242 lines
6.8 KiB
Python
242 lines
6.8 KiB
Python
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"""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 = mydata.__dict__.items()
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... items.sort()
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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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__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 almost all platforms do have support for
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# 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 on most boxes.
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class _localbase(object):
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__slots__ = '_local__key', '_local__args', '_local__lock'
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def __new__(cls, *args, **kw):
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self = object.__new__(cls)
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key = '_local__key', 'thread.local.' + str(id(self))
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object.__setattr__(self, '_local__key', key)
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object.__setattr__(self, '_local__args', (args, kw))
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object.__setattr__(self, '_local__lock', RLock())
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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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# 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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dict = object.__getattribute__(self, '__dict__')
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currentThread().__dict__[key] = dict
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return self
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def _patch(self):
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key = object.__getattribute__(self, '_local__key')
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d = currentThread().__dict__.get(key)
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if d is None:
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d = {}
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currentThread().__dict__[key] = d
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object.__setattr__(self, '__dict__', d)
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# we have a new instance dict, so call out __init__ if we have
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# one
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cls = type(self)
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if cls.__init__ is not object.__init__:
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args, kw = object.__getattribute__(self, '_local__args')
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cls.__init__(self, *args, **kw)
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else:
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object.__setattr__(self, '__dict__', d)
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class local(_localbase):
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def __getattribute__(self, name):
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lock = object.__getattribute__(self, '_local__lock')
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lock.acquire()
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try:
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_patch(self)
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return object.__getattribute__(self, name)
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finally:
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lock.release()
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def __setattr__(self, name, value):
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lock = object.__getattribute__(self, '_local__lock')
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lock.acquire()
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try:
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_patch(self)
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return object.__setattr__(self, name, value)
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finally:
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lock.release()
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def __delattr__(self, name):
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lock = object.__getattribute__(self, '_local__lock')
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lock.acquire()
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try:
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_patch(self)
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return object.__delattr__(self, name)
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finally:
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lock.release()
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def __del__(self):
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import threading
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key = object.__getattribute__(self, '_local__key')
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try:
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threads = list(threading.enumerate())
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except:
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# If enumerate fails, as it seems to do during
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# shutdown, we'll skip cleanup under the assumption
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# that there is nothing to clean up.
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return
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for thread in threads:
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try:
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__dict__ = thread.__dict__
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except AttributeError:
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# Thread is dying, rest in peace.
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continue
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if key in __dict__:
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try:
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del __dict__[key]
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except KeyError:
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pass # didn't have anything in this thread
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from threading import currentThread, RLock
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