On 17 sep, 19:10, <[EMAIL PROTECTED]> wrote:
> I'd like some advices from more experienced python users. I want to
> have a class in my python application that does not allow multiple
> instance with same "name".
>
> I want to have a class (call it kls) that behave at least following way:
>
> 1) New instance has to have a property called 'name'
> 2) When instance is attemped to created, e.g., x=kls(name='myname'), and
> there already exists an instance with obj.name =='myname', that
> pre-existing instance is returned, instead of making new one.
For all of this I would use the __new__ method.
> 3) A class property 'all' for class gives me a list of all the
> instances. So kls.all lets me iterates through all instances.
> 4) When all the hard-link to an instance is deleted, the instance should
> be deleted, just like an instance from any regular class does.
I'd store a WeakValueDictionary using the name as a key. Instead of a
property 'all', I'd use a method 'get_all_instances()' that should
call values() on the weak dictionary.
> My question is if i can find metaclass or something that allows me to
> create such class easily. I tried to search such thing on internet, but
> I even don't know how to call this kind of thing. It is in a sense like
> singleton, but less restrictive.
Have you tried the Python Cookbook?
> Assuming that I have to write it on my own, what should I do? I tried
> to implement it using weakref.WeakValueDictionary and metaclass, but
> instance doesn't disappear when I think it should disappear. I am also
> wondering if it is easier to keeping {name:id(obj)} would be a better
> solution.
A WeakValueDictionary should work. If the instance is not deleted when
you expect, maybe there are other references somewhere, or the garbage
collector didn't run yet.
Ok, finally I wrote an example:
py> from weakref import WeakValueDictionary
py>
py> class OneOfAKind(object):
... _instances = WeakValueDictionary()
... def __new__(cls, name):
... inst = cls._instances.get(name, None)
... if inst is None:
... inst = cls._instances[name] = super(OneOfAKind,
cls).__new__(cls
)
... inst.name = name
... return inst
... @classmethod
... def get_all_instances(cls):
... return cls._instances.values()
... __str__ = __repr__ = lambda self: '%s(%r)' %
(self.__class__.__name__, s
elf.name)
...
py> a = OneOfAKind('A')
py> b = OneOfAKind('A')
py> assert b is a
py> c = OneOfAKind('C')
py> assert c is not a
py> print OneOfAKind.get_all_instances()
[OneOfAKind('A'), OneOfAKind('C')]
py> del c
py> import gc
py> gc.collect()
17
py> print OneOfAKind.get_all_instances()
[OneOfAKind('A')]
py> del a
py> del b
py> gc.collect()
0
py> print OneOfAKind.get_all_instances()
[]
py>
--
Gabriel Genellina
--
http://mail.python.org/mailman/listinfo/python-list
