django/utils/_threading_local.py

   1 """Thread-local objects
   2
   3 (Note that this module provides a Python version of thread
   4  threading.local class.  Depending on the version of Python you're
   5  using, there may be a faster one available.  You should always import
   6  the local class from 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 # Threading import is at end
 137
 138 class _localbase(object):
 139     __slots__ = '_local__key', '_local__args', '_local__lock'
 140
 141     def __new__(cls, *args, **kw):
 142         self = object.__new__(cls)
 143         key = '_local__key', 'thread.local.' + str(id(self))
 144         object.__setattr__(self, '_local__key', key)
 145         object.__setattr__(self, '_local__args', (args, kw))
 146         object.__setattr__(self, '_local__lock', RLock())
 147
 148         if args or kw and (cls.__init__ is object.__init__):
 149             raise TypeError("Initialization arguments are not supported")
 150
 151         # We need to create the thread dict in anticipation of
 152         # __init__ being called, to make sure we don't call it
 153         # again ourselves.
 154         dict = object.__getattribute__(self, '__dict__')
 155         currentThread().__dict__[key] = dict
 156
 157         return self
 158
 159 def _patch(self):
 160     key = object.__getattribute__(self, '_local__key')
 161     d = currentThread().__dict__.get(key)
 162     if d is None:
 163         d = {}
 164         currentThread().__dict__[key] = d
 165         object.__setattr__(self, '__dict__', d)
 166
 167         # we have a new instance dict, so call out __init__ if we have
 168         # one
 169         cls = type(self)
 170         if cls.__init__ is not object.__init__:
 171             args, kw = object.__getattribute__(self, '_local__args')
 172             cls.__init__(self, *args, **kw)
 173     else:
 174         object.__setattr__(self, '__dict__', d)
 175
 176 class local(_localbase):
 177
 178     def __getattribute__(self, name):
 179         lock = object.__getattribute__(self, '_local__lock')
 180         lock.acquire()
 181         try:
 182             _patch(self)
 183             return object.__getattribute__(self, name)
 184         finally:
 185             lock.release()
 186
 187     def __setattr__(self, name, value):
 188         lock = object.__getattribute__(self, '_local__lock')
 189         lock.acquire()
 190         try:
 191             _patch(self)
 192             return object.__setattr__(self, name, value)
 193         finally:
 194             lock.release()
 195
 196     def __delattr__(self, name):
 197         lock = object.__getattribute__(self, '_local__lock')
 198         lock.acquire()
 199         try:
 200             _patch(self)
 201             return object.__delattr__(self, name)
 202         finally:
 203             lock.release()
 204
 205
 206     def __del__():
 207         threading_enumerate = enumerate
 208         __getattribute__ = object.__getattribute__
 209
 210         def __del__(self):
 211             key = __getattribute__(self, '_local__key')
 212
 213             try:
 214                 threads = list(threading_enumerate())
 215             except:
 216                 # if enumerate fails, as it seems to do during
 217                 # shutdown, we'll skip cleanup under the assumption
 218                 # that there is nothing to clean up
 219                 return
 220
 221             for thread in threads:
 222                 try:
 223                     __dict__ = thread.__dict__
 224                 except AttributeError:
 225                     # Thread is dying, rest in peace
 226                     continue
 227
 228                 if key in __dict__:
 229                     try:
 230                         del __dict__[key]
 231                     except KeyError:
 232                         pass # didn't have anything in this thread
 233
 234         return __del__
 235     __del__ = __del__()
 236
 237 try:
 238     from threading import currentThread, enumerate, RLock
 239 except ImportError:
 240     from dummy_threading import currentThread, enumerate, RLock