[Python-checkins] bpo-37868: Improve is_dataclass for instances. (GH-15325)
Eric V. Smith
webhook-mailer at python.org
Tue Aug 20 01:40:32 EDT 2019
https://github.com/python/cpython/commit/b0f4dab8735f692bcfedcf0fa9a25e238a554bab
commit: b0f4dab8735f692bcfedcf0fa9a25e238a554bab
branch: master
author: Eric V. Smith <ericvsmith at users.noreply.github.com>
committer: GitHub <noreply at github.com>
date: 2019-08-20T01:40:28-04:00
summary:
bpo-37868: Improve is_dataclass for instances. (GH-15325)
files:
A Misc/NEWS.d/next/Library/2019-08-17-22-33-54.bpo-37868.hp64fi.rst
M Lib/dataclasses.py
M Lib/test/test_dataclasses.py
diff --git a/Lib/dataclasses.py b/Lib/dataclasses.py
index f778a27912de..9020c905d117 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 ea42904b003d..037bf4c22142 100755
--- a/Lib/test/test_dataclasses.py
+++ b/Lib/test/test_dataclasses.py
@@ -1300,6 +1300,32 @@ class D:
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 000000000000..7f342e1ee354
--- /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.
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