Added support for synthesized __slots__ symbol in dataclasses that are constructed using the slots=True argument. This addresses https://github.com/microsoft/pylance-release/issues/4147.

packages/pyright-internal/src/analyzer/dataClasses.ts

@@ -625,6 +625,26 @@ export function synthesizeDataClassMethods(
         classType.details.localSlotsNames = localDataClassEntries.map((entry) => entry.name);
     }
 
+    // Should we synthesize a __slots__ symbol?
+    if (ClassType.isGeneratedDataClassSlots(classType)) {
+        let iterableType = evaluator.getTypingType(node, 'Iterable') ?? UnknownType.create();
+
+        if (isInstantiableClass(iterableType)) {
+            iterableType = ClassType.cloneAsInstance(
+                ClassType.cloneForSpecialization(
+                    iterableType,
+                    [evaluator.getBuiltInObject(node, 'str')],
+                    /* isTypeArgumentExplicit */ true
+                )
+            );
+        }
+
+        symbolTable.set(
+            '__slots__',
+            Symbol.createWithType(SymbolFlags.ClassMember | SymbolFlags.ClassVar, iterableType)
+        );
+    }
+
     // If this dataclass derived from a NamedTuple, update the NamedTuple with
     // the specialized entry types.
     updateNamedTupleBaseClass(

packages/pyright-internal/src/tests/samples/dataclass17.py

@@ -44,3 +44,26 @@ class D:
 
 
 D(1, "bar")
+
+
+@dataclass(slots=True)
+class E:
+    a: int
+
+
+E.__slots__
+E(1).__slots__
+
+reveal_type(E.__slots__, expected_text="Iterable[str]")
+
+
+@dataclass
+class F:
+    a: int
+
+
+# This should generate an error.
+F.__slots__
+
+# This should generate an error.
+F(1).__slots__

packages/pyright-internal/src/tests/typeEvaluator4.test.ts

@@ -621,7 +621,7 @@ test('DataClass17', () => {
     configOptions.defaultPythonVersion = PythonVersion.V3_10;
     const analysisResults = TestUtils.typeAnalyzeSampleFiles(['dataclass17.py'], configOptions);
 
-    TestUtils.validateResults(analysisResults, 3);
+    TestUtils.validateResults(analysisResults, 5);
 });
 
 test('DataClass18', () => {