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[ty] Use field-specifier return type as the default type for the field #20915

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Merged
merged 1 commit into from
Oct 16, 2025
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[ty] Use field-specifier return type as the default type for the field #20915

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10 changes: 6 additions & 4 deletions crates/ty_python_semantic/resources/mdtest/dataclasses/fields.md
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Original file line number Diff line number Diff line change
Expand Up @@ -11,7 +11,7 @@ class Member:
role: str = field(default="user")
tag: str | None = field(default=None, init=False)

# revealed: (self: Member, name: str, role: str = Literal["user"]) -> None
# revealed: (self: Member, name: str, role: str = str) -> None
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@sharkdp sharkdp Oct 16, 2025

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This happens due to literal promotion, I guess.

reveal_type(Member.__init__)

alice = Member(name="Alice", role="admin")
Expand All @@ -37,7 +37,7 @@ class Data:
content: list[int] = field(default_factory=list)
timestamp: datetime = field(default_factory=datetime.now, init=False)

# revealed: (self: Data, content: list[int] = list[Unknown]) -> None
# revealed: (self: Data, content: list[int] = list[int]) -> None
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@sharkdp sharkdp Oct 16, 2025

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This shows that our previous approach, calling default_factory manually, was apparently too naive.

reveal_type(Data.__init__)

data = Data([1, 2, 3])
Expand All @@ -63,7 +63,8 @@ class Person:
age: int | None = field(default=None, kw_only=True)
role: str = field(default="user", kw_only=True)

# revealed: (self: Person, name: str, *, age: int | None = None, role: str = Literal["user"]) -> None
# TODO: this would ideally show a default value of `None` for `age`
# revealed: (self: Person, name: str, *, age: int | None = int | None, role: str = str) -> None
reveal_type(Person.__init__)

alice = Person(role="admin", name="Alice")
Expand All @@ -82,7 +83,8 @@ def get_default() -> str:

reveal_type(field(default=1)) # revealed: dataclasses.Field[Literal[1]]
reveal_type(field(default=None)) # revealed: dataclasses.Field[None]
reveal_type(field(default_factory=get_default)) # revealed: dataclasses.Field[str]
# TODO: this could ideally be `dataclasses.Field[str]` with a better generics solver
reveal_type(field(default_factory=get_default)) # revealed: dataclasses.Field[Unknown]
```

## dataclass_transform field_specifiers
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17 changes: 10 additions & 7 deletions crates/ty_python_semantic/src/types/call/bind.rs
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Expand Up @@ -977,13 +977,16 @@ impl<'db> Bindings<'db> {
let kw_only =
overload.parameter_type_by_name("kw_only").unwrap_or(None);

let default_ty = match (default, default_factory) {
(Some(default_ty), _) => Some(default_ty),
(_, Some(default_factory_ty)) => default_factory_ty
.try_call(db, &CallArguments::none())
.ok()
.map(|binding| binding.return_type(db)),
_ => None,
// `dataclasses.field` and field-specifier functions of commonly used
// libraries like `pydantic`, `attrs`, and `SQLAlchemy` all return
// the default type for the field (or `Any`) instead of an actual `Field`
// instance, even if this is not what happens at runtime (see also below).
// We still make use of this fact and pretend that all field specifiers
// return the type of the default value:
let default_ty = if default.is_some() || default_factory.is_some() {
Some(overload.return_ty)
} else {
None
};

let init = init
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