Add __dataclass_fields__ and __attrs_attrs__ to dataclasses (#8578)

Fixes #6568.

mypy/plugin.py

@@ -254,6 +254,18 @@ def named_type(self, qualified_name: str, args: Optional[List[Type]] = None) ->
         """Construct an instance of a builtin type with given type arguments."""
         raise NotImplementedError
 
+    @abstractmethod
+    def named_type_or_none(self,
+                           qualified_name: str,
+                           args: Optional[List[Type]] = None) -> Optional[Instance]:
+        """Construct an instance of a type with given type arguments.
+
+        Return None if a type could not be constructed for the qualified
+        type name. This is possible when the qualified name includes a
+        module name and the module has not been imported.
+        """
+        raise NotImplementedError
+
     @abstractmethod
     def parse_bool(self, expr: Expression) -> Optional[bool]:
         """Parse True/False literals."""

mypy/plugins/attrs.py

@@ -21,8 +21,8 @@
     deserialize_and_fixup_type
 )
 from mypy.types import (
-    Type, AnyType, TypeOfAny, CallableType, NoneType, TypeVarType,
-    Overloaded, UnionType, FunctionLike, get_proper_type
+    TupleType, Type, AnyType, TypeOfAny, CallableType, NoneType, TypeVarType,
+    Overloaded, UnionType, FunctionLike, get_proper_type,
 )
 from mypy.typeops import make_simplified_union, map_type_from_supertype
 from mypy.typevars import fill_typevars
@@ -300,6 +300,8 @@ def attr_class_maker_callback(ctx: 'mypy.plugin.ClassDefContext',
             ctx.api.defer()
             return
 
+    _add_attrs_magic_attribute(ctx, raw_attr_types=[info[attr.name].type for attr in attributes])
+
     # Save the attributes so that subclasses can reuse them.
     ctx.cls.info.metadata['attrs'] = {
         'attributes': [attr.serialize() for attr in attributes],
@@ -703,6 +705,27 @@ def _add_init(ctx: 'mypy.plugin.ClassDefContext', attributes: List[Attribute],
     adder.add_method('__init__', args, NoneType())
 
 
+def _add_attrs_magic_attribute(ctx: 'mypy.plugin.ClassDefContext',
+                               raw_attr_types: 'List[Optional[Type]]') -> None:
+    attr_name = '__attrs_attrs__'
+    any_type = AnyType(TypeOfAny.explicit)
+    attributes_types: 'List[Type]' = [
+        ctx.api.named_type_or_none('attr.Attribute', [attr_type or any_type]) or any_type
+        for attr_type in raw_attr_types
+    ]
+    fallback_type = ctx.api.named_type('__builtins__.tuple', [
+        ctx.api.named_type_or_none('attr.Attribute', [any_type]) or any_type,
+    ])
+    var = Var(name=attr_name, type=TupleType(attributes_types, fallback=fallback_type))
+    var.info = ctx.cls.info
+    var._fullname = ctx.cls.info.fullname + '.' + attr_name
+    ctx.cls.info.names[attr_name] = SymbolTableNode(
+        kind=MDEF,
+        node=var,
+        plugin_generated=True,
+    )
+
+
 class MethodAdder:
     """Helper to add methods to a TypeInfo.
 

mypy/plugins/dataclasses.py

@@ -13,7 +13,10 @@
     add_method, _get_decorator_bool_argument, deserialize_and_fixup_type,
 )
 from mypy.typeops import map_type_from_supertype
-from mypy.types import Type, Instance, NoneType, TypeVarType, CallableType, get_proper_type
+from mypy.types import (
+    Type, Instance, NoneType, TypeVarType, CallableType, get_proper_type,
+    AnyType, TypeOfAny,
+)
 from mypy.server.trigger import make_wildcard_trigger
 
 # The set of decorators that generate dataclasses.
@@ -187,6 +190,8 @@ def transform(self) -> None:
 
         self.reset_init_only_vars(info, attributes)
 
+        self._add_dataclass_fields_magic_attribute()
+
         info.metadata['dataclass'] = {
             'attributes': [attr.serialize() for attr in attributes],
             'frozen': decorator_arguments['frozen'],
@@ -417,6 +422,23 @@ def _is_kw_only_type(self, node: Optional[Type]) -> bool:
             return False
         return node_type.type.fullname == 'dataclasses.KW_ONLY'
 
+    def _add_dataclass_fields_magic_attribute(self) -> None:
+        attr_name = '__dataclass_fields__'
+        any_type = AnyType(TypeOfAny.explicit)
+        field_type = self._ctx.api.named_type_or_none('dataclasses.Field', [any_type]) or any_type
+        attr_type = self._ctx.api.named_type('__builtins__.dict', [
+            self._ctx.api.named_type('__builtins__.str'),
+            field_type,
+        ])
+        var = Var(name=attr_name, type=attr_type)
+        var.info = self._ctx.cls.info
+        var._fullname = self._ctx.cls.info.fullname + '.' + attr_name
+        self._ctx.cls.info.names[attr_name] = SymbolTableNode(
+            kind=MDEF,
+            node=var,
+            plugin_generated=True,
+        )
+
 
 def dataclass_class_maker_callback(ctx: ClassDefContext) -> None:
     """Hooks into the class typechecking process to add support for dataclasses.

test-data/unit/check-attr.test

@@ -1390,3 +1390,15 @@ class B(A):
 
 reveal_type(B)  # N: Revealed type is "def (foo: builtins.int) -> __main__.B"
 [builtins fixtures/bool.pyi]
+
+[case testAttrsClassHasAttributeWithAttributes]
+import attr
+
+@attr.s
+class A:
+    b: int = attr.ib()
+    c: str = attr.ib()
+
+reveal_type(A.__attrs_attrs__)  # N: Revealed type is "Tuple[attr.Attribute[builtins.int], attr.Attribute[builtins.str]]"
+
+[builtins fixtures/attr.pyi]