From b0f4dab8735f692bcfedcf0fa9a25e238a554bab Mon Sep 17 00:00:00 2001 From: "Eric V. Smith" Date: Tue, 20 Aug 2019 01:40:28 -0400 Subject: [PATCH] bpo-37868: Improve is_dataclass for instances. (GH-15325) --- Lib/dataclasses.py | 5 ++-- Lib/test/test_dataclasses.py | 26 +++++++++++++++++++ .../2019-08-17-22-33-54.bpo-37868.hp64fi.rst | 3 +++ 3 files changed, 32 insertions(+), 2 deletions(-) create mode 100644 Misc/NEWS.d/next/Library/2019-08-17-22-33-54.bpo-37868.hp64fi.rst diff --git a/Lib/dataclasses.py b/Lib/dataclasses.py index f778a27912..9020c905d1 100644 --- a/Lib/dataclasses.py +++ b/Lib/dataclasses.py @@ -1015,13 +1015,14 @@ def fields(class_or_instance): def _is_dataclass_instance(obj): """Returns True if obj is an instance of a dataclass.""" - return not isinstance(obj, type) and hasattr(obj, _FIELDS) + return hasattr(type(obj), _FIELDS) def is_dataclass(obj): """Returns True if obj is a dataclass or an instance of a dataclass.""" - return hasattr(obj, _FIELDS) + cls = obj if isinstance(obj, type) else type(obj) + return hasattr(cls, _FIELDS) def asdict(obj, *, dict_factory=dict): diff --git a/Lib/test/test_dataclasses.py b/Lib/test/test_dataclasses.py index ea42904b00..037bf4c221 100755 --- a/Lib/test/test_dataclasses.py +++ b/Lib/test/test_dataclasses.py @@ -1300,6 +1300,32 @@ class TestCase(unittest.TestCase): self.assertTrue(is_dataclass(d.d)) self.assertFalse(is_dataclass(d.e)) + def test_is_dataclass_when_getattr_always_returns(self): + # See bpo-37868. + class A: + def __getattr__(self, key): + return 0 + self.assertFalse(is_dataclass(A)) + a = A() + + # Also test for an instance attribute. + class B: + pass + b = B() + b.__dataclass_fields__ = [] + + for obj in a, b: + with self.subTest(obj=obj): + self.assertFalse(is_dataclass(obj)) + + # Indirect tests for _is_dataclass_instance(). + with self.assertRaisesRegex(TypeError, 'should be called on dataclass instances'): + asdict(obj) + with self.assertRaisesRegex(TypeError, 'should be called on dataclass instances'): + astuple(obj) + with self.assertRaisesRegex(TypeError, 'should be called on dataclass instances'): + replace(obj, x=0) + 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. diff --git a/Misc/NEWS.d/next/Library/2019-08-17-22-33-54.bpo-37868.hp64fi.rst b/Misc/NEWS.d/next/Library/2019-08-17-22-33-54.bpo-37868.hp64fi.rst new file mode 100644 index 0000000000..7f342e1ee3 --- /dev/null +++ b/Misc/NEWS.d/next/Library/2019-08-17-22-33-54.bpo-37868.hp64fi.rst @@ -0,0 +1,3 @@ +Fix dataclasses.is_dataclass when given an instance that never raises +AttributeError in __getattr__. That is, an object that returns something +for __dataclass_fields__ even if it's not a dataclass. -- 2.40.0