Support descriptors in dataclass transform (#15006)

Infer `__init__` argument types from the signatures of descriptor
`__set__` methods, if present. We can't (easily) perform full type
inference in a plugin, so we cheat and use a simplified implementation
that should still cover most use cases. Here we assume that `__set__` is
not decorated or overloaded, in particular.

Fixes #14868.

mypy/plugins/dataclasses.py

@@ -6,7 +6,7 @@
 from typing_extensions import Final
 
 from mypy import errorcodes, message_registry
-from mypy.expandtype import expand_type
+from mypy.expandtype import expand_type, expand_type_by_instance
 from mypy.nodes import (
     ARG_NAMED,
     ARG_NAMED_OPT,
@@ -23,6 +23,7 @@
     Context,
     DataclassTransformSpec,
     Expression,
+    FuncDef,
     IfStmt,
     JsonDict,
     NameExpr,
@@ -98,7 +99,7 @@ def __init__(
         self.has_default = has_default
         self.line = line
         self.column = column
-        self.type = type
+        self.type = type  # Type as __init__ argument
         self.info = info
         self.kw_only = kw_only
         self.is_neither_frozen_nor_nonfrozen = is_neither_frozen_nor_nonfrozen
@@ -535,9 +536,12 @@ def collect_attributes(self) -> list[DataclassAttribute] | None:
             elif not isinstance(stmt.rvalue, TempNode):
                 has_default = True
 
-            if not has_default:
-                # Make all non-default attributes implicit because they are de-facto set
-                # on self in the generated __init__(), not in the class body.
+            if not has_default and self._spec is _TRANSFORM_SPEC_FOR_DATACLASSES:
+                # Make all non-default dataclass attributes implicit because they are de-facto
+                # set on self in the generated __init__(), not in the class body. On the other
+                # hand, we don't know how custom dataclass transforms initialize attributes,
+                # so we don't treat them as implicit. This is required to support descriptors
+                # (https://github.com/python/mypy/issues/14868).
                 sym.implicit = True
 
             is_kw_only = kw_only
@@ -578,6 +582,7 @@ def collect_attributes(self) -> list[DataclassAttribute] | None:
                     )
 
             current_attr_names.add(lhs.name)
+            init_type = self._infer_dataclass_attr_init_type(sym, lhs.name, stmt)
             found_attrs[lhs.name] = DataclassAttribute(
                 name=lhs.name,
                 alias=alias,
@@ -586,7 +591,7 @@ def collect_attributes(self) -> list[DataclassAttribute] | None:
                 has_default=has_default,
                 line=stmt.line,
                 column=stmt.column,
-                type=sym.type,
+                type=init_type,
                 info=cls.info,
                 kw_only=is_kw_only,
                 is_neither_frozen_nor_nonfrozen=_has_direct_dataclass_transform_metaclass(
@@ -755,6 +760,50 @@ def _get_bool_arg(self, name: str, default: bool) -> bool:
             return require_bool_literal_argument(self._api, expression, name, default)
         return default
 
+    def _infer_dataclass_attr_init_type(
+        self, sym: SymbolTableNode, name: str, context: Context
+    ) -> Type | None:
+        """Infer __init__ argument type for an attribute.
+
+        In particular, possibly use the signature of __set__.
+        """
+        default = sym.type
+        if sym.implicit:
+            return default
+        t = get_proper_type(sym.type)
+
+        # Perform a simple-minded inference from the signature of __set__, if present.
+        # We can't use mypy.checkmember here, since this plugin runs before type checking.
+        # We only support some basic scanerios here, which is hopefully sufficient for
+        # the vast majority of use cases.
+        if not isinstance(t, Instance):
+            return default
+        setter = t.type.get("__set__")
+        if setter:
+            if isinstance(setter.node, FuncDef):
+                super_info = t.type.get_containing_type_info("__set__")
+                assert super_info
+                if setter.type:
+                    setter_type = get_proper_type(
+                        map_type_from_supertype(setter.type, t.type, super_info)
+                    )
+                else:
+                    return AnyType(TypeOfAny.unannotated)
+                if isinstance(setter_type, CallableType) and setter_type.arg_kinds == [
+                    ARG_POS,
+                    ARG_POS,
+                    ARG_POS,
+                ]:
+                    return expand_type_by_instance(setter_type.arg_types[2], t)
+                else:
+                    self._api.fail(
+                        f'Unsupported signature for "__set__" in "{t.type.name}"', context
+                    )
+            else:
+                self._api.fail(f'Unsupported "__set__" in "{t.type.name}"', context)
+
+        return default
+
 
 def add_dataclass_tag(info: TypeInfo) -> None:
     # The value is ignored, only the existence matters.

