[Tutor] singleton pattern

[Roeland Rengelink]

alan.gauld@bt.com wrote:
> 

[snip]

> > * the SingletonFactory
> >
> > class SingletonFactory:
> >     singleton = None
> >     def __call__(self):
> >         if SingletonFactory.singleton is None:
> >             SingletonFactory.singleton = Singleton()
> >         return SingletonFactory.singleton
> 
> And this is what I was doing in my second post except
> that I was allowing multiple Factories to be created
> each one managing a singleton instance of the class
> specified in the ctor. But I then moved the actual
> construction of the singleton into the makeInstance
> method and used an exception to pass back the actual
> singleton instance rather than fake access thru'
> the Factory class.
> 
> > >>> SF = SingletonFactory()
> > >>> x = SF()
> > >>> y = SF()
> > >>> id(x) == id(y)
> > 1
> 
> I guess my question is whether it wouyld be easier to
> assign the instance attribute directly to x,y:
> 
> >>> x = SF().instance
> >>> y = SF().instance
> 

that's what 

>>> x = SF()
>>> y = SF()

allready do

Remember SF is an instance of SingletonFactory, SF().instance makes no
sense

x = SF()

is equivalent to

x = SF.__call__()

which is equivalent to

x = SF.singleton    # if SF.singleton != None)

or

SF.singleton = Singleton() # if SF.singleton == None
x = SF.singleton

> And also what happens using this model if we create more than one
> SignletonFactory instance:
> 
> SF = SingletonFactorty()
> MF = SingletonFactorty()
> x = MF()
> y = SF()
> 

SF.singleton == MF.singleton

That's why I assigned to SingletonFactory.singleton, rather than
self.singleton

SingletonFactory in this sense behaves like a singleton itself (i.e.
different ids, but identical state)

> are x and y now pointing at the same object?
> 

yes


Have fun,

Roeland

-- 
r.b.rigilink@chello.nl

"Half of what I say is nonsense. Unfortunately I don't know which half"