Bengt Richter wrote:
...
>
> class Foo(object):
> class __metaclass__(type):
> def __setattr__(cls, name, value):
> if type(cls.__dict__.get(name)).__name__ == 'Descriptor':
> raise AttributeError, 'setting Foo.%s to %r is not allowed'
> %(name, value)
> type.__setattr__(cls, name, value)
> @classproperty
> def TheAnswer(cls):
> return "The Answer according to %s is 42" % cls.__name__
> @classproperty
> def AnotherAnswer(cls):
> return "Another Answer according to %s is 43" % cls.__name__
>
or, simply put the read-only descriptor in the metaclass:
Python 2.4 (#60, Nov 30 2004, 11:49:19) [MSC v.1310 32 bit (Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> def classproperty(function):
... class Descriptor(object):
... def __get__(self, obj, objtype):
... return function(objtype)
... def __set__(self, obj, value):
... raise AttributeError, "can't set class attribute"
... return Descriptor()
...
>>> class A(object):
... class __metaclass__(type):
... @classproperty
... def TheAnswer(cls):
... return "The Answer according to %s is 42" % cls.__name__
...
>>> A.TheAnswer
'The Answer according to __metaclass__ is 42'
>>> A.TheAnswer = 3
Traceback (most recent call last):
File "<input>", line 1, in ?
File "<input>", line 6, in __set__
AttributeError: can't set class attribute
>>> class B(A): pass
...
>>> B.TheAnswer
'The Answer according to __metaclass__ is 42'
>>>
this means that the getter doesn't automatically get a reference to the class
(since it is a method of metaclass), which may or may not matter, depending on
the application
Michael
--
http://mail.python.org/mailman/listinfo/python-list
