Reject isinstance() with TypedDict and NewType (#3654)

These can't be used for runtime type checking at runtime.

Author: JukkaL

mypy/checkexpr.py

@@ -201,11 +201,16 @@ def visit_call_expr(self, e: CallExpr, allow_none_return: bool = False) -> Type:
                     except KeyError:
                         # Undefined names should already be reported in semantic analysis.
                         node = None
-                if (isinstance(typ, IndexExpr)
-                        and isinstance(typ.analyzed, (TypeApplication, TypeAliasExpr))
+                if ((isinstance(typ, IndexExpr)
+                        and isinstance(typ.analyzed, (TypeApplication, TypeAliasExpr)))
                         # node.kind == TYPE_ALIAS only for aliases like It = Iterable[int].
-                        or isinstance(typ, NameExpr) and node and node.kind == nodes.TYPE_ALIAS):
+                        or (isinstance(typ, NameExpr) and node and node.kind == nodes.TYPE_ALIAS)):
                     self.msg.type_arguments_not_allowed(e)
+                if isinstance(typ, RefExpr) and isinstance(typ.node, TypeInfo):
+                    if typ.node.typeddict_type:
+                        self.msg.fail(messages.CANNOT_ISINSTANCE_TYPEDDICT, e)
+                    elif typ.node.is_newtype:
+                        self.msg.fail(messages.CANNOT_ISINSTANCE_NEWTYPE, e)
         self.try_infer_partial_type(e)
         callee_type = self.accept(e.callee, always_allow_any=True)
         if (self.chk.options.disallow_untyped_calls and

mypy/messages.py

@@ -88,6 +88,8 @@
 NON_BOOLEAN_IN_CONDITIONAL = 'Condition must be a boolean'
 DUPLICATE_TYPE_SIGNATURES = 'Function has duplicate type signatures'
 GENERIC_INSTANCE_VAR_CLASS_ACCESS = 'Access to generic instance variables via class is ambiguous'
+CANNOT_ISINSTANCE_TYPEDDICT = 'Cannot use isinstance() with a TypedDict type'
+CANNOT_ISINSTANCE_NEWTYPE = 'Cannot use isinstance() with a NewType type'
 
 ARG_CONSTRUCTOR_NAMES = {
     ARG_POS: "Arg",

mypy/test/testextensions.py

@@ -84,11 +84,11 @@ def test_typeddict_errors(self):
         self.assertEqual(TypedDict.__module__, 'mypy_extensions')
         jim = Emp(name='Jim', id=1)
         with self.assertRaises(TypeError):
-            isinstance({}, Emp)
+            isinstance({}, Emp)  # type: ignore
         with self.assertRaises(TypeError):
-            isinstance(jim, Emp)
+            isinstance(jim, Emp)  # type: ignore
         with self.assertRaises(TypeError):
-            issubclass(dict, Emp)
+            issubclass(dict, Emp)  # type: ignore
         with self.assertRaises(TypeError):
             TypedDict('Hi', x=1)
         with self.assertRaises(TypeError):

test-data/unit/check-newtype.test

@@ -336,3 +336,11 @@ class C(B): pass  # E: Cannot subclass NewType
 from typing import NewType
 Any = NewType('Any', int)
 Any(5)
+
+[case testNewTypeAndIsInstance]
+from typing import NewType
+T = NewType('T', int)
+d: object
+if isinstance(d, T):  # E: Cannot use isinstance() with a NewType type
+    reveal_type(d) # E: Revealed type is '__main__.T'
+[builtins fixtures/isinstancelist.pyi]

test-data/unit/check-typeddict.test

@@ -717,8 +717,14 @@ def set_coordinate(p: TaggedPoint, key: str, value: int) -> None:
 
 -- isinstance
 
--- TODO: Implement support for this case.
---[case testCannotIsInstanceTypedDictType]
+[case testTypedDictAndInstance]
+from mypy_extensions import TypedDict
+D = TypedDict('D', {'x': int})
+d: object
+if isinstance(d, D):  # E: Cannot use isinstance() with a TypedDict type
+    reveal_type(d) # E: Revealed type is '__main__.D'
+[builtins fixtures/isinstancelist.pyi]
+
 
 -- scoping
 [case testTypedDictInClassNamespace]

test-data/unit/fixtures/isinstancelist.pyi

@@ -8,6 +8,7 @@ class type:
 
 class tuple: pass
 class function: pass
+class ellipsis: pass
 
 def isinstance(x: object, t: Union[type, Tuple]) -> bool: pass
 def issubclass(x: object, t: Union[type, Tuple]) -> bool: pass