Lib/_threading_local.py

   1 """Thread-local objects.
   2
   3 (Note that this module provides a Python version of the threading.local
   4  class.  Depending on the version of Python you're using, there may be a
   5  faster one available.  You should always import the `local` class from
   6  `threading`.)
   7
   8 Thread-local objects support the management of thread-local data.
   9 If you have data that you want to be local to a thread, simply create
  10 a thread-local object and use its attributes:
  11
  12   >>> mydata = local()
  13   >>> mydata.number = 42
  14   >>> mydata.number
  15   42
  16
  17 You can also access the local-object's dictionary:
  18
  19   >>> mydata.__dict__
  20   {'number': 42}
  21   >>> mydata.__dict__.setdefault('widgets', [])
  22   []
  23   >>> mydata.widgets
  24   []
  25
  26 What's important about thread-local objects is that their data are
  27 local to a thread. If we access the data in a different thread:
  28
  29   >>> log = []
  30   >>> def f():
  31   ...     items = mydata.__dict__.items()
  32   ...     items.sort()
  33   ...     log.append(items)
  34   ...     mydata.number = 11
  35   ...     log.append(mydata.number)
  36
  37   >>> import threading
  38   >>> thread = threading.Thread(target=f)
  39   >>> thread.start()
  40   >>> thread.join()
  41   >>> log
  42   [[], 11]
  43
  44 we get different data.  Furthermore, changes made in the other thread
  45 don't affect data seen in this thread:
  46
  47   >>> mydata.number
  48   42
  49
  50 Of course, values you get from a local object, including a __dict__
  51 attribute, are for whatever thread was current at the time the
  52 attribute was read.  For that reason, you generally don't want to save
  53 these values across threads, as they apply only to the thread they
  54 came from.
  55
  56 You can create custom local objects by subclassing the local class:
  57
  58   >>> class MyLocal(local):
  59   ...     number = 2
  60   ...     initialized = False
  61   ...     def __init__(self, **kw):
  62   ...         if self.initialized:
  63   ...             raise SystemError('__init__ called too many times')
  64   ...         self.initialized = True
  65   ...         self.__dict__.update(kw)
  66   ...     def squared(self):
  67   ...         return self.number ** 2
  68
  69 This can be useful to support default values, methods and
  70 initialization.  Note that if you define an __init__ method, it will be
  71 called each time the local object is used in a separate thread.  This
  72 is necessary to initialize each thread's dictionary.
  73
  74 Now if we create a local object:
  75
  76   >>> mydata = MyLocal(color='red')
  77
  78 Now we have a default number:
  79
  80   >>> mydata.number
  81   2
  82
  83 an initial color:
  84
  85   >>> mydata.color
  86   'red'
  87   >>> del mydata.color
  88
  89 And a method that operates on the data:
  90
  91   >>> mydata.squared()
  92   4
  93
  94 As before, we can access the data in a separate thread:
  95
  96   >>> log = []
  97   >>> thread = threading.Thread(target=f)
  98   >>> thread.start()
  99   >>> thread.join()
 100   >>> log
 101   [[('color', 'red'), ('initialized', True)], 11]
 102
 103 without affecting this thread's data:
 104
 105   >>> mydata.number
 106   2
 107   >>> mydata.color
 108   Traceback (most recent call last):
 109   ...
