Mercurial > cpython-lru_cache-weakref

annotate Lib/_threading_local.py @ 87690:66c1c9f32567 default tip

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Add use_weakref option to functools.lru_cache The use_weakref option makes lru_cache create weak references to the function arguments when creating the cache key, which prevents them from staying in the memory forever even after all other references to them are gone. This is a proof of concept for http://bugs.python.org/issue19859
author Radomir Dopieralski <devel@sheep.art.pl>
date Tue, 03 Dec 2013 16:02:48 +0100
parents 3c9232a684ae
children
Ignore whitespace changes - Everywhere: Within whitespace: At end of lines:
rev   line source
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1 """Thread-local objects.
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2
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3 (Note that this module provides a Python version of the threading.local
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4 class. Depending on the version of Python you're using, there may be a
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5 faster one available. You should always import the `local` class from
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6 `threading`.)
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7
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8 Thread-local objects support the management of thread-local data.
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9 If you have data that you want to be local to a thread, simply create
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10 a thread-local object and use its attributes:
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11
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12 >>> mydata = local()
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13 >>> mydata.number = 42
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14 >>> mydata.number
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15 42
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16
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17 You can also access the local-object's dictionary:
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18
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19 >>> mydata.__dict__
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20 {'number': 42}
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21 >>> mydata.__dict__.setdefault('widgets', [])
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22 []
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23 >>> mydata.widgets
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24 []
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25
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26 What's important about thread-local objects is that their data are
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27 local to a thread. If we access the data in a different thread:
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28
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29 >>> log = []
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30 >>> def f():
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31 ... items = sorted(mydata.__dict__.items())
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32 ... log.append(items)
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33 ... mydata.number = 11
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34 ... log.append(mydata.number)
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35
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36 >>> import threading
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37 >>> thread = threading.Thread(target=f)
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38 >>> thread.start()
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39 >>> thread.join()
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40 >>> log
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41 [[], 11]
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42
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43 we get different data. Furthermore, changes made in the other thread
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44 don't affect data seen in this thread:
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45
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46 >>> mydata.number
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47 42
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48
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49 Of course, values you get from a local object, including a __dict__
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50 attribute, are for whatever thread was current at the time the
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51 attribute was read. For that reason, you generally don't want to save
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52 these values across threads, as they apply only to the thread they
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53 came from.
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54
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55 You can create custom local objects by subclassing the local class:
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56
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57 >>> class MyLocal(local):
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58 ... number = 2
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59 ... initialized = False
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60 ... def __init__(self, **kw):
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61 ... if self.initialized:
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62 ... raise SystemError('__init__ called too many times')
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63 ... self.initialized = True
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64 ... self.__dict__.update(kw)
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65 ... def squared(self):
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66 ... return self.number ** 2
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67
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68 This can be useful to support default values, methods and
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69 initialization. Note that if you define an __init__ method, it will be
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70 called each time the local object is used in a separate thread. This
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71 is necessary to initialize each thread's dictionary.
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72
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73 Now if we create a local object:
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74
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75 >>> mydata = MyLocal(color='red')
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76
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77 Now we have a default number:
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78
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79 >>> mydata.number
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80 2
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81
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82 an initial color:
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83
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84 >>> mydata.color
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85 'red'
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86 >>> del mydata.color
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87
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88 And a method that operates on the data:
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89
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90 >>> mydata.squared()
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91 4
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92
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93 As before, we can access the data in a separate thread:
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94
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95 >>> log = []
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96 >>> thread = threading.Thread(target=f)
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97 >>> thread.start()
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98 >>> thread.join()
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99 >>> log
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100 [[('color', 'red'), ('initialized', True)], 11]
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101
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102 without affecting this thread's data:
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103
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104 >>> mydata.number
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105 2
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106 >>> mydata.color
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107 Traceback (most recent call last):
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108 ...
