gh-92261: Disallow iteration of Union (and other _SpecialForms)

Lib/test/test_typing.py

@@ -7334,6 +7334,37 @@ def test_all_exported_names(self):
         self.assertSetEqual(computed_all, actual_all)
 
 
+class TypeIterationTests(BaseTestCase):
+    _EXPECTED_ERROR_BY_TYPE = {
+        Any: "'_AnyMeta' object is not iterable",
+        Union: "'_SpecialForm' object is not iterable",
+        Union[str, int]: "'_UnionGenericAlias' object is not iterable",
+        Union[str, T]: "'_UnionGenericAlias' object is not iterable",
+        List: "'_SpecialGenericAlias' object is not iterable",
+        Tuple: "'_TupleType' object is not iterable",
+        Callable: "'_CallableType' object is not iterable",
+        Callable[..., T]: "'_CallableGenericAlias' object is not iterable",
+        Callable[[T], str]: "'_CallableGenericAlias' object is not iterable",
+        Annotated: "'type' object is not iterable",
+        Annotated[T, '']: "'_AnnotatedAlias' object is not iterable",
+    }
+
+    def test_cannot_iterate(self):
+        for test_type, expected_error in self._EXPECTED_ERROR_BY_TYPE.items():
+            with self.subTest(type=test_type):
+                expected_error_regex = '^{}$'.format(expected_error)
+                with self.assertRaisesRegex(TypeError, expected_error_regex):
+                    iter(test_type)
+                with self.assertRaisesRegex(TypeError, expected_error_regex):
+                    list(test_type)
+                with self.assertRaisesRegex(TypeError, expected_error_regex):
+                    for _ in test_type:
+                        pass
+
+    def test_is_not_instance_of_iterable(self):
+        for type_to_test in self._EXPECTED_ERROR_BY_TYPE.keys():
+            self.assertNotIsInstance(type_to_test, collections.abc.Iterable)
+
 
 if __name__ == '__main__':
     main()

Lib/typing.py

@@ -405,9 +405,24 @@ def __deepcopy__(self, memo):
         return self
 
 
+class _NotIterable:
+    """Mixin to prevent iteration, without being compatible with Iterable.
+
+    That is, we could do:
+        def __iter__(self): raise TypeError()
+    But this would make users of this mixin duck type-compatible with
+    collections.abc.Iterable - isinstance(foo, Iterable) would be True.
+
+    Luckily, we can instead prevent iteration by setting __iter__ to None, which
+    is treated specially.
+    """
+
+    __iter__ = None
+
+
 # Internal indicator of special typing constructs.
 # See __doc__ instance attribute for specific docs.
-class _SpecialForm(_Final, _root=True):
+class _SpecialForm(_Final, _NotIterable, _root=True):
     __slots__ = ('_name', '__doc__', '_getitem')
 
     def __init__(self, getitem):
@@ -1498,7 +1513,7 @@ def __iter__(self):
 # 1 for List and 2 for Dict.  It may be -1 if variable number of
 # parameters are accepted (needs custom __getitem__).
 
-class _SpecialGenericAlias(_BaseGenericAlias, _root=True):
+class _SpecialGenericAlias(_NotIterable, _BaseGenericAlias, _root=True):
     def __init__(self, origin, nparams, *, inst=True, name=None):
         if name is None:
             name = origin.__name__
@@ -1541,7 +1556,7 @@ def __or__(self, right):
     def __ror__(self, left):
         return Union[left, self]
 
-class _CallableGenericAlias(_GenericAlias, _root=True):
+class _CallableGenericAlias(_NotIterable, _GenericAlias, _root=True):
     def __repr__(self):
         assert self._name == 'Callable'
         args = self.__args__
@@ -1606,7 +1621,7 @@ def __getitem__(self, params):
         return self.copy_with(params)
 
 
-class _UnionGenericAlias(_GenericAlias, _root=True):
+class _UnionGenericAlias(_NotIterable, _GenericAlias, _root=True):
     def copy_with(self, params):
         return Union[params]
 
@@ -2046,7 +2061,7 @@ def _proto_hook(other):
             cls.__init__ = _no_init_or_replace_init
 
 
-class _AnnotatedAlias(_GenericAlias, _root=True):
+class _AnnotatedAlias(_NotIterable, _GenericAlias, _root=True):
     """Runtime representation of an annotated type.
 
     At its core 'Annotated[t, dec1, dec2, ...]' is an alias for the type 't'

Misc/NEWS.d/next/Core and Builtins/2022-05-03-20-12-18.gh-issue-92261.aigLnb.rst

@@ -0,0 +1 @@
+Fix hang when trying to iterate over a ``typing.Union``.