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783eea722b
is friendlier towards reference cycles. This change is not normally visible since an optimized C implementation (_thread._local) is used instead.
246 lines
7.2 KiB
Python
246 lines
7.2 KiB
Python
"""Thread-local objects.
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(Note that this module provides a Python version of the threading.local
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class. Depending on the version of Python you're using, there may be a
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faster one available. You should always import the `local` class from
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`threading`.)
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Thread-local objects support the management of thread-local data.
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If you have data that you want to be local to a thread, simply create
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a thread-local object and use its attributes:
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>>> mydata = local()
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>>> mydata.number = 42
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>>> mydata.number
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42
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You can also access the local-object's dictionary:
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>>> mydata.__dict__
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{'number': 42}
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>>> mydata.__dict__.setdefault('widgets', [])
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[]
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>>> mydata.widgets
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[]
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What's important about thread-local objects is that their data are
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local to a thread. If we access the data in a different thread:
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>>> log = []
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>>> def f():
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... items = sorted(mydata.__dict__.items())
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... log.append(items)
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... mydata.number = 11
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... log.append(mydata.number)
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>>> import threading
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>>> thread = threading.Thread(target=f)
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>>> thread.start()
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>>> thread.join()
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>>> log
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[[], 11]
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we get different data. Furthermore, changes made in the other thread
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don't affect data seen in this thread:
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>>> mydata.number
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42
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Of course, values you get from a local object, including a __dict__
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attribute, are for whatever thread was current at the time the
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attribute was read. For that reason, you generally don't want to save
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these values across threads, as they apply only to the thread they
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came from.
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You can create custom local objects by subclassing the local class:
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>>> class MyLocal(local):
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... number = 2
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... initialized = False
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... def __init__(self, **kw):
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... if self.initialized:
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... raise SystemError('__init__ called too many times')
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... self.initialized = True
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... self.__dict__.update(kw)
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... def squared(self):
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... return self.number ** 2
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This can be useful to support default values, methods and
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initialization. Note that if you define an __init__ method, it will be
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called each time the local object is used in a separate thread. This
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is necessary to initialize each thread's dictionary.
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Now if we create a local object:
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>>> mydata = MyLocal(color='red')
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Now we have a default number:
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>>> mydata.number
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2
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an initial color:
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>>> mydata.color
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'red'
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>>> del mydata.color
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And a method that operates on the data:
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>>> mydata.squared()
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4
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As before, we can access the data in a separate thread:
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>>> log = []
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>>> thread = threading.Thread(target=f)
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>>> thread.start()
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>>> thread.join()
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>>> log
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[[('color', 'red'), ('initialized', True)], 11]
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without affecting this thread's data:
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>>> mydata.number
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2
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>>> mydata.color
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Traceback (most recent call last):
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...
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AttributeError: 'MyLocal' object has no attribute 'color'
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Note that subclasses can define slots, but they are not thread
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local. They are shared across threads:
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>>> class MyLocal(local):
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... __slots__ = 'number'
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>>> mydata = MyLocal()
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>>> mydata.number = 42
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>>> mydata.color = 'red'
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So, the separate thread:
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>>> thread = threading.Thread(target=f)
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>>> thread.start()
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>>> thread.join()
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affects what we see:
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>>> mydata.number
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11
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>>> del mydata
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"""
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from weakref import ref
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from contextlib import contextmanager
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__all__ = ["local"]
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# We need to use objects from the threading module, but the threading
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# module may also want to use our `local` class, if support for locals
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# isn't compiled in to the `thread` module. This creates potential problems
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# with circular imports. For that reason, we don't import `threading`
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# until the bottom of this file (a hack sufficient to worm around the
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# potential problems). Note that all platforms on CPython do have support
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# for locals in the `thread` module, and there is no circular import problem
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# then, so problems introduced by fiddling the order of imports here won't
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# manifest.
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class _localimpl:
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"""A class managing thread-local dicts"""
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__slots__ = 'key', 'dicts', 'localargs', 'locallock', '__weakref__'
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def __init__(self):
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# The key used in the Thread objects' attribute dicts.
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# We keep it a string for speed but make it unlikely to clash with
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# a "real" attribute.
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self.key = '_threading_local._localimpl.' + str(id(self))
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# { id(Thread) -> (ref(Thread), thread-local dict) }
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self.dicts = {}
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def get_dict(self):
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"""Return the dict for the current thread. Raises KeyError if none
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defined."""
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thread = current_thread()
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return self.dicts[id(thread)][1]
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def create_dict(self):
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"""Create a new dict for the current thread, and return it."""
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localdict = {}
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key = self.key
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thread = current_thread()
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idt = id(thread)
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def local_deleted(_, key=key):
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# When the localimpl is deleted, remove the thread attribute.
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thread = wrthread()
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if thread is not None:
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del thread.__dict__[key]
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def thread_deleted(_, idt=idt):
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# When the thread is deleted, remove the local dict.
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# Note that this is suboptimal if the thread object gets
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# caught in a reference loop. We would like to be called
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# as soon as the OS-level thread ends instead.
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local = wrlocal()
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if local is not None:
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dct = local.dicts.pop(idt)
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wrlocal = ref(self, local_deleted)
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wrthread = ref(thread, thread_deleted)
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thread.__dict__[key] = wrlocal
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self.dicts[idt] = wrthread, localdict
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return localdict
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@contextmanager
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def _patch(self):
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impl = object.__getattribute__(self, '_local__impl')
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try:
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dct = impl.get_dict()
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except KeyError:
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dct = impl.create_dict()
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args, kw = impl.localargs
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self.__init__(*args, **kw)
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with impl.locallock:
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object.__setattr__(self, '__dict__', dct)
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yield
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class local:
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__slots__ = '_local__impl', '__dict__'
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def __new__(cls, *args, **kw):
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if (args or kw) and (cls.__init__ is object.__init__):
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raise TypeError("Initialization arguments are not supported")
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self = object.__new__(cls)
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impl = _localimpl()
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impl.localargs = (args, kw)
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impl.locallock = RLock()
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object.__setattr__(self, '_local__impl', impl)
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# We need to create the thread dict in anticipation of
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# __init__ being called, to make sure we don't call it
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# again ourselves.
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impl.create_dict()
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return self
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def __getattribute__(self, name):
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with _patch(self):
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return object.__getattribute__(self, name)
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def __setattr__(self, name, value):
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if name == '__dict__':
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raise AttributeError(
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"%r object attribute '__dict__' is read-only"
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% self.__class__.__name__)
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with _patch(self):
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return object.__setattr__(self, name, value)
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def __delattr__(self, name):
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if name == '__dict__':
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raise AttributeError(
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"%r object attribute '__dict__' is read-only"
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% self.__class__.__name__)
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with _patch(self):
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return object.__delattr__(self, name)
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from threading import current_thread, RLock
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