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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.
This commit is contained in:
Antoine Pitrou 2010-09-07 22:06:17 +00:00
parent 2673c5bf5e
commit 783eea722b
3 changed files with 84 additions and 84 deletions
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154
Lib/_threading_local.py
View File

@ -132,6 +132,9 @@ affects what we see:
>>> 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 @@ __all__ = ["local"]
# 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

9
Lib/test/test_threading_local.py
View File

@ -184,11 +184,6 @@ class BaseLocalTest:
"""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 @@ class ThreadLocalTest(unittest.TestCase, BaseLocalTest):
gc.collect()
self.assertIs(wr(), None)
class ThreadLocalTest(unittest.TestCase, BaseLocalTest):
_local = _thread._local
class PyThreadingLocalTest(unittest.TestCase, BaseLocalTest):
_local = _threading_local.local

5
Misc/NEWS
View File

@ -13,6 +13,11 @@ Core and Builtins
Library
-------
- 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.
- Issue #6394: os.getppid() is now supported on Windows. Note that it will
still return the id of the parent process after it has exited. This process
id may even have been reused by another unrelated process.