[Tutor] What is __weakref__ ?

[Karim]

Hello Steven,

> (1) slots = [] doesn't do anything special. You have misspelled 
> __slots__.
Yes sorry my mistake :-[


> (2) Classes don't become read only just because you add __slots__ to 
> them. All you prevent is adding extra attributes, and why would you 
> wish to do that?

I know this is ugly but until now it is the only way (with this side 
effect) I found to declare Enums class that I _understand_:

*class CategoryType(object):
     """Implements the enumeration and prevent associated newly created
     instances to redefine enums value via special class variable 
'__slots__'
     definition. It makes also, instances have no attributes at all.
     """
     __slots__ = ()

     TRANSISTOR, MOS, BIPOLAR, CAPACITOR, RESISTOR, DIODE, ESD, PAD, \
     COMPARATOR, OPAMP, ADC, DAC, PLL, OTHER = range(14)

     def toString ( self, litteral ):
         """Convert the litteral integer number to its associated string 
representation."""
         if litteral == self.TRANSISTOR:
           return 'transistor'
         elif litteral == self.MOS:
           return 'mos'
         elif litteral == self.BIPOLAR:*
[...]

I discovered recently that I could configure __setattr__(self, name, 
value) by raising AttributeError to prevent to set up a new attribute value.
Or use an conditional statement to avoid to define it twice.
If you have good advice for enum (I hope I could understand) you welcome 
;-) .

Thus I use __slots__, __dict__ (in __setattr__) and I wanted to about 
much for __weakref__
> Have you read the Fine Manual?
>
> http://docs.python.org/reference/datamodel.html#slots
>

In fact, I already read that. But I am searching for doc about what is a 
weak reference.

>
> >>> class K1(object):
> ...     __slots__ = []  # Don't support weak-refs.
> ...
> >>> class K2(object):
> ...    __slots__ = '__weakref__'  # Do support weak-refs.
> ...
> >>> import weakref
> >>> k1 = K1()
> >>> k2 = K2()
> >>> weakref.ref(k1)
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
> TypeError: cannot create weak reference to 'K1' object
> >>> weakref.ref(k2)
> <weakref at 0xb7c6661c; to 'K2' at 0xb7ee762c>

Same thing I don't know what to do with this object weakref :-[ as I 
don't know its meaning. That was my true question in fact;

Regards

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