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             threads = list(threading.enumerate())

   222         except:

   223             # If enumerate fails, as it seems to do during

   224             # shutdown, we'll skip cleanup under the assumption

   225             # that there is nothing to clean up.

   226             return

   227 

   228         for thread in threads:

   229             try:

   230                 __dict__ = thread.__dict__

   231             except AttributeError:

   232                 # Thread is dying, rest in peace.

   233                 continue

   234 

   235             if key in __dict__:

   236                 try:

   237                     del __dict__[key]

   238                 except KeyError:

   239                     pass # didn't have anything in this thread

   240 

   241 from threading import current_thread, RLock