gh-142414: Remove singleton design pattern in _crossinterp.py

Lib/concurrent/interpreters/_crossinterp.py

@@ -5,71 +5,23 @@ class ItemInterpreterDestroyed(Exception):
     """Raised when trying to get an item whose interpreter was destroyed."""
 
 
-class classonly:
-    """A non-data descriptor that makes a value only visible on the class.
-
-    This is like the "classmethod" builtin, but does not show up on
-    instances of the class.  It may be used as a decorator.
-    """
-
-    def __init__(self, value):
-        self.value = value
-        self.getter = classmethod(value).__get__
-        self.name = None
-
-    def __set_name__(self, cls, name):
-        if self.name is not None:
-            raise TypeError('already used')
-        self.name = name
-
-    def __get__(self, obj, cls):
-        if obj is not None:
-            raise AttributeError(self.name)
-        # called on the class
-        return self.getter(None, cls)
-
-
 class UnboundItem:
     """Represents a cross-interpreter item no longer bound to an interpreter.
 
     An item is unbound when the interpreter that added it to the
     cross-interpreter container is destroyed.
     """
 
-    __slots__ = ()
-
-    @classonly
-    def singleton(cls, kind, module, name='UNBOUND'):
-        doc = cls.__doc__
-        if doc:
-            doc = doc.replace(
-                'cross-interpreter container', kind,
-            ).replace(
-                'cross-interpreter', kind,
-            )
-        subclass = type(
-            f'Unbound{kind.capitalize()}Item',
-            (cls,),
-            {
-                "_MODULE": module,
-                "_NAME": name,
-                "__doc__": doc,
-            },
-        )
-        return object.__new__(subclass)
-
-    _MODULE = __name__
-    _NAME = 'UNBOUND'
-
-    def __new__(cls):
-        raise Exception(f'use {cls._MODULE}.{cls._NAME}')
+    def __init__(self, kind, module, name='UNBOUND'):
+        self._kind = kind
+        self._module = module
+        self._name = name
 
     def __repr__(self):
-        return f'{self._MODULE}.{self._NAME}'
-#        return f'interpreters._queues.UNBOUND'
+        return f'{self._module}.{self._name}'
 
 
-UNBOUND = object.__new__(UnboundItem)
+UNBOUND = UnboundItem('cross-interpreter', __name__)
 UNBOUND_ERROR = object()
 UNBOUND_REMOVE = object()
 
@@ -84,6 +36,8 @@ def __repr__(self):
 
 def serialize_unbound(unbound):
     op = unbound
+    if isinstance(op, UnboundItem):
+        return _UNBOUND_CONSTANT_TO_FLAG[UNBOUND],
     try:
         flag = _UNBOUND_CONSTANT_TO_FLAG[op]
     except KeyError:

Lib/concurrent/interpreters/_queues.py

@@ -46,12 +46,10 @@ class ItemInterpreterDestroyed(QueueError,
 _PICKLED = 1
 
 
-UNBOUND = _crossinterp.UnboundItem.singleton('queue', __name__)
+UNBOUND = _crossinterp.UnboundItem('queue', __name__)
 
 
 def _serialize_unbound(unbound):
-    if unbound is UNBOUND:
-        unbound = _crossinterp.UNBOUND
     return _crossinterp.serialize_unbound(unbound)
 
 

Lib/test/support/channels.py

@@ -28,12 +28,10 @@ class ItemInterpreterDestroyed(ChannelError,
     """Raised from get() and get_nowait()."""
 
 
-UNBOUND = _crossinterp.UnboundItem.singleton('queue', __name__)
+UNBOUND = _crossinterp.UnboundItem('queue', __name__)
 
 
 def _serialize_unbound(unbound):
-    if unbound is UNBOUND:
-        unbound = _crossinterp.UNBOUND
     return _crossinterp.serialize_unbound(unbound)
 
 

Lib/test/test_interpreters/test_queues.py

@@ -8,6 +8,7 @@
 # Raise SkipTest if subinterpreters not supported.
 _queues = import_helper.import_module('_interpqueues')
 from concurrent import interpreters
+from concurrent.futures import InterpreterPoolExecutor
 from concurrent.interpreters import _queues as queues, _crossinterp
 from .utils import _run_output, TestBase as _TestBase
 
@@ -93,6 +94,14 @@ def test_bind_release(self):
             with self.assertRaises(queues.QueueError):
                 _queues.release(qid)
 
+    def test_interpreter_pool_executor_after_reload(self):
+        # Regression test for gh-142414 (KeyError in serialize_unbound).
+        importlib.reload(queues)
+        code = "import struct"
+        with InterpreterPoolExecutor(max_workers=1) as executor:
+            results = executor.map(exec, [code] * 1)
+            self.assertEqual(list(results), [None] * 1)
+
 
 class QueueTests(TestBase):
 

Misc/NEWS.d/next/Library/2025-12-10-15-27-58.gh-issue-142414.zTHgP-.rst

@@ -0,0 +1 @@
+Fix spurious KeyError when concurrent.interpreters is reloaded after import.