[Tutor] thread-safe singleton ?

[Raymond Hettinger]

My first thought is to just use a lock.


Raymond Hettinger


----- Original Message ----- 
From: "Praveen Kumar" <pk1u@yahoo.com>
To: <tutor@python.org>
Sent: Friday, May 31, 2002 6:35 PM
Subject: [Tutor] thread-safe singleton ?


> 
> Are either/both of the below singleton implementations
> thread-safe ? I'm thinking not, b/c I think there are
> cases where multiple threads can execute the statement
> that sets the singleton instance, eg:
> 
> 
>             Singleton.__instance = Singleton.__impl()
> 
>         Singleton.__single = self  
> 
> I'm new to python, so perhaps I'm missing something.
> Thanks,
> 
> pk
> 
> -----
> 
> http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/52558
> 
> class Singleton:
>     """ A python singleton """
> 
>     class __impl:
>         """ Implementation of the singleton interface
> """
> 
>         def spam(self):
>             """ Test method, return singleton id """
>             return id(self)
> 
>     # storage for the instance reference
>     __instance = None
> 
>     def __init__(self):
>         """ Create singleton instance """
>         # Check whether we already have an instance
>         if Singleton.__instance is None:
>             # Create and remember instance
>             Singleton.__instance = Singleton.__impl()
> 
>         # Store instance reference as the only member
> in the handle
>         self.__dict__['_Singleton__instance'] =
> Singleton.__instance
> 
>     def __getattr__(self, attr):
>         """ Delegate access to implementation """
>         return getattr(self.__instance, attr)
> 
>     def __setattr__(self, attr, value):
>         """ Delegate access to implementation """
>         return setattr(self.__instance, attr, value)
> 
> 
> # Test it
> s1 = Singleton()
> print id(s1), s1.spam()
> 
> s2 = Singleton()
> print id(s2), s2.spam()
> 
> # Sample output, the second (inner) id is constant:
> # 8172684 8176268
> # 8168588 8176268
> 
> 
> -----------------------------
> 
> http://www.python.org/workshops/1997-10/proceedings/savikko.html
> 
> class Singleton:
>     __single = None
>     def __init__( self ):
>         if Singleton.__single:
>             raise Singleton.__single
>         Singleton.__single = self  
> 
> def Handle( x = Singleton ):
>     try:
>         single = x()
>     except Singleton, s:
>         single = s
>     return single    
> 
> 
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