Is it possible to call a class but without a new instance created?

[Vincent Vande Vyvre]

Le 18/06/18 à 06:48, Jach Fong a écrit :
> After looking into the \tkiniter\font.py source file, triggered by Jim's
> hint on my previous subject "Why an object changes its "address" between
> adjacent calls?", I get more confused.
>
> Below was quoted from the font.py:
> ------------------------
> def nametofont(name):
>     """Given the name of a tk named font, returns a Font representation.
>     """
>     return Font(name=name, exists=True)
>
> class Font:
>     """Represents a named font.
>     Constructor options are:
>     ...
>     exists -- does a named font by this name already exist?
>        Creates a new named font if False, points to the existing font 
> if True.
>     ...
>     """
>
>     def __init__(self, root=None, font=None, name=None, exists=False,
>                  **options):
>         ...
> ----------------------
> From my understanding, the __init__ method was called after the instance
> was created, that's why the __init__ has a self parameter, right? Then,
> how is it possible "...points to the existing font if True"? I didn't
> see any clue in __init__ doing this.
>
> I also make a test of my own and it fails too.
>
> >>> class A:
> ...     objs = []
> ...     def __init__(self, exists=False):
> ...             if exists:  self = self.objs[0]
> ...             else:  self.objs.append(self)
> ...
> >>> a0 = A()
> >>> id(a0)
> 35569968
> >>> a1 = A(exists=True)
> >>> id(a1)
> 35572336
>
> What I expect is that id(a0) and id(a1) has the same value. They 
> should points to the same object.
>
>
> Best Regards,
> Jach Fong
>
>
>
>
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Hi,


What you try to do is called a /singleton./

A classic example :

class Foo:
     _instance = None
     def __new__(cls, *args, **kwargs):
         if cls._instance is None:
             cls._instance = super(Foo, cls).__new__(cls, *args, **kwargs)
         return cls._instance

     def __init__(self, ...):

         ...


Vincent

Send at Mon, 18 Jun 2018 09:17:21 +0200