Example of a weak reference.

weakref-example.ipynb

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 "worksheets": [
  {
   "cells": [
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "import weakref\n",
      "\n",
      "class Client(object):\n",
      "    def __init__(self, name):\n",
      "        self.name = name\n",
      "\n",
      "def callback(obj):\n",
      "    print('client is going away')\n",
      "        \n",
      "client = Client('foobar')\n",
      "ref = weakref.proxy(client, callback)\n",
      "\n",
      "client.name"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [
      {
       "metadata": {},
       "output_type": "pyout",
       "prompt_number": 8,
       "text": [
        "'foobar'"
       ]
      }
     ],
     "prompt_number": 8
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "ref.name"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [
      {
       "metadata": {},
       "output_type": "pyout",
       "prompt_number": 9,
       "text": [
        "'foobar'"
       ]
      }
     ],
     "prompt_number": 9
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "del(client)"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [
      {
       "output_type": "stream",
       "stream": "stdout",
       "text": [
        "client is going away\n"
       ]
      }
     ],
     "prompt_number": 10
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "ref.name"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [
      {
       "ename": "ReferenceError",
       "evalue": "weakly-referenced object no longer exists",
       "output_type": "pyerr",
       "traceback": [
        "\u001b[0;31m---------------------------------------------------------------------------\u001b[0m\n\u001b[0;31mReferenceError\u001b[0m                            Traceback (most recent call last)",
        "\u001b[0;32m<ipython-input-12-65ab57b7f808>\u001b[0m in \u001b[0;36m<module>\u001b[0;34m()\u001b[0m\n\u001b[0;32m----> 1\u001b[0;31m \u001b[0mref\u001b[0m\u001b[0;34m.\u001b[0m\u001b[0mname\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0m",
        "\u001b[0;31mReferenceError\u001b[0m: weakly-referenced object no longer exists"
       ]
      }
     ],
     "prompt_number": 12
    }
   ],
   "metadata": {}
  }
 ]
}