[ty] Infer class attributes assigned by metaclass initialization#25342
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charliermarsh merged 9 commits intoMay 27, 2026
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Typing conformance resultsNo changes detected ✅Current numbersThe percentage of diagnostics emitted that were expected errors held steady at 89.70%. The percentage of expected errors that received a diagnostic held steady at 86.99%. The number of fully passing files held steady at 91/134. |
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Significant changesClick to expand detailed breakdownprefect
sphinx
trio
flake8
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| Lint rule | Added | Removed | Changed |
|---|---|---|---|
invalid-assignment |
0 | 0 | 1 |
| Total | 0 | 0 | 1 |
Raw diff:
core (https://github.com/home-assistant/core)
- homeassistant/components/bluetooth/passive_update_processor.py:97:30 error[invalid-assignment] Object of type `object` is not assignable to `type[EntityDescription]`
+ homeassistant/components/bluetooth/passive_update_processor.py:97:30 error[invalid-assignment] Object of type `type` is not assignable to `type[EntityDescription]`
charliermarsh
commented
May 23, 2026
| self.extend_with_class_members(db, ty, metaclass.class_literal(db)); | ||
| if let Some((metaclass, _)) = metaclass.static_class_literal(db) { | ||
| self.extend_with_instance_members(db, ty, metaclass); | ||
| } |
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This here adds instance members defined during metaclass initialization, like:
class Meta(type):
def __init__(cls, ...):
cls.generated = 1(Which wasn't happening before.)
sharkdp
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May 27, 2026
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sharkdp
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May 27, 2026
| ); | ||
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| if let Some(metaclass_instance) = self.to_meta_type(db).to_instance(db) { | ||
| let metaclass_attr = metaclass_instance.instance_member(db, name); |
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Hm, interesting. I guess I never thought about it that way, but I guess it all makes sense: this works because the cls in the metaclasses' __init__ is recognized as the self attribute.
class Meta(type):
def __init__(cls, name: str, bases: tuple[type, ...], namespace: dict[str, object]) -> None:
cls.attr: int = 1| .. | ||
| }) | ||
| ) { | ||
| class_attr.or_fall_back_to(db, || metaclass_attr) |
Contributor
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Can the fallback ever be relevant here if we match on Place::Defined above? If not, can we simplify this by skipping the metaclass instance member lookup entirely?
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I think it is possible to be Place::Defined but Definedness::PossiblyUndefined, as in;
class Meta(type):
def __init__(cls, ...) -> None:
cls.attr: int | str = 1
def _(flag: bool):
class C(metaclass=Meta):
if flag:
attr: str = "class value"
reveal_type(C.attr) # int | str| metaclass_attr.or_fall_back_to(db, || class_attr) | ||
| } | ||
| } else { | ||
| class_attr |
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What if the metaclass_attr is possibly defined? Shouldn't we create a union type in that case?
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anishgirianish
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to anishgirianish/ruff
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May 28, 2026
…ral-sh#25342) ## Summary Prior to this change, we didn't recognize class-object attributes populated by a metaclass during class initialization. For example: ```python class Meta(type): def __init__(cls, name: str, bases: tuple[type, ...], namespace: dict[str, object]) -> None: cls.attr: int = 1 class C(metaclass=Meta): ... reveal_type(C.attr) # revealed: int ``` We now recognize attributes that a metaclass adds to a class while creating it. If the metaclass overwrites an existing class-body value, we use the overwritten value's type. If the class provides an annotation for the generated attribute, we preserve that annotation as its public type. For example, if the class body initially gives attr a value, the metaclass assignment happens later at runtime and overwrites it: ```python class Meta(type): def __init__(cls, name, bases, namespace): cls.attr: int = 1 class C(metaclass=Meta): attr = "initial value" reveal_type(C.attr) # int ``` Closes astral-sh/ty#1138.
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Summary
Prior to this change, we didn't recognize class-object attributes populated by a metaclass during class initialization. For example:
We now recognize attributes that a metaclass adds to a class while creating it. If the metaclass overwrites an existing class-body value, we use the overwritten value's type. If the class provides an annotation for the generated attribute, we preserve that annotation as its public type.
For example, if the class body initially gives attr a value, the metaclass assignment happens later at runtime and overwrites it:
Closes astral-sh/ty#1138.