 110   AttributeError: 'MyLocal' object has no attribute 'color'
 111
 112 Note that subclasses can define slots, but they are not thread
 113 local. They are shared across threads:
 114
 115   >>> class MyLocal(local):
 116   ...     __slots__ = 'number'
 117
 118   >>> mydata = MyLocal()
 119   >>> mydata.number = 42
 120   >>> mydata.color = 'red'
 121
 122 So, the separate thread:
 123
 124   >>> thread = threading.Thread(target=f)
 125   >>> thread.start()
 126   >>> thread.join()
 127
 128 affects what we see:
 129
 130   >>> mydata.number
 131   11
 132
 133 >>> del mydata
 134 """
 135
 136 __all__ = ["local"]
 137
 138 # We need to use objects from the threading module, but the threading
 139 # module may also want to use our `local` class, if support for locals
 140 # isn't compiled in to the `thread` module.  This creates potential problems
 141 # with circular imports.  For that reason, we don't import `threading`
 142 # until the bottom of this file (a hack sufficient to worm around the
 143 # potential problems).  Note that almost all platforms do have support for
 144 # locals in the `thread` module, and there is no circular import problem
 145 # then, so problems introduced by fiddling the order of imports here won't
 146 # manifest on most boxes.
 147
 148 class _localbase(object):
 149     __slots__ = '_local__key', '_local__args', '_local__lock'
 150
 151     def __new__(cls, *args, **kw):
 152         self = object.__new__(cls)
 153         key = '_local__key', 'thread.local.' + str(id(self))
 154         object.__setattr__(self, '_local__key', key)
 155         object.__setattr__(self, '_local__args', (args, kw))
 156         object.__setattr__(self, '_local__lock', RLock())
 157
 158         if args or kw and (cls.__init__ is object.__init__):
 159             raise TypeError("Initialization arguments are not supported")
 160
 161         # We need to create the thread dict in anticipation of
 162         # __init__ being called, to make sure we don't call it
 163         # again ourselves.
 164         dict = object.__getattribute__(self, '__dict__')
 165         current_thread().__dict__[key] = dict
 166
 167         return self
 168
 169 def _patch(self):
 170     key = object.__getattribute__(self, '_local__key')
 171     d = current_thread().__dict__.get(key)
 172     if d is None:
 173         d = {}
 174         current_thread().__dict__[key] = d
 175         object.__setattr__(self, '__dict__', d)
 176
 177         # we have a new instance dict, so call out __init__ if we have
 178         # one
 179         cls = type(self)
 180         if cls.__init__ is not object.__init__:
 181             args, kw = object.__getattribute__(self, '_local__args')
 182             cls.__init__(self, *args, **kw)
 183     else:
 184         object.__setattr__(self, '__dict__', d)
 185
 186 class local(_localbase):
 187
 188     def __getattribute__(self, name):
 189         lock = object.__getattribute__(self, '_local__lock')
 190         lock.acquire()
 191         try:
 192             _patch(self)
 193             return object.__getattribute__(self, name)
 194         finally:
 195             lock.release()
 196
 197     def __setattr__(self, name, value):
 198         lock = object.__getattribute__(self, '_local__lock')
 199         lock.acquire()
 200         try:
 201             _patch(self)
 202             return object.__setattr__(self, name, value)
 203         finally:
 204             lock.release()
 205
 206     def __delattr__(self, name):
 207         lock = object.__getattribute__(self, '_local__lock')
 208         lock.acquire()
 209         try:
 210             _patch(self)
 211             return object.__delattr__(self, name)
 212         finally:
 213             lock.release()
 214
 215     def __del__(self):
 216         import threading
 217
 218         key = object.__getattribute__(self, '_local__key')
 219
 220         try:
 221             # We use the non-locking API since we might already hold the lock
 222             # (__del__ can be called at any point by the cyclic GC).
 223             threads = threading._enumerate()
 224         except:
 225             # If enumerating the current threads fails, as it seems to do
 226             # during shutdown, we'll skip cleanup under the assumption
 227             # that there is nothing to clean up.
 228             return
 229
 230         for thread in threads:
 231             try:
 232                 __dict__ = thread.__dict__
 233             except AttributeError:
 234                 # Thread is dying, rest in peace.
 235                 continue
 236
 237             if key in __dict__:
 238                 try:
 239                     del __dict__[key]
 240                 except KeyError:
 241                     pass # didn't have anything in this thread
 242
 243 from threading import current_thread, RLock