test-data/unit/check-dataclass-transform.test

@@ -807,3 +807,217 @@ reveal_type(bar.base)  # N: Revealed type is "builtins.int"
 
 [typing fixtures/typing-full.pyi]
 [builtins fixtures/dataclasses.pyi]
+
+[case testDataclassTransformSimpleDescriptor]
+# flags: --python-version 3.11
+
+from typing import dataclass_transform, overload, Any
+
+@dataclass_transform()
+def my_dataclass(cls): ...
+
+class Desc:
+    @overload
+    def __get__(self, instance: None, owner: Any) -> Desc: ...
+    @overload
+    def __get__(self, instance: object, owner: Any) -> str: ...
+    def __get__(self, instance: object | None, owner: Any) -> Desc | str: ...
+
+    def __set__(self, instance: Any, value: str) -> None: ...
+
+@my_dataclass
+class C:
+    x: Desc
+    y: int
+
+C(x='x', y=1)
+C(x=1, y=1)  # E: Argument "x" to "C" has incompatible type "int"; expected "str"
+reveal_type(C(x='x', y=1).x)  # N: Revealed type is "builtins.str"
+reveal_type(C(x='x', y=1).y)  # N: Revealed type is "builtins.int"
+reveal_type(C.x)  # N: Revealed type is "__main__.Desc"
+
+[typing fixtures/typing-full.pyi]
+[builtins fixtures/dataclasses.pyi]
+
+[case testDataclassTransformUnannotatedDescriptor]
+# flags: --python-version 3.11
+
+from typing import dataclass_transform, overload, Any
+
+@dataclass_transform()
+def my_dataclass(cls): ...
+
+class Desc:
+    @overload
+    def __get__(self, instance: None, owner: Any) -> Desc: ...
+    @overload
+    def __get__(self, instance: object, owner: Any) -> str: ...
+    def __get__(self, instance: object | None, owner: Any) -> Desc | str: ...
+
+    def __set__(*args, **kwargs): ...
+
+@my_dataclass
+class C:
+    x: Desc
+    y: int
+
+C(x='x', y=1)
+C(x=1, y=1)
+reveal_type(C(x='x', y=1).x)  # N: Revealed type is "builtins.str"
+reveal_type(C(x='x', y=1).y)  # N: Revealed type is "builtins.int"
+reveal_type(C.x)  # N: Revealed type is "__main__.Desc"
+
+[typing fixtures/typing-full.pyi]
+[builtins fixtures/dataclasses.pyi]
+
+[case testDataclassTransformGenericDescriptor]
+# flags: --python-version 3.11
+
+from typing import dataclass_transform, overload, Any, TypeVar, Generic
+
+@dataclass_transform()
+def my_dataclass(frozen: bool = False): ...
+
+T = TypeVar("T")
+
+class Desc(Generic[T]):
+    @overload
+    def __get__(self, instance: None, owner: Any) -> Desc[T]: ...
+    @overload
+    def __get__(self, instance: object, owner: Any) -> T: ...
+    def __get__(self, instance: object | None, owner: Any) -> Desc | T: ...
+
+    def __set__(self, instance: Any, value: T) -> None: ...
+
+@my_dataclass()
+class C:
+    x: Desc[str]
+
+C(x='x')
+C(x=1)  # E: Argument "x" to "C" has incompatible type "int"; expected "str"
+reveal_type(C(x='x').x)  # N: Revealed type is "builtins.str"
+reveal_type(C.x)  # N: Revealed type is "__main__.Desc[builtins.str]"
+
+@my_dataclass()
+class D(C):
+    y: Desc[int]
+
+d = D(x='x', y=1)
+reveal_type(d.x)  # N: Revealed type is "builtins.str"
+reveal_type(d.y)  # N: Revealed type is "builtins.int"
+reveal_type(D.x)  # N: Revealed type is "__main__.Desc[builtins.str]"
+reveal_type(D.y)  # N: Revealed type is "__main__.Desc[builtins.int]"
+
+@my_dataclass(frozen=True)
+class F:
+    x: Desc[str] = Desc()
+
+F(x='x')
