Singleton implementation problems

[George Sakkis]

On Jul 3, 6:58 pm, Urizev <uri... at gmail.com> wrote:

> Hi everyone
>
> I have developed the singleton implementation. However I have found a
> strange behaviour when using from different files. The code is
> attached.
>
> Executing main
> new MySet object
> No singleton instance
> New singleton:
> <__main__.Singleton instance at 0x2b98be474a70>
> new MySet object
> There is a singlenton instance
> new Member
> new MySet object
> No singleton instance
> New singleton:
> <myset.Singleton instance at 0x2b98be474d88>
> new Member
> new MySet object
> There is a singlenton instance
> new Member
> new MySet object
> There is a singlenton instance
>
> I do not know why, but it creates two instances of the singleton. Does
> anybody know why?

Because __init__() is called to initialize the state of an object
*after* it has already been created. You should create a "new-style"
class and define __new__() instead. Here's a working version:

class Singleton(object):   # new-style class
    def __new__(cls, *args, **kwds):
        # return the singleton (if already created)
        try: return cls.__dict__['__singleton']
        except KeyError: # raised only the first time for a given
class
            # create the singleton and store it to the class namespace
            singleton = object.__new__(cls, *args, **kwds)
            setattr(cls, '__singleton', singleton)
            return singleton

class X(Singleton):
   def __init__(self, a): self.a = a

assert X(1) is X(2)


Note however that the classic Singleton pattern is usually frowned
upon in Python; the preferred approach is to use (module level)
globals. Also search for the "Borg pattern".

George