On Tue, 15 Mar 2005 20:21:19 +0100, bruno modulix <[EMAIL PROTECTED]> wrote:
>Hi
>
>How can I make a *class* attribute read-only ?
>
>The answer must be pretty obvious but I just can't find it (it's late
>and I've spent all day on metaclasses, descriptors and the like, which,
>as fun as it is, may have side-effects on intellectual abilities...)
>
>*The context:*
>
># library code
>class AbstractBaseClass(object):
> # snip some stuff here,
> # a part of it depending on the derived classes
> # defining class attribute class_private_attrib
>
># client code
>class SubClass(AbstractBaseClass):
> class_private_attrib = "my private attrib"
> # snip
>
>
>*What I'm looking for: (if possible)*
>
> >>SubClass.class_private_attrib
>"my private attrib"
> >>SubClass.class_private_attrib = "toto"
>AttributeError : SubClass.class_private_attrib is read only
> >>s = SubClass()
> >>s.class_private_attribute = "toto"
>AttributeError : SubClass.class_private_attrib is read only
>
>*What I've tried: (simplified as possible)*
>
>class ReadOnlyDescriptor(object):
> def __init__(self, name, initval=None):
> self._val = initval
> self._name = name
>
> def __get__(self, obj, objtype):
> print 'Retrieving', self._name
> return self._val
>
> def __set__(self, obj, val):
> raise AttributeError, \
> "%s.%s is ReadOnly" % (obj.__class.__.__name__, self._name)
>
>class SubClass(object):
> class_private_attrib = ReadOnlyDescriptor("class_private_attrib",
> "my private attrib")
> # snip
>
>*What i get:*
> >>SubClass.class_private_attrib
>Retrieving class_private_attrib
>"my private attrib"
> >>SubClass.class_private_attrib = "toto"
> >>SubClass.class_private_attrib
>"toto"
> >>SubClass.__dict__['class_private_attrib']
>"toto"
> >> s = SubClass()
> >> s.class_private_attrib
>"toto" # of course :(
>
>*What I understand:*
>Ok, I've re-read the manual, noticed that data descriptors __set__()
>method was only called when an instance attribute is set (which is
>obvious from the prototypes of the methods). My solution is plain wrong
>and I should have guess without ever trying. duh :(
>
>
>Now please have mercy, you Noble Pythoneers : what's the trick to
>prevent client code to accidentally mess with the class's dict ? (most
>client code - apart from subclass definitions - shouldn't even bother
>about the existence of this attribute, it's there for the library
>internal usage)
>
>NB : in the real code I'm also messing with the AbstractBaseClass's
>meta_class for other stuff (so it's not a problem if the solution
>involves metaclasses), but I've tested with the simplified example
>above, which exhibits the same problem.
>
Does this help, or did I misunderstand?
>>> class Base(object):
... class __metaclass__(type):
... def __setattr__(cls, name, value):
... raise AttributeError, 'setting %r to %r not allowed' %(name,
value)
...
>>> class Sub(Base):
... def m(self): print 'method m called'
... x = 123
...
>>> obj = Sub()
Instance attributes work normally:
>>> obj.x
123
>>> obj.x = 456
>>> obj.x
456
>>> del obj.x
If not shadowed, the class var is found
>>> Sub.x
123
But it is read-only:
>>> Sub.x = 456
Traceback (most recent call last):
File "<stdin>", line 1, in ?
File "<stdin>", line 4, in __setattr__
AttributeError: setting 'x' to 456 not allowed
>>> Base.x = 456
Traceback (most recent call last):
File "<stdin>", line 1, in ?
File "<stdin>", line 4, in __setattr__
AttributeError: setting 'x' to 456 not allowed
>>> Sub.anything = 'something'
Traceback (most recent call last):
File "<stdin>", line 1, in ?
File "<stdin>", line 4, in __setattr__
AttributeError: setting 'anything' to 'something' not allowed
Regards,
Bengt Richter
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