+F(x=1)  # E: Argument "x" to "F" has incompatible type "int"; expected "str"
+reveal_type(F(x='x').x)  # N: Revealed type is "builtins.str"
+reveal_type(F.x)  # N: Revealed type is "__main__.Desc[builtins.str]"
+
+[typing fixtures/typing-full.pyi]
+[builtins fixtures/dataclasses.pyi]
+
+[case testDataclassTransformGenericDescriptorWithInheritance]
+# flags: --python-version 3.11
+
+from typing import dataclass_transform, overload, Any, TypeVar, Generic
+
+@dataclass_transform()
+def my_dataclass(cls): ...
+
+T = TypeVar("T")
+
+class Desc(Generic[T]):
+    @overload
+    def __get__(self, instance: None, owner: Any) -> Desc[T]: ...
+    @overload
+    def __get__(self, instance: object, owner: Any) -> T: ...
+    def __get__(self, instance: object | None, owner: Any) -> Desc | T: ...
+
+    def __set__(self, instance: Any, value: T) -> None: ...
+
+class Desc2(Desc[str]):
+    pass
+
+@my_dataclass
+class C:
+    x: Desc2
+
+C(x='x')
+C(x=1)  # E: Argument "x" to "C" has incompatible type "int"; expected "str"
+reveal_type(C(x='x').x)  # N: Revealed type is "builtins.str"
+reveal_type(C.x)  # N: Revealed type is "__main__.Desc[builtins.str]"
+
+[typing fixtures/typing-full.pyi]
+[builtins fixtures/dataclasses.pyi]
+
+[case testDataclassTransformDescriptorWithDifferentGetSetTypes]
+# flags: --python-version 3.11
+
+from typing import dataclass_transform, overload, Any
+
+@dataclass_transform()
+def my_dataclass(cls): ...
+
+class Desc:
+    @overload
+    def __get__(self, instance: None, owner: Any) -> int: ...
+    @overload
+    def __get__(self, instance: object, owner: Any) -> str: ...
+    def __get__(self, instance, owner): ...
+
+    def __set__(self, instance: Any, value: bytes) -> None: ...
+
+@my_dataclass
+class C:
+    x: Desc
+
+c = C(x=b'x')
+C(x=1)  # E: Argument "x" to "C" has incompatible type "int"; expected "bytes"
+reveal_type(c.x)  # N: Revealed type is "builtins.str"
+reveal_type(C.x)  # N: Revealed type is "builtins.int"
+c.x = b'x'
+c.x = 1  # E: Incompatible types in assignment (expression has type "int", variable has type "bytes")
+
+[typing fixtures/typing-full.pyi]
+[builtins fixtures/dataclasses.pyi]
+
+[case testDataclassTransformUnsupportedDescriptors]
+# flags: --python-version 3.11
+
+from typing import dataclass_transform, overload, Any
+
+@dataclass_transform()
+def my_dataclass(cls): ...
+
+class Desc:
+    @overload
+    def __get__(self, instance: None, owner: Any) -> int: ...
+    @overload
+    def __get__(self, instance: object, owner: Any) -> str: ...
+    def __get__(self, instance, owner): ...
+
+    def __set__(*args, **kwargs) -> None: ...
+
+class Desc2:
+    @overload
+    def __get__(self, instance: None, owner: Any) -> int: ...
+    @overload
+    def __get__(self, instance: object, owner: Any) -> str: ...
+    def __get__(self, instance, owner): ...
+
+    @overload
+    def __set__(self, instance: Any, value: bytes) -> None: ...
+    @overload
+    def __set__(self) -> None: ...
+    def __set__(self, *args, **kawrga) -> None: ...
+
+@my_dataclass
+class C:
+    x: Desc  # E: Unsupported signature for "__set__" in "Desc"
+    y: Desc2  # E: Unsupported "__set__" in "Desc2"
+[typing fixtures/typing-full.pyi]
+[builtins fixtures/dataclasses.pyi]