{
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  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "# 弱引用\n",
    "\n",
    "Python 会自动进行内存管理（对大多数对象进行引用计数并使用 {term}`garbage collection` 来清除循环引用）。当某个对象的最后一个引用被移除后不久就会释放其所占用的内存。\n",
    "\n",
    "此方式对大多数应用来说都适用，但偶尔也必须在对象持续被其他对象所使用时跟踪它们。 不幸的是，跟踪它们将创建一个会令其永久化的引用。{mod}`weakref` 模块提供的工具可以不必创建引用就能跟踪对象。当对象不再需要时，它将自动从一个弱引用表中被移除，并为弱引用对象触发一个回调。 典型应用包括对创建开销较大的对象进行缓存："
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 1,
   "metadata": {},
   "outputs": [],
   "source": [
    "import weakref, gc\n",
    "class A:\n",
    "    def __init__(self, value):\n",
    "        self.value = value\n",
    "    def __repr__(self):\n",
    "        return str(self.value)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 2,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "10\n",
      "343\n"
     ]
    },
    {
     "ename": "KeyError",
     "evalue": "'primary'",
     "output_type": "error",
     "traceback": [
      "\u001b[1;31m---------------------------------------------------------------------------\u001b[0m",
      "\u001b[1;31mKeyError\u001b[0m                                  Traceback (most recent call last)",
      "\u001b[1;32m~\\AppData\\Local\\Temp/ipykernel_18000/460714087.py\u001b[0m in \u001b[0;36m<module>\u001b[1;34m\u001b[0m\n\u001b[0;32m      7\u001b[0m \u001b[0mprint\u001b[0m\u001b[1;33m(\u001b[0m\u001b[0mgc\u001b[0m\u001b[1;33m.\u001b[0m\u001b[0mcollect\u001b[0m\u001b[1;33m(\u001b[0m\u001b[1;33m)\u001b[0m\u001b[1;33m)\u001b[0m                \u001b[1;31m# run garbage collection right away\u001b[0m\u001b[1;33m\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0;32m      8\u001b[0m \u001b[1;33m\u001b[0m\u001b[0m\n\u001b[1;32m----> 9\u001b[1;33m \u001b[0md\u001b[0m\u001b[1;33m[\u001b[0m\u001b[1;34m'primary'\u001b[0m\u001b[1;33m]\u001b[0m                \u001b[1;31m# entry was automatically removed\u001b[0m\u001b[1;33m\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0m",
      "\u001b[1;32m~\\.conda\\envs\\xpp\\lib\\weakref.py\u001b[0m in \u001b[0;36m__getitem__\u001b[1;34m(self, key)\u001b[0m\n\u001b[0;32m    135\u001b[0m         \u001b[1;32mif\u001b[0m \u001b[0mself\u001b[0m\u001b[1;33m.\u001b[0m\u001b[0m_pending_removals\u001b[0m\u001b[1;33m:\u001b[0m\u001b[1;33m\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0;32m    136\u001b[0m             \u001b[0mself\u001b[0m\u001b[1;33m.\u001b[0m\u001b[0m_commit_removals\u001b[0m\u001b[1;33m(\u001b[0m\u001b[1;33m)\u001b[0m\u001b[1;33m\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[1;32m--> 137\u001b[1;33m         \u001b[0mo\u001b[0m \u001b[1;33m=\u001b[0m \u001b[0mself\u001b[0m\u001b[1;33m.\u001b[0m\u001b[0mdata\u001b[0m\u001b[1;33m[\u001b[0m\u001b[0mkey\u001b[0m\u001b[1;33m]\u001b[0m\u001b[1;33m(\u001b[0m\u001b[1;33m)\u001b[0m\u001b[1;33m\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0m\u001b[0;32m    138\u001b[0m         \u001b[1;32mif\u001b[0m \u001b[0mo\u001b[0m \u001b[1;32mis\u001b[0m \u001b[1;32mNone\u001b[0m\u001b[1;33m:\u001b[0m\u001b[1;33m\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0;32m    139\u001b[0m             \u001b[1;32mraise\u001b[0m \u001b[0mKeyError\u001b[0m\u001b[1;33m(\u001b[0m\u001b[0mkey\u001b[0m\u001b[1;33m)\u001b[0m\u001b[1;33m\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n",
      "\u001b[1;31mKeyError\u001b[0m: 'primary'"
     ]
    }
   ],
   "source": [
    "a = A(10)                   # create a reference\n",
    "d = weakref.WeakValueDictionary()\n",
    "d['primary'] = a            # does not create a reference\n",
    "print(d['primary'])                # fetch the object if it is still alive\n",
    "\n",
    "del a                       # remove the one reference\n",
    "print(gc.collect())                # run garbage collection right away\n",
    "\n",
    "d['primary']                # entry was automatically removed"
   ]
  }
 ],
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