Added support for the X is C and X is not C type guard pattern (w…

docs/type-concepts-advanced.md

@@ -62,6 +62,7 @@ In addition to assignment-based type narrowing, Pyright supports the following t
 * `type(x) is T` and `type(x) is not T`
 * `type(x) == T` and `type(x) != T`
 * `x is E` and `x is not E` (where E is a literal enum or bool)
+* `x is C` and `x is not C` (where C is a class)
 * `x == L` and `x != L` (where L is an expression that evaluates to a literal type)
 * `x.y is None` and `x.y is not None` (where x is a type that is distinguished by a field with a None)
 * `x.y is E` and `x.y is not E` (where E is a literal enum or bool and x is a type that is distinguished by a field with a literal type)

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

@@ -222,12 +222,12 @@ export function getTypeNarrowingCallback(
                 }
             }
 
-            // Look for "X is Y" or "X is not Y" where Y is a an enum or bool literal.
             if (isOrIsNotOperator) {
                 if (ParseTreeUtils.isMatchingExpression(reference, testExpression.leftExpression)) {
                     const rightTypeResult = evaluator.getTypeOfExpression(testExpression.rightExpression);
                     const rightType = rightTypeResult.type;
 
+                    // Look for "X is Y" or "X is not Y" where Y is a an enum or bool literal.
                     if (
                         isClassInstance(rightType) &&
                         (ClassType.isEnumClass(rightType) || ClassType.isBuiltIn(rightType, 'bool')) &&
@@ -246,9 +246,19 @@ export function getTypeNarrowingCallback(
                             };
                         };
                     }
+
+                    // Look for X is <class> or X is not <class>.
+                    if (isInstantiableClass(rightType)) {
+                        return (type: Type) => {
+                            return {
+                                type: narrowTypeForClassComparison(evaluator, type, rightType, adjIsPositiveTest),
+                                isIncomplete: !!rightTypeResult.isIncomplete,
+                            };
+                        };
+                    }
                 }
 
-                // Look for X[<literal>] is <literal> or X[<literal>] is not <literal>
+                // Look for X[<literal>] is <literal> or X[<literal>] is not <literal>.
                 if (
                     testExpression.leftExpression.nodeType === ParseNodeType.Index &&
                     testExpression.leftExpression.items.length === 1 &&
@@ -2078,6 +2088,54 @@ function narrowTypeForTypeIs(evaluator: TypeEvaluator, type: Type, classType: Cl
     );
 }
 
+// Attempts to narrow a type based on a comparison with a class using "is" or
+// "is not". This pattern is sometimes used for sentinels.
+function narrowTypeForClassComparison(
+    evaluator: TypeEvaluator,
+    referenceType: Type,
+    classType: ClassType,
+    isPositiveTest: boolean
+): Type {
+    return mapSubtypes(referenceType, (subtype) => {
+        const concreteSubtype = evaluator.makeTopLevelTypeVarsConcrete(subtype);
+
+        if (isPositiveTest) {
+            if (isNoneInstance(concreteSubtype)) {
+                return undefined;
+            }
+
+            if (isClassInstance(concreteSubtype) && TypeBase.isInstance(subtype)) {
+                return undefined;
+            }
+
+            if (isInstantiableClass(concreteSubtype) && ClassType.isFinal(concreteSubtype)) {
+                if (
+                    !ClassType.isSameGenericClass(concreteSubtype, classType) &&
+                    !isIsinstanceFilterSuperclass(
+                        evaluator,
+                        concreteSubtype,
+                        classType,
+                        classType,
+                        /* isInstanceCheck */ false
+                    )
+                ) {
+                    return undefined;
+                }
+            }
+        } else {
+            if (
+                isInstantiableClass(concreteSubtype) &&
+                ClassType.isSameGenericClass(classType, concreteSubtype) &&
+                ClassType.isFinal(classType)
+            ) {
+                return undefined;
+            }
+        }
+
+        return subtype;
+    });
+}
+
 // Attempts to narrow a type (make it more constrained) based on a comparison
 // (equal or not equal) to a literal value. It also handles "is" or "is not"
 // operators if isIsOperator is true.

packages/pyright-internal/src/tests/samples/typeNarrowingIsClass1.py

@@ -0,0 +1,58 @@
+# This sample tests type narrowing for conditional
+# statements of the form X is <class> or X is not <class>.
+
+from typing import Any, TypeVar, final
+
+
+@final
+class A:
+    ...
+
+
+@final
+class B:
+    ...
+
+
+class C:
+    ...
+
+
+def func1(x: type[A] | type[B] | None | int):
+    if x is A:
+        reveal_type(x, expected_text="type[A]")
+    else:
+        reveal_type(x, expected_text="type[B] | int | None")
+
+
+def func2(x: type[A] | type[B] | None | int, y: type[A]):
+    if x is not y:
+        reveal_type(x, expected_text="type[B] | int | None")
+    else:
+        reveal_type(x, expected_text="type[A]")
+
+
+def func3(x: type[A] | type[B] | Any):
+    if x is A:
+        reveal_type(x, expected_text="type[A] | Any")
+    else:
+        reveal_type(x, expected_text="type[B] | Any")
+
+
+def func4(x: type[A] | type[B] | type[C]):
+    if x is C:
+        reveal_type(x, expected_text="type[C]")
+    else:
+        reveal_type(x, expected_text="type[A] | type[B] | type[C]")
+
+
+T = TypeVar("T")
+
+
+def func5(x: type[A] | type[B] | type[T]) -> type[A] | type[B] | type[T]:
+    if x is A:
+        reveal_type(x, expected_text="type[A] | type[T@func5]")
+    else:
+        reveal_type(x, expected_text="type[B] | type[T@func5]")
+
+    return x

packages/pyright-internal/src/tests/typeEvaluator1.test.ts

@@ -315,6 +315,12 @@ test('TypeNarrowingIsNone2', () => {
     TestUtils.validateResults(analysisResults, 0);
 });
 
+test('TypeNarrowingIsClass1', () => {
+    const analysisResults = TestUtils.typeAnalyzeSampleFiles(['typeNarrowingIsClass1.py']);
+
+    TestUtils.validateResults(analysisResults, 0);
+});
+
 test('TypeNarrowingIsNoneTuple1', () => {
     const analysisResults = TestUtils.typeAnalyzeSampleFiles(['typeNarrowingIsNoneTuple1.py']);