metaclass.ipynb

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     "text": [
      "--- metaclass ---\n"
     ]
    }
   ],
   "source": [
    "print '--- metaclass ---'\n",
    "class Meta(type):\n",
    "    def __new__(mcs, name, bases, attrs):\n",
    "        \"\"\"Meta.__new__\"\"\"\n",
    "        print 'Meta.__new__', mcs, name, bases, attrs\n",
    "\n",
    "        # create properties dynamically\n",
    "        properties = attrs.pop('properties', [])\n",
    "        for prop in properties:\n",
    "            attrs[prop] = prop  # make a property!\n",
    "\n",
    "        return super(Meta, mcs).__new__(mcs, name, bases, attrs)\n",
    "    \n",
    "    def __init__(cls, name, bases, attrs):\n",
    "        \"\"\"Meta.__init__\"\"\"\n",
    "        print 'Meta.__init__', cls, name, bases, attrs\n",
    "        super(Meta, cls).__init__(name, bases, attrs)\n",
    "        \n",
    "    def __call__(cls, *args, **kwargs):\n",
    "        \"\"\"Meta.__call__\"\"\"\n",
    "        print 'Meta.__call__', cls, args, kwargs\n",
    "        \n",
    "        properties = kwargs.pop('properties', [])\n",
    "        if isinstance(properties, str):\n",
    "            properties = properties.split()\n",
    "        \n",
    "        # `properties` not passed... Use default behavior\n",
    "        if not properties:\n",
    "            return super(Meta, cls).__call__(*args, **kwargs)\n",
    "        \n",
    "        # Special behavior: construct a new class with `properties` attr\n",
    "        metaclass = cls.__class__\n",
    "        print '<<< BEGIN temporary class'\n",
    "        temp_class = metaclass(cls.__name__, (cls,),  dict(properties=properties))\n",
    "        print '>>> END temporary class'\n",
    "        \n",
    "        return temp_class"
   ]
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     "text": [
      "--- class ---\n",
      "Meta.__new__ <class '__main__.Meta'> AClass (<type 'object'>,) {'__module__': '__main__', '__metaclass__': <class '__main__.Meta'>}\n",
      "Meta.__init__ <class '__main__.AClass'> AClass (<type 'object'>,) {'__module__': '__main__', '__metaclass__': <class '__main__.Meta'>}\n"
     ]
    }
   ],
   "source": [
    "print '--- class ---'\n",
    "class AClass(object):\n",
    "    __metaclass__ = Meta"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 3,
   "metadata": {
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   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "T.__new__(S, ...) -> a new object with type S, a subtype of T\n"
     ]
    }
   ],
   "source": [
    "print AClass.__new__.__doc__"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 4,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "--- object ---\n",
      "Meta.__call__ <class '__main__.AClass'> () {}\n",
      "<class '__main__.AClass'>\n"
     ]
    }
   ],
   "source": [
    "print '--- object ---'\n",
    "anobject = AClass()\n",
    "print type(anobject)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 5,
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    "collapsed": false
   },
   "outputs": [
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     "text": [
      "--- class ---\n",
      "Meta.__call__ <class '__main__.AClass'> () {'properties': ['a', 'b']}\n",
      "<<< BEGIN temporary class\n",
      "Meta.__new__ <class '__main__.Meta'> AClass (<class '__main__.AClass'>,) {'properties': ['a', 'b']}\n",
      "Meta.__init__ <class '__main__.AClass'> AClass (<class '__main__.AClass'>,) {'a': 'a', 'b': 'b'}\n",
      ">>> END temporary class\n",
      "<class '__main__.Meta'>\n"
     ]
    }
   ],
   "source": [
    "print '--- class ---'\n",
    "AnotherClass = AClass(properties=['a', 'b'])\n",
    "print type(AnotherClass)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 6,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "--- object ---\n",
      "Meta.__call__ <class '__main__.AClass'> () {}\n",
      "<class '__main__.AClass'>\n",
      "a\n",
      "b\n"
     ]
    }
   ],
   "source": [
    "print '--- object ---'\n",
    "x = AnotherClass()\n",
    "print type(x)\n",
    "print x.a\n",
    "print x.b"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 7,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "name": "stdout",
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     "text": [
      "--- class ---\n",
      "Meta.__new__ <class '__main__.Meta'> YetAnotherClass (<class '__main__.AClass'>,) {'__module__': '__main__', 'properties': ['c', 'd']}\n",
      "Meta.__init__ <class '__main__.YetAnotherClass'> YetAnotherClass (<class '__main__.AClass'>,) {'__module__': '__main__', 'c': 'c', 'd': 'd'}\n",
      "--- object ---\n",
      "Meta.__call__ <class '__main__.YetAnotherClass'> () {}\n",
      "c\n",
      "d\n"
     ]
    }
   ],
   "source": [
    "print '--- class ---'\n",
    "class YetAnotherClass(AClass):\n",
    "    properties = ['c', 'd']\n",
    "\n",
    "print '--- object ---'\n",
    "yetanotherobject = YetAnotherClass()\n",
    "print yetanotherobject.c\n",
    "print yetanotherobject.d"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 8,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "--- class ---\n",
      "Meta.__call__ <class '__main__.AClass'> () {'properties': ['e', 'f']}\n",
      "<<< BEGIN temporary class\n",
      "Meta.__new__ <class '__main__.Meta'> AClass (<class '__main__.AClass'>,) {'properties': ['e', 'f']}\n",
      "Meta.__init__ <class '__main__.AClass'> AClass (<class '__main__.AClass'>,) {'e': 'e', 'f': 'f'}\n",
      ">>> END temporary class\n",
      "Meta.__new__ <class '__main__.Meta'> OneMoreClass (<class '__main__.AClass'>,) {'__module__': '__main__'}\n",
      "Meta.__init__ <class '__main__.OneMoreClass'> OneMoreClass (<class '__main__.AClass'>,) {'__module__': '__main__'}\n",
      "--- object ---\n",
      "Meta.__call__ <class '__main__.OneMoreClass'> () {}\n",
      "e\n",
      "f\n"
     ]
    }
   ],
   "source": [
    "print '--- class ---'\n",
    "class OneMoreClass(AClass(properties=['e', 'f'])):\n",
    "    pass\n",
    "\n",
    "\n",
    "print '--- object ---'\n",
    "onemoreobject = OneMoreClass()\n",
    "print onemoreobject.e\n",
    "print onemoreobject.f"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
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    "collapsed": true
   },
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   "source": []
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