05-advanced-python/05.12-decorators.ipynb

{
 "cells": [
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "# 修饰符"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## 函数是一种对象"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "在 `Python` 中，函数是也是一种对象。"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 1,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "<type 'function'>\n"
     ]
    }
   ],
   "source": [
    "def foo(x):\n",
    "    print x\n",
    "    \n",
    "print(type(foo))"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "查看函数拥有的方法："
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 2,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "['__call__',\n",
       " '__class__',\n",
       " '__closure__',\n",
       " '__code__',\n",
       " '__defaults__',\n",
       " '__delattr__',\n",
       " '__dict__',\n",
       " '__doc__',\n",
       " '__format__',\n",
       " '__get__',\n",
       " '__getattribute__',\n",
       " '__globals__',\n",
       " '__hash__',\n",
       " '__init__',\n",
       " '__module__',\n",
       " '__name__',\n",
       " '__new__',\n",
       " '__reduce__',\n",
       " '__reduce_ex__',\n",
       " '__repr__',\n",
       " '__setattr__',\n",
       " '__sizeof__',\n",
       " '__str__',\n",
       " '__subclasshook__',\n",
       " 'func_closure',\n",
       " 'func_code',\n",
       " 'func_defaults',\n",
       " 'func_dict',\n",
       " 'func_doc',\n",
       " 'func_globals',\n",
       " 'func_name']"
      ]
     },
     "execution_count": 2,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "dir(foo)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "在这些方法中，`__call__` 是最重要的一种方法： "
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 3,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "42\n"
     ]
    }
   ],
   "source": [
    "foo.__call__(42)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "相当于："
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 4,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "42\n"
     ]
    }
   ],
   "source": [
    "foo(42)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "因为函数是对象，所以函数可以作为参数传入另一个函数："
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 5,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "def bar(f, x):\n",
    "    x += 1\n",
    "    f(x)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 6,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "5\n"
     ]
    }
   ],
   "source": [
    "bar(foo, 4)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## 修饰符"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "修饰符是这样的一种函数，它接受一个函数作为输入，通常输出也是一个函数："
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 7,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "def dec(f):\n",
    "    print 'I am decorating function', id(f)\n",
    "    return f"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "将 `len` 函数作为参数传入这个修饰符函数："
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 8,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "I am decorating function 33716168\n"
     ]
    }
   ],
   "source": [
    "declen = dec(len)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "使用这个新生成的函数："
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 9,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "3"
      ]
     },
     "execution_count": 9,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "declen([10,20,30])"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "上面的例子中，我们仅仅返回了函数的本身，也可以利用这个函数生成一个新的函数，看一个新的例子："
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 10,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "def loud(f):\n",
    "    def new_func(*args, **kw):\n",
    "        print 'calling with', args, kw\n",
    "        rtn = f(*args, **kw)\n",
    "        print 'return value is', rtn\n",
    "        return rtn\n",
    "    return new_func"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 11,
   "metadata": {
    "collapsed": