrue(
is_dataclass(z_instance),
"z_instance should be a dataclass because it is an instance of Z",
)

def test_helper_fields_with_class_instance(self):
# Check that we can call fields() on either a class or instance,
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