# Copyright (C) 2009 AG Projects. See LICENSE for details.
#

"""Miscellaneous utility descriptors"""

__all__ = ['ThreadLocal', 'WriteOnceAttribute', 'isdescriptor']

import weakref
from threading import local


class ThreadLocal(object):
    """Descriptor that allows objects to have thread specific attributes"""
    thread_local = local()
    def __init__(self, type, *args, **kw):
        self.type = type
        self.args = args
        self.kw = kw
    def __get__(self, obj, objtype):
        if obj is None:
            return self
        obj_id = id(obj)
        self_id = id(self)
        try:
            return self.thread_local.__dict__[(obj_id, self_id)][0]
        except KeyError:
            instance = self.type(*self.args, **self.kw)
            thread_dict = self.thread_local.__dict__
            ref = weakref.ref(obj, lambda weak_ref: thread_dict.pop((obj_id, self_id)))
            thread_dict[(obj_id, self_id)] = (instance, ref)
            return instance
    def __set__(self, obj, value):
        obj_id = id(obj)
        self_id = id(self)
        thread_dict = self.thread_local.__dict__
        if (obj_id, self_id) in thread_dict:
            ref = thread_dict[(obj_id, self_id)][1]
        else:
            ref = weakref.ref(obj, lambda weak_ref: thread_dict.pop((obj_id, self_id)))
        thread_dict[(obj_id, self_id)] = (value, ref)
    def __delete__(self, obj):
        raise AttributeError("attribute cannot be deleted")


class WriteOnceAttribute(object):
    """Descriptor that allows objects to have write once attributes.

       It should be noted that the descriptor only enforces this when directly
       accessing the object's attribute. It is still possible to modify/delete
       such an attribute by messing around with the descriptor's internal data.
    """
    def __init__(self):
        self.values = {}
    def __get__(self, obj, type):
        if obj is None:
            return self
        try:
            return self.values[id(obj)][0]
        except KeyError:
            raise AttributeError("attribute is not set")
    def __set__(self, obj, value):
        obj_id = id(obj)
        if obj_id in self.values:
            raise AttributeError("attribute is read-only")
        self.values[obj_id] = (value, weakref.ref(obj, lambda weak_ref: self.values.pop(obj_id)))
    def __delete__(self, obj):
        raise AttributeError("attribute cannot be deleted")


def isdescriptor(object):
    """Test if `object' is a descriptor"""
    return bool(set(('__get__', '__set__', '__delete__')).intersection(dir(object)))