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""" Implements a singleton mixin class """
# The following code was written by Gary Robinson
# (grobinson@transpose.com) and placed in the public domain. His
# copyright notice is as follows:
# By Gary Robinson, grobinson@transpose.com. No rights reserved --
# placed in the public domain -- which is only reasonable considering
# how much it owes to other people's version which are in the
# public domain. The idea of using a metaclass came from
# a comment on Gary's blog (see
# http://www.garyrobinson.net/2004/03/python_singleto.html#comments).
# Other improvements came from comments and email from other
# people who saw it online. (See the blog post and comments
# for further credits.)
# Not guaranteed to be fit for any particular purpose. Use at your
# own risk.
class SingletonException(Exception):
pass
class MetaSingleton(type):
def __new__(metaclass, strName, tupBases, dict):
if dict.has_key('__new__'):
raise SingletonException, 'Can not override __new__ in a Singleton'
return super(MetaSingleton,metaclass).__new__(metaclass, strName, tupBases, dict)
def __call__(cls, *lstArgs, **dictArgs):
raise SingletonException, 'Singletons may only be instantiated through getInstance()'
class Singleton(object):
__metaclass__ = MetaSingleton
def getInstance(cls, *lstArgs):
"""
Call this to instantiate an instance or retrieve the existing instance.
If the singleton requires args to be instantiated, include them the first
time you call getInstance.
"""
if cls._isInstantiated():
if len(lstArgs) != 0:
raise SingletonException, 'If no supplied args, singleton must already be instantiated, or __init__ must require no args'
else:
if cls._getConstructionArgCountNotCountingSelf() > 0 and len(lstArgs) <= 0:
raise SingletonException, 'If the singleton requires __init__ args, supply them on first instantiation'
instance = cls.__new__(cls)
instance.__init__(*lstArgs)
cls.cInstance = instance
return cls.cInstance
getInstance = classmethod(getInstance)
def _isInstantiated(cls):
return hasattr(cls, 'cInstance')
_isInstantiated = classmethod(_isInstantiated)
def _getConstructionArgCountNotCountingSelf(cls):
return cls.__init__.im_func.func_code.co_argcount - 1
_getConstructionArgCountNotCountingSelf = classmethod(_getConstructionArgCountNotCountingSelf)
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