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[3.9] bpo-45663: Fix is_dataclass() for dataclasses which are subclasses of types.GenericAlias (GH-29294) #29926

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miss-islington merged 1 commit into from
Dec 5, 2021
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[3.9] bpo-45663: Fix is_dataclass() for dataclasses which are subclasses of types.GenericAlias (GH-29294) #29926

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bpo-45663: Fix is_dataclass() for dataclasses which are subclasses of… 
… types.GenericAlias (GH-29294)

(cherry picked from commit 446be16)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
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@serhiy-storchaka @miss-islington
serhiy-storchaka authored and miss-islington committed Dec 5, 2021
commit c6ef4d8bf07ca4add65eccfbe774ed0018c6fd12
2 changes: 1 addition & 1 deletion Lib/dataclasses.py
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Expand Up @@ -1047,7 +1047,7 @@ def _is_dataclass_instance(obj):
def is_dataclass(obj):
"""Returns True if obj is a dataclass or an instance of a
dataclass."""
cls = obj if isinstance(obj, type) else type(obj)
cls = obj if isinstance(obj, type) and not isinstance(obj, GenericAlias) else type(obj)
return hasattr(cls, _FIELDS)


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12 changes: 12 additions & 0 deletions Lib/test/test_dataclasses.py
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Expand Up @@ -7,6 +7,7 @@
import pickle
import inspect
import builtins
import types
import unittest
from unittest.mock import Mock
from typing import ClassVar, Any, List, Union, Tuple, Dict, Generic, TypeVar, Optional, Protocol
Expand Down Expand Up @@ -1348,6 +1349,17 @@ class B:
with self.assertRaisesRegex(TypeError, 'should be called on dataclass instances'):
replace(obj, x=0)

def test_is_dataclass_genericalias(self):
@dataclass
class A(types.GenericAlias):
origin: type
args: type
self.assertTrue(is_dataclass(A))
a = A(list, int)
self.assertTrue(is_dataclass(type(a)))
self.assertTrue(is_dataclass(a))


def test_helper_fields_with_class_instance(self):
# Check that we can call fields() on either a class or instance,
# and get back the same thing.
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2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Library/2021-10-28-23-11-59.bpo-45663.J90N5R.rst
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@@ -0,0 +1,2 @@
Fix :func:`dataclasses.is_dataclass` for dataclasses which are subclasses of
:class:`types.GenericAlias`.