Misleading "Not an instance or subtype" error when assigning to property of frozen, slotted dataclass #143969
Description
Bug report
Bug description:
When a dataclass is frozen, attempting to assign to a property (with a setter or without) fails as expected:
from dataclasses import dataclass
@dataclass(frozen=True)
class Example:
a: int
@property
def prop(self):
return None
example = Example(10)
example.prop = 0 File "test_dataclass.py", line 12, in <module>
example.prop = 0
^^^^^^^^^^^^
File "<string>", line 15, in __setattr__
dataclasses.FrozenInstanceError: cannot assign to field 'prop'
And when the class is slotted, if the property has no setter, it also fails the way one would expect:
from dataclasses import dataclass
@dataclass(slots=True)
class Example:
a: int
@property
def prop(self):
return None
example = Example(10)
example.prop = 0Traceback (most recent call last):
File "test_dataclass.py", line 12, in <module>
example.prop = 0
^^^^^^^^^^^^
AttributeError: property 'prop' of 'Example' object has no setter
(In this case, of course, adding a setter makes it work fine.)
What's unexpected (and confusing) is when the class is both frozen and slotted:
from dataclasses import dataclass
@dataclass(frozen=True, slots=True)
class Example:
a: int
@property
def prop(self):
return None
example = Example(10)
example.prop = 0Traceback (most recent call last):
File "test_dataclass.py", line 12, in <module>
example.prop = 0
^^^^^^^^^^^^
File "<string>", line 16, in __setattr__
TypeError: super(type, obj): obj (instance of Example) is not an instance or subtype of type (Example).
One would, of course, expect an instance of Example to in fact be an instance of Example, but maybe this is somehow a collision between the initial vs. dataclass-ified class definitions?
CPython versions tested on:
3.13
Operating systems tested on:
macOS