"""Thread-local objects.
(Note that this module provides a Python version of the threading.local
class. Depending on the version of Python you're using, there may be a
faster one available. You should always import the `local` class from
`threading`.)
"""
from weakref import ref
from contextlib import contextmanager
__all__ = ["local"]
# We need to use objects from the threading module, but the threading
# module may also want to use our `local` class, if support for locals
# 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 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.
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):
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.
impl.create_dict()
return self
def __getattribute__(self, name):
with _patch(self):
return object.__getattribute__(self, name)
def __setattr__(self, name, value):
if name == '__dict__':
raise AttributeError(
"%r object attribute '__dict__' is read-only"
% self.__class__.__name__)
with _patch(self):
return object.__setattr__(self, name, value)
def __delattr__(self, name):
if name == '__dict__':
raise AttributeError(
"%r object attribute '__dict__' is read-only"
% self.__class__.__name__)
with _patch(self):
return object.__delattr__(self, name)
from threading import current_thread, RLock
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