Support issubclass for ClassVar data members

Doc/library/typing.rst

@@ -1247,6 +1247,11 @@ These are not used in annotations. They are building blocks for creating generic
 
    .. versionadded:: 3.8
 
+   .. versionchanged:: 3.11
+      Protocols with data members annotated with :data:`ClassVar` now support
+      :func:`issubclass` checks. Subclasses must set these data members to pass.
+
+
 Other special directives
 """"""""""""""""""""""""
 

Doc/whatsnew/3.11.rst

@@ -219,6 +219,14 @@ sqlite3
   (Contributed by Erlend E. Aasland in :issue:`44688`.)
 
 
+typing
+------
+
+* Runtime protocols with data members now support :func:`issubclass` as long
+  as those members are annotated with :data:`typing.ClassVar`.
+  (Contributed by Ken Jin in :issue:`44975`).
+
+
 Removed
 =======
 * :class:`smtpd.MailmanProxy` is now removed as it is unusable without

Lib/test/test_typing.py

@@ -1610,6 +1610,18 @@ class P(Protocol):
         with self.assertRaisesRegex(TypeError, "@runtime_checkable"):
             isinstance(1, P)
 
+    def test_runtime_issubclass_with_classvar_data_members(self):
+        @runtime_checkable
+        class P(Protocol):
+            x: ClassVar[int] = 1
+
+        class C: pass
+
+        class D:
+            x = 1
+        self.assertNotIsSubclass(C, P)
+        self.assertIsSubclass(D, P)
+
 
 class GenericTests(BaseTestCase):
 

Lib/typing.py

@@ -1396,8 +1396,15 @@ def _get_protocol_attrs(cls):
 
 
 def _is_callable_members_only(cls):
+    attr_names = _get_protocol_attrs(cls)
+    annotations = getattr(cls, '__annotations__', {})
     # PEP 544 prohibits using issubclass() with protocols that have non-method members.
-    return all(callable(getattr(cls, attr, None)) for attr in _get_protocol_attrs(cls))
+    for attr_name in attr_names:
+        attr = getattr(cls, attr_name, None)
+        if not (callable(attr)
+            or (getattr(annotations.get(attr_name), '__name__', None) == 'ClassVar')):
+            return False
+    return True
 
 
 def _no_init(self, *args, **kwargs):
@@ -1511,8 +1518,9 @@ def _proto_hook(other):
             if not _is_callable_members_only(cls):
                 if _allow_reckless_class_checks():
                     return NotImplemented
-                raise TypeError("Protocols with non-method members"
-                                " don't support issubclass()")
+                raise TypeError("Protocol members must be methods or data"
+                                " attributes annotated with ClassVar to support"
+                                " issubclass()")
             if not isinstance(other, type):
                 # Same error message as for issubclass(1, int).
                 raise TypeError('issubclass() arg 1 must be a class')

Misc/NEWS.d/next/Library/2021-08-22-16-58-02.bpo-44975.ub3Vxk.rst

@@ -0,0 +1,2 @@
+Runtime protocols with data members now support :func:`issubclass` as long
+as those members are annotated with :data:`typing.ClassVar`.