Tips/2016-03-22-Shallow-and-Deep-Copy.ipynb

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    "## Python 知之深浅\n",
    "\n",
    "Python 中的对象分为两种：可变对象（mutable）和不可变对象（immutable）。不可变对象包括int，float，long，str，tuple等，可变对象包括list，set，dict等。在 Python 中，赋值（assignment, `=`）的过程仅仅是：\n",
    "\n",
    "1. 创建一个（某个值的）对象；\n",
    "2. 将变量名指向（引用）这个对象。\n",
    "\n",
    "这就像 C 语言中指针的概念，只不过更灵活地是 Python 中的变量随时可以指向其它对象（不分类型），其它变量也可以指向这一对象。如果这一对象是可变的，那么对其中一个引用变量的改变会影响其它变量："
   ]
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   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "[1, 2]\n",
      "{'b': 1, 'a': 0}\n"
     ]
    }
   ],
   "source": [
    "lst = [1, 2, 3]\n",
    "s = lst\n",
    "s.pop()\n",
    "print(lst)\n",
    "\n",
    "d = {'a': 0}\n",
    "e = d\n",
    "e['b'] = 1\n",
    "print(d)"
   ]
  },
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   "metadata": {},
   "source": [
    "如果你不是刻意想要这样做（实际也很少会要这样操作），那么就可能导致一些意想不到的错误（尤其是在传递参数给函数的时候）。为了解决这一麻烦，最简单的方法就是不直接变量指向现有的对象，而是生成一份新的 copy 赋值给新的变量，有很多种语法可以实现："
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 2,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "[1, 2, 3, '#0']\n",
      "{'dd': '#0', 'a': 0}\n"
     ]
    }
   ],
   "source": [
    "lst = [1,2,3]\n",
    "\n",
    "llst = [lst,\n",
    "        lst[:],\n",
    "        lst.copy(),\n",
    "        [*lst]] # invalid in 2.7\n",
    "for i, v in enumerate(llst):\n",
    "    v.append(\"#{}\".format(i))\n",
    "print(lst)\n",
    "\n",
    "d = {\"a\": 0}\n",
    "dd = [d,\n",
    "      d.copy(),\n",
    "      {**d}] # invalid in 2.7\n",
    "for i, v in enumerate(dd):\n",
    "    v['dd'] = \"#{}\".format(i)\n",
    "print(d)"
   ]
  },
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   "metadata": {},
   "source": [
    "### `deep` vs `shallow`\n",
    "\n",
    "上面给出的这些 copy 的例子比较简单，都没有嵌套的情况出现，如果这里的可变对象中还包含其它可变对象，结果会怎样呢："
   ]
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   "execution_count": 3,
   "metadata": {
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   "outputs": [
    {
     "name": "stdout",
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     "text": [
      "[0, 1, [2, 3, '#0', '#1', '#2', '#3']]\n",
      "{'a': {'b': [0, '#0', '#1', '#2']}}\n"
     ]
    }
   ],
   "source": [
    "lst = [0, 1, [2, 3]]\n",
    "\n",
    "llst = [lst,\n",
    "        lst[:],\n",
    "        lst.copy(),\n",
    "        [*lst]]\n",
    "for i, v in enumerate(llst):\n",
    "    v[2].append(\"#{}\".format(i))\n",
    "print(lst)\n",
    "\n",
    "d = {\"a\": {\"b\": [0]}}\n",
    "dd = [d,\n",
    "      d.copy(),\n",
    "      {**d}]\n",
    "for i, v in enumerate(dd):\n",
    "    v['a']['b'].append(\"#{}\".format(i))\n",
    "print(d)"
   ]
  },
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   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "这些 copy 的方法称为**浅拷贝（shallow copy）**，它相比直接赋值更进了一步生成了新的对象，但是对于嵌套的对象仍然采用了赋值的方法来创建；如果要再进一步，则需要**深拷贝（deep copy）**，由标准库 `copy` 提供："
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 4,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "[0, 1, [2, 3, 4]]\n",
      "[0, 1, [2, 3]]\n",
      "{'a': {'b': [0, 1]}}\n",
      "{'a': {'b': [0]}}\n"
     ]
    }
   ],
   "source": [
    "from copy import deepcopy\n",
    "\n",
    "lst = [0, 1, [2, 3]]\n",
    "lst2 = deepcopy(lst)\n",
    "lst2[2].append(4)\n",
    "print(lst2)\n",
    "print(lst)\n",
    "\n",
    "d = {\"a\": {\"b\": [0]}}\n",
    "d2 = deepcopy(d)\n",
    "d2[\"a\"][\"b\"].append(1)\n",
    "print(d2)\n",
    "print(d)"
   ]
  },
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   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "清楚了赋值（引用）、copy 还是 `deepcopy` 之间的区别才能更好地避免意想不到的错误，同样也可以利用它们的特性去实现一些 little tricks，例如我们在 [0x04 闭包与作用域](https://github.com/rainyear/pytips/blob/master/Tips/2016-03-10-Scope-and-Closure.ipynb) 中利用可变对象的特性实现 `nonlocal` 的功能。关于可变对象的引用、传递等既是 Python 的基本属性，同时又因为隐藏在背后的“暗箱操作”而容易引起误解，想要深入了解可以进一步阅读参考链接的文章，我也会在后面的文章中继续一边学习、一边补充更多这方面的知识。"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### 参考\n",
    "\n",
    "1. [python基础（5）：深入理解 python 中的赋值、引用、拷贝、作用域](http://my.oschina.net/leejun2005/blog/145911)"
   ]
  }
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