Ch8 Object References, Mutability, and Recycling.ipynb

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     "data": {
      "text/plain": "([1, 2, 3], [1, 2])"
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     "execution_count": 2,
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   "source": [
    "a = [1, 2]\n",
    "b = [1, 2]\n",
    "c = (a, b)\n",
    "a.append(3)\n",
    "c"
   ]
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   "cell_type": "code",
   "execution_count": 2,
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     "text": [
      "False\n",
      "True\n",
      "True\n"
     ]
    }
   ],
   "source": [
    "l1 = [3, [66, 55, 44], (7, 8, 9)]\n",
    "l2 = list(l1)\n",
    "l2[0] += 1\n",
    "for i in range(len(l1)):\n",
    "    print(id(l1[i]) == id(l2[i]))"
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      "text/plain": "True"
     },
     "execution_count": 1,
     "metadata": {},
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   ],
   "source": [
    "import weakref\n",
    "\n",
    "s1 = {1, 2, 3}\n",
    "s2 = s1\n",
    "\n",
    "\n",
    "def bye():\n",
    "    print('Gone')\n",
    "\n",
    "\n",
    "ender = weakref.finalize(s1, bye)\n",
    "ender.alive"
   ],
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  },
  {
   "cell_type": "code",
   "execution_count": 5,
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "Gone\n"
     ]
    },
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     "data": {
      "text/plain": "False"
     },
     "execution_count": 5,
     "metadata": {},
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   "source": [
    "\n",
    "s2 = 'spam'\n",
    "ender.alive"
   ],
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    }
   }
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  {
   "cell_type": "code",
   "execution_count": 7,
   "outputs": [
    {
     "data": {
      "text/plain": "{0, 1}"
     },
     "execution_count": 7,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "a_set = {0, 1}\n",
    "wref = weakref.ref(a_set)\n",
    "wref()"
   ],
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    }
   }
  },
  {
   "cell_type": "code",
   "execution_count": 10,
   "outputs": [
    {
     "data": {
      "text/plain": "['1', '2']"
     },
     "execution_count": 10,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "class Chess:\n",
    "    def __init__(self, kind):\n",
    "        self.kind = kind\n",
    "\n",
    "    def __repr__(self):\n",
    "        return f'Chess{self.kind}'\n",
    "\n",
    "\n",
    "stock = weakref.WeakValueDictionary()\n",
    "catalog = [Chess('1'), Chess('2')]\n",
    "\n",
    "for chess in catalog:\n",
    "    stock[chess.kind] = chess\n",
    "\n",
    "sorted(stock.keys())"
   ],
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    }
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  },
  {
   "cell_type": "code",
   "execution_count": 11,
   "outputs": [
    {
     "data": {
      "text/plain": "['2']"
     },
     "execution_count": 11,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "del catalog\n",
    "sorted(stock.keys())"
   ],
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     "name": "#%%\n"
    }
   }
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  {
   "cell_type": "code",
   "execution_count": 12,
   "outputs": [
    {
     "ename": "TypeError",
     "evalue": "cannot create weak reference to 'list' object",
     "output_type": "error",
     "traceback": [
      "\u001B[1;31m---------------------------------------------------------------------------\u001B[0m",
      "\u001B[1;31mTypeError\u001B[0m                                 Traceback (most recent call last)",
      "\u001B[1;32m<ipython-input-12-04e3b07427be>\u001B[0m in \u001B[0;36m<module>\u001B[1;34m\u001B[0m\n\u001B[0;32m      1\u001B[0m \u001B[0ma\u001B[0m\u001B[1;33m=\u001B[0m\u001B[1;33m[\u001B[0m\u001B[1;36m1\u001B[0m\u001B[1;33m]\u001B[0m\u001B[1;33m\u001B[0m\u001B[1;33m\u001B[0m\u001B[0m\n\u001B[1;32m----> 2\u001B[1;33m \u001B[0mweakref\u001B[0m\u001B[1;33m.\u001B[0m\u001B[0mref\u001B[0m\u001B[1;33m(\u001B[0m\u001B[0ma\u001B[0m\u001B[1;33m)\u001B[0m\u001B[1;33m\u001B[0m\u001B[1;33m\u001B[0m\u001B[0m\n\u001B[0m",
      "\u001B[1;31mTypeError\u001B[0m: cannot create weak reference to 'list' object"
     ]
    }
   ],
   "source": [
    "a=[1]\n",
    "weakref.ref(a)"
   ],
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