[Tutor] Descriptors and type declaration order

[Emile van Sebille]

On 7/14/2011 2:44 AM Knacktus said...
> Hi guys,
>
> I've got the following (not working) code:
>
> class Attribute(object):
>
>      def __init__(self, attribute_name, att_type_name):
>          self._attribute_name = attribute_name
>          try:
>              self._attribute_type = globals()[att_type_name]
>          except KeyError:
>              self._attribute_type = getattr(globals()["__builtins__"],
> att_type_name)
>
>      def __get__(self, obj, obj_type):
>          return getattr(obj, self._attribute_name)
>
>      def __set__(self, obj, value):
>          if isinstance(value, self._attribute_type):
>              setattr(obj, self._attribute_name, value)
>          else:
>              raise ItemTypeException(self._attribute_type, type(value))
>
> class BaseItem(object):
>
>      ident = Attribute("ident", "int")
>      owner = Attribute("owner", "str")
>      item = Attribute("item", "BaseItem")
>
> if __name__ == "__main__":
>      print "About to create an item"
>      test = BaseItem()
>      print "OK"
>
> The problem is that the descriptors are created when the module is
> evaluated. But at this time the class BaseItem is not known yet. Any ideas?


You want to embed an instance of a class as an attribute of the same 
class? Does this help:

class BaseItem(object):

      ident = Attribute("ident", "int")
      owner = Attribute("owner", "str")


BaseItem.item = Attribute("item", "BaseItem")


Emile