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#
# Copyright (c) 2010 Intel Corporation
#
# Permission is hereby granted, free of charge, to any person obtaining a
# copy of this software and associated documentation files (the "Software"),
# to deal in the Software without restriction, including without limitation
# the rights to use, copy, modify, merge, publish, distribute, sublicense,
# and/or sell copies of the Software, and to permit persons to whom the
# Software is furnished to do so, subject to the following conditions:
#
# The above copyright notice and this permission notice (including the next
# paragraph) shall be included in all copies or substantial portions of the
# Software.
#
# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
# THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
# FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
# IN THE SOFTWARE.
#
import threading
class Singleton(object):
'''
Modeled after http://www.python.org/download/releases/2.2.3/descrintro/*__new__
A thread-safe (mostly -- see NOTE) Singleton class pattern.
NOTE: deleting a singleton instance (i.e. Singleton::delInstance) does not guarantee that something
else is currently using it. To reduce this risk, a program should not hold a reference to the
instance. Rather, use the create/construct syntax (see example below) to access the instance. Yet,
this still does not guarantee that this type of usage will result in a desired effect in a
multithreaded program.
You've been warned so use the singleton pattern wisely!
Example:
class MySingletonClass(Singleton):
def init(self):
print "in MySingletonClass::init()", self
def foo(self):
print "in MySingletonClass::foo()", self
MySingletonClass().foo()
MySingletonClass().foo()
MySingletonClass().foo()
---> output will look something like this:
in MySingletonClass::init() <__main__.MySingletonClass object at 0x7ff5b322f3d0>
in MySingletonClass::foo() <__main__.MySingletonClass object at 0x7ff5b322f3d0>
in MySingletonClass::foo() <__main__.MySingletonClass object at 0x7ff5b322f3d0>
in MySingletonClass::foo() <__main__.MySingletonClass object at 0x7ff5b322f3d0>
'''
lock = threading.RLock()
def __new__(cls, *args, **kwargs):
try:
cls.lock.acquire()
it = cls.__dict__.get('__it__')
if it is not None:
return it
cls.__it__ = it = object.__new__(cls)
it.init(*args, **kwargs)
return it
finally: # this always gets called, even when returning from within the try block
cls.lock.release()
def init(self, *args, **kwargs):
'''
Derived classes should override this method to do its initializations
The derived class should not implement a '__init__' method.
'''
pass
@classmethod
def delInstance(cls):
cls.lock.acquire()
try:
if cls.__dict__.get('__it__') is not None:
del cls.__it__
finally:
cls.lock.release()
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