property collections

[gzeljko]

If you insist on this interface:

default = -1

class PC:
  def __init__(self):
    self.__dict__["index"] = default
  def __getitem__(self, item):
    self.__dict__["index"] = item
    return self
  def __getattr__(self, attr):
    index = self.index
    self.__dict__["index"] = default
    if self.__dict__.has_key((index, attr)):
      return self.__dict__[(index, attr)]
    return self.__dict__[(default, attr)]
  def __setattr__(self, attr, value):
    index = self.index
    self.__dict__["index"] = default
    self.__dict__[(index, attr)] = value

gzeljko

----- Original Message ----- 
From: Robin Becker <robin at jessikat.fsnet.co.uk>
Newsgroups: comp.lang.python
To: <python-list at python.org>
Sent: Monday, May 21, 2001 11:48 PM
Subject: Re: property collections


> In article <6qlmnqfsrn.fsf at abnoba.intevation.de>, Bernhard Herzog
> <bh at intevation.de> writes
> >Robin Becker <robin at jessikat.fsnet.co.uk> writes:
> ...
> 
> 
> This looks more interesting. The problem for me is that the actual items
> are of a particular PropertyHolder class, but I think it's possible to
> create these on the fly.
> 
> Thanks for the code.
> 
> >This seems to work without cyclic references:
> >
> >class ItemProxy:
> >
> >    def __init__(self, parent, index):
> >        self.__dict__["_parent"] = parent
> >        self.__dict__["_index"] = index
> >
> >    def __getattr__(self, attr):
> >        return self._parent.get_attribute(self._index, attr)
> >
> >    def __setattr__(self, attr, value):
> >        self._parent.set_attribute(self._index, attr, value)
> >
> >class Parent:
> >
> >    def __init__(self):
> >        self.items = {}
> >
> >    def __getitem__(self, index):
> >        return ItemProxy(self, index)
> >
> >    def set_attribute(self, index, attr, value):
> >        item = self.items.get(index)
> >        if item is None:
> >            item = self.items[index] = {}
> >        item[attr] = value
> >
> >    def get_attribute(self, index, attr):
> >        item = self.items.get(index)
> >        if item is not None and item.has_key(attr):
> >            return item[attr]
> >        return getattr(self, attr)
> >
> >
> >Sample session:
> >
> >$ python1.5 -i itemproxy.py 
> >>>> p = Parent()
> >>>> p.a = 1
> >>>> p.b = 2
> >>>> p[1].a = 0.5
> >>>> p[1].a
> >0.5
> >>>> p[1].b
> >2
> >>>> p[2].a
> >1
> >>>> p[2].b
> >2
> >>>> 
> >
> >
> >The trick here is that the parent does not have any references to the
> >objects returned by the __getitem__ method, so there are no cyclic
> >references.
> >
> >
> 
> -- 
> Robin Becker
> -- 
> http://mail.python.org/mailman/listinfo/python-list
>