a metaclass for python that keep tracks of created classes and instances. So, when one modifies a class, all the instantiated instances will refer to this new class

all_instances.ipynb

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     "text": [
      "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/matplotlib/__init__.py:872: UserWarning: axes.color_cycle is deprecated and replaced with axes.prop_cycle; please use the latter.\n",
      "  warnings.warn(self.msg_depr % (key, alt_key))\n"
     ]
    }
   ],
   "source": [
    "%matplotlib notebook"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 145,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "import weakref\n",
    "from collections import defaultdict\n",
    "import types"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 26,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "class Dict(dict):\n",
    "    pass"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 146,
   "metadata": {
    "collapsed": false
   },
   "outputs": [],
   "source": [
    "class class_holder(type):\n",
    "    classes = Dict()\n",
    "    all_instances = defaultdict(list)\n",
    "    def __init__(cls, name, bases, nmspc):\n",
    "        super(class_holder, cls).__init__(name, bases, nmspc)\n",
    "        \n",
    "        def instances(cls):\n",
    "            return [ref() for ref in class_holder.all_instances[name]]\n",
    "        \n",
    "        def pre_init(self, *args, **kargs):\n",
    "            answer  = old_init(self, *args, **kargs)\n",
    "            class_holder.all_instances[name].append(weakref.ref(self))\n",
    "            return answer\n",
    "        old_init = cls.__init__\n",
    "        cls.__init__ = pre_init\n",
    "        cls.all_instances = types.MethodType(instances, cls, cls.__class__) \n",
    "        def adjust_instance(instance):\n",
    "            instance.__class__ = cls\n",
    "            return instance\n",
    "        def adjust_all_instances(instances):\n",
    "            to_remove = []\n",
    "            for id, reference in enumerate(instances):\n",
    "                instance = reference()\n",
    "                if instance:\n",
    "                    adjust_instance(instance)\n",
    "                else: \n",
    "                    to_remove.append(id)\n",
    "            [instances.remove(i) for i in to_remove[::-1]]\n",
    "        adjust_all_instances(class_holder.all_instances[name])\n",
    "        class_holder.classes[name] = weakref.ref(cls) "
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 90,
   "metadata": {
    "collapsed": false
   },
   "outputs": [],
   "source": [
    "a = []"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 91,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "[9, 8, 7, 6, 5, 4, 3, 2, 1, 0]"
      ]
     },
     "execution_count": 91,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "range(10)[::-1]"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "[].remove"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 147,
   "metadata": {
    "collapsed": false
   },
   "outputs": [],
   "source": [
    "class Simple1(object):\n",
    "    __metaclass__ = class_holder\n",
    "    def foo(self):\n",
    "        print 'hola'\n",
    "    def bar(self): \n",
    "        print 'chau'"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 148,
   "metadata": {
    "collapsed": false
   },
   "outputs": [],
   "source": [
    "simple = Simple1()\n"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 149,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "hola\n"
     ]
    }
   ],
   "source": [
    "simple.foo()"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 150,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "chau\n"
     ]
    }
   ],
   "source": [
    "simple.bar()"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 151,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "class Simple1(object):\n",
    "    __metaclass__ = class_holder\n",
    "    def foo(self):\n",
    "        print 'hola'\n",
    "    def bar(self): \n",
    "        print 'goodbye'"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 152,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "goodbye\n"
     ]
    }
   ],
   "source": [
    "simple.bar()"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 153,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "<bound method Simple1.bar of <__main__.Simple1 object at 0x112026850>>"
      ]
     },
     "execution_count": 153,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "simple.bar"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 154,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "<__main__.Simple1 at 0x112026850>"
      ]
     },
     "execution_count": 154,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "simple"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 155,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "[<__main__.Simple1 at 0x112026850>]"
      ]
     },
     "execution_count": 155,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "Simple1.all_instances()"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 156,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "naranja = Simple1()"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 157,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "[<__main__.Simple1 at 0x112026850>, <__main__.Simple1 at 0x112026f90>]"
      ]
     },
     "execution_count": 157,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "Simple1.all_instances()"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 158,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "!open ."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": []
  }
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