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109 AttributeError: 'MyLocal' object has no attribute 'color'
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110
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111 Note that subclasses can define slots, but they are not thread
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112 local. They are shared across threads:
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113
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114 >>> class MyLocal(local):
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115 ... __slots__ = 'number'
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116
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117 >>> mydata = MyLocal()
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118 >>> mydata.number = 42
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119 >>> mydata.color = 'red'
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120
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121 So, the separate thread:
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122
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123 >>> thread = threading.Thread(target=f)
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124 >>> thread.start()
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125 >>> thread.join()
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126
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127 affects what we see:
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128
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129 >>> mydata.number
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130 11
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131
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132 >>> del mydata
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133 """
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134
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135 from weakref import ref
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136 from contextlib import contextmanager
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137
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138 __all__ = ["local"]
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139
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140 # We need to use objects from the threading module, but the threading
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141 # module may also want to use our `local` class, if support for locals
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142 # isn't compiled in to the `thread` module. This creates potential problems
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143 # with circular imports. For that reason, we don't import `threading`
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144 # until the bottom of this file (a hack sufficient to worm around the
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145 # potential problems). Note that all platforms on CPython do have support
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146 # for locals in the `thread` module, and there is no circular import problem
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147 # then, so problems introduced by fiddling the order of imports here won't
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148 # manifest.
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149
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150 class _localimpl:
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151 """A class managing thread-local dicts"""
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152 __slots__ = 'key', 'dicts', 'localargs', 'locallock', '__weakref__'
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153
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154 def __init__(self):
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155 # The key used in the Thread objects' attribute dicts.
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156 # We keep it a string for speed but make it unlikely to clash with
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157 # a "real" attribute.
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158 self.key = '_threading_local._localimpl.' + str(id(self))
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159 # { id(Thread) -> (ref(Thread), thread-local dict) }
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160 self.dicts = {}
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161
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162 def get_dict(self):
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163 """Return the dict for the current thread. Raises KeyError if none
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164 defined."""
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165 thread = current_thread()
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166 return self.dicts[id(thread)][1]
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167
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168 def create_dict(self):
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169 """Create a new dict for the current thread, and return it."""
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170 localdict = {}
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171 key = self.key
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172 thread = current_thread()
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173 idt = id(thread)
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174 def local_deleted(_, key=key):
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175 # When the localimpl is deleted, remove the thread attribute.
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176 thread = wrthread()
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177 if thread is not None:
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178 del thread.__dict__[key]
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179 def thread_deleted(_, idt=idt):
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180 # When the thread is deleted, remove the local dict.
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181 # Note that this is suboptimal if the thread object gets
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182 # caught in a reference loop. We would like to be called
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183 # as soon as the OS-level thread ends instead.
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184 local = wrlocal()
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185 if local is not None:
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186 dct = local.dicts.pop(idt)
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187 wrlocal = ref(self, local_deleted)
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188 wrthread = ref(thread, thread_deleted)
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189 thread.__dict__[key] = wrlocal
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190 self.dicts[idt] = wrthread, localdict
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191 return localdict
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192
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193
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194 @contextmanager
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195 def _patch(self):
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196 impl = object.__getattribute__(self, '_local__impl')
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197 try:
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198 dct = impl.get_dict()
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199 except KeyError:
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200 dct = impl.create_dict()
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201 args, kw = impl.localargs
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202 self.__init__(*args, **kw)
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203 with impl.locallock:
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204 object.__setattr__(self, '__dict__', dct)
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205 yield
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206
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207
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208 class local:
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209 __slots__ = '_local__impl', '__dict__'
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210
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211 def __new__(cls, *args, **kw):
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212 if (args or kw) and (cls.__init__ is object.__init__):
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213 raise TypeError("Initialization arguments are not supported")
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214 self = object.__new__(cls)
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215 impl = _localimpl()
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216 impl.localargs = (args, kw)
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217 impl.locallock = RLock()
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218 object.__setattr__(self, '_local__impl', impl)
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219 # We need to create the thread dict in anticipation of
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220 # __init__ being called, to make sure we don't call it
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221 # again ourselves.
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222 impl.create_dict()
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223 return self
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224
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225 def __getattribute__(self, name):
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226 with _patch(self):
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227 return object.__getattribute__(self, name)
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228
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229 def __setattr__(self, name, value):
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230 if name == '__dict__':
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231 raise AttributeError(
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232 "%r object attribute '__dict__' is read-only"
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233 % self.__class__.__name__)
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234 with _patch(self):
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235 return object.__setattr__(self, name, value)
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236
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237 def __delattr__(self, name):
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238 if name == '__dict__':
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239 raise AttributeError(
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240 "%r object attribute '__dict__' is read-only"
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241 % self.__class__.__name__)
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242 with _patch(self):
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243 return object.__delattr__(self, name)
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244
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245
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246 from threading import current_thread, RLock