Issue #9707: Rewritten reference implementation of threading.local which
is friendlier towards reference cycles. This change is not normally
visible since an optimized C implementation (_thread._local) is used
instead.
diff --git a/Lib/_threading_local.py b/Lib/_threading_local.py
index 3af7807..4ec4828 100644
--- a/Lib/_threading_local.py
+++ b/Lib/_threading_local.py
@@ -132,6 +132,9 @@
>>> del mydata
"""
+from weakref import ref
+from contextlib import contextmanager
+
__all__ = ["local"]
# We need to use objects from the threading module, but the threading
@@ -139,112 +142,105 @@
# isn't compiled in to the `thread` module. This creates potential problems
# with circular imports. For that reason, we don't import `threading`
# until the bottom of this file (a hack sufficient to worm around the
-# potential problems). Note that almost all platforms do have support for
-# locals in the `thread` module, and there is no circular import problem
+# potential problems). Note that all platforms on CPython do have support
+# for locals in the `thread` module, and there is no circular import problem
# then, so problems introduced by fiddling the order of imports here won't
-# manifest on most boxes.
+# manifest.
-class _localbase(object):
- __slots__ = '_local__key', '_local__args', '_local__lock'
+class _localimpl:
+ """A class managing thread-local dicts"""
+ __slots__ = 'key', 'dicts', 'localargs', 'locallock', '__weakref__'
+
+ def __init__(self):
+ # The key used in the Thread objects' attribute dicts.
+ # We keep it a string for speed but make it unlikely to clash with
+ # a "real" attribute.
+ self.key = '_threading_local._localimpl.' + str(id(self))
+ # { id(Thread) -> (ref(Thread), thread-local dict) }
+ self.dicts = {}
+
+ def get_dict(self):
+ """Return the dict for the current thread. Raises KeyError if none
+ defined."""
+ thread = current_thread()
+ return self.dicts[id(thread)][1]
+
+ def create_dict(self):
+ """Create a new dict for the current thread, and return it."""
+ localdict = {}
+ key = self.key
+ thread = current_thread()
+ idt = id(thread)
+ def local_deleted(_, key=key):
+ # When the localimpl is deleted, remove the thread attribute.
+ thread = wrthread()
+ if thread is not None:
+ del thread.__dict__[key]
+ def thread_deleted(_, idt=idt):
+ # When the thread is deleted, remove the local dict.
+ # Note that this is suboptimal if the thread object gets
+ # caught in a reference loop. We would like to be called
+ # as soon as the OS-level thread ends instead.
+ local = wrlocal()
+ if local is not None:
+ dct = local.dicts.pop(idt)
+ wrlocal = ref(self, local_deleted)
+ wrthread = ref(thread, thread_deleted)
+ thread.__dict__[key] = wrlocal
+ self.dicts[idt] = wrthread, localdict
+ return localdict
+
+
+@contextmanager
+def _patch(self):
+ impl = object.__getattribute__(self, '_local__impl')
+ try:
+ dct = impl.get_dict()
+ except KeyError:
+ dct = impl.create_dict()
+ args, kw = impl.localargs
+ self.__init__(*args, **kw)
+ with impl.locallock:
+ object.__setattr__(self, '__dict__', dct)
+ yield
+
+
+class local:
+ __slots__ = '_local__impl', '__dict__'
def __new__(cls, *args, **kw):
- self = object.__new__(cls)
- key = '_local__key', 'thread.local.' + str(id(self))
- object.__setattr__(self, '_local__key', key)
- object.__setattr__(self, '_local__args', (args, kw))
- object.__setattr__(self, '_local__lock', RLock())
-
if (args or kw) and (cls.__init__ is object.__init__):
raise TypeError("Initialization arguments are not supported")
-
+ self = object.__new__(cls)
+ impl = _localimpl()
+ impl.localargs = (args, kw)
+ impl.locallock = RLock()
+ object.__setattr__(self, '_local__impl', impl)
# We need to create the thread dict in anticipation of
# __init__ being called, to make sure we don't call it
# again ourselves.
- dict = object.__getattribute__(self, '__dict__')
- current_thread().__dict__[key] = dict
-
+ impl.create_dict()
return self
-def _patch(self):
- key = object.__getattribute__(self, '_local__key')
- d = current_thread().__dict__.get(key)
- if d is None:
- d = {}
- current_thread().__dict__[key] = d
- object.__setattr__(self, '__dict__', d)
-
- # we have a new instance dict, so call out __init__ if we have
- # one
- cls = type(self)
- if cls.__init__ is not object.__init__:
- args, kw = object.__getattribute__(self, '_local__args')
- cls.__init__(self, *args, **kw)
- else:
- object.__setattr__(self, '__dict__', d)
-
-class local(_localbase):
-
def __getattribute__(self, name):
- lock = object.__getattribute__(self, '_local__lock')
- lock.acquire()
- try:
- _patch(self)
+ with _patch(self):
return object.__getattribute__(self, name)
- finally:
- lock.release()
def __setattr__(self, name, value):
if name == '__dict__':
raise AttributeError(
"%r object attribute '__dict__' is read-only"
% self.__class__.__name__)
- lock = object.__getattribute__(self, '_local__lock')
- lock.acquire()
- try:
- _patch(self)
+ with _patch(self):
return object.__setattr__(self, name, value)
- finally:
- lock.release()
def __delattr__(self, name):
if name == '__dict__':
raise AttributeError(
"%r object attribute '__dict__' is read-only"
% self.__class__.__name__)
- lock = object.__getattribute__(self, '_local__lock')
- lock.acquire()
- try:
- _patch(self)
+ with _patch(self):
return object.__delattr__(self, name)
- finally:
- lock.release()
- def __del__(self):
- import threading
-
- key = object.__getattribute__(self, '_local__key')
-
- try:
- # We use the non-locking API since we might already hold the lock
- # (__del__ can be called at any point by the cyclic GC).
- threads = threading._enumerate()
- except:
- # If enumerating the current threads fails, as it seems to do
- # during shutdown, we'll skip cleanup under the assumption
- # that there is nothing to clean up.
- return
-
- for thread in threads:
- try:
- __dict__ = thread.__dict__
- except AttributeError:
- # Thread is dying, rest in peace.
- continue
-
- if key in __dict__:
- try:
- del __dict__[key]
- except KeyError:
- pass # didn't have anything in this thread
from threading import current_thread, RLock
diff --git a/Lib/test/test_threading_local.py b/Lib/test/test_threading_local.py
index acf37a0..c886a25 100644
--- a/Lib/test/test_threading_local.py
+++ b/Lib/test/test_threading_local.py
@@ -184,11 +184,6 @@
"""To test that subclasses behave properly."""
self._test_dict_attribute(LocalSubclass)
-
-class ThreadLocalTest(unittest.TestCase, BaseLocalTest):
- _local = _thread._local
-
- # Fails for the pure Python implementation
def test_cycle_collection(self):
class X:
pass
@@ -201,6 +196,10 @@
gc.collect()
self.assertIs(wr(), None)
+
+class ThreadLocalTest(unittest.TestCase, BaseLocalTest):
+ _local = _thread._local
+
class PyThreadingLocalTest(unittest.TestCase, BaseLocalTest):
_local = _threading_local.local