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Dataclass "init" method should expect the type from a custom descriptor's "set" method #13856
Description
This issue is copied, mostly verbatim, from here: microsoft/pyright#3245.
If a @dataclass field is initialized with a descriptor as a default value, mypy should use the value type of the __set__ method for the descriptor when determining the type of the corresponding parameter within the synthesized __init__ method.
Types are also evaluated incorrectly in this case.
from dataclasses import dataclass
from typing import Any, cast
class MyDescriptor:
def __get__(self, __obj: object | None, __owner: Any) -> "int | MyDescriptor":
if __obj is None:
return self
return cast(Any, __obj)._x
def __set__(self, __obj: object | None, __value: int) -> None:
if __obj is not None:
cast(Any, __obj)._x = __value
@dataclass
class Foo:
y: MyDescriptor = MyDescriptor()
f = Foo(3) # Currently a false positive error in mypy
print(f.y) # 3
f.y = 4
print(f.y) # 4
print(Foo.y) # MyDescriptor object
reveal_type(f.y) # int | MyDescriptor, which is wrong
reveal_type(Foo.y) # int | MyDescriptor, which is wrongTo Reproduce
Run mypy against the above script.
Expected Behavior
No Errors
Actual Behavior
test.py:23: error: Argument 1 to "Foo" has incompatible type "int"; expected "MyDescriptor"
test.py:30: note: Revealed type is "Union[builtins.int, test.MyDescriptor]"
test.py:31: note: Revealed type is "Union[builtins.int, test.MyDescriptor]"
Found 1 error in 1 file (checked 1 source file)
Your Environment
- OS: Ubuntu 22.04.1
- Mypy version used: 0.982
- Mypy command-line flags: None
- Mypy configuration options from
mypy.ini(and other config files): None - Python version used: 3.10.6
Activity
pappasam commentedon Oct 11, 2022
@erictraut fyi, I migrated your issue from pyright here
debonte commentedon Mar 10, 2023
FYI, the behavior of
dataclasswith descriptor-typed fields was discussed in python/cpython#91330 and is documented here.glyph commentedon Mar 10, 2023
I think this confusion goes a bit deeper than
__set__; Mypy seems to have the wrong idea aboutdataclass-like types in generalnon-dataclass PEP 681 classes such as attrs. Consider this brief comparison of attrs and dataclasses:Side-by-side, type-check and output looks like this (emojis added to indicate agreement / disagreement between type-time and runtime):
Updated to include annotations about which one's which, which shows that the problem is with attrs' annotations more than it is with dataclasses, although "attribute error" vs. descriptor-type is not great.
glyph commentedon Mar 10, 2023
As I was playing with this I discovered a more specific problem in this neighborhood with Callable #14869
document no-pep681 workarounds
document no-pep681 workarounds
feat: avoid use of Ref, make use of descriptor to add reactive behavior
feat: avoid use of Ref, make use of descriptor to add reactive behavior
pappasam commentedon Apr 18, 2023
Just tested out with
mypy 1.2and this appears to have been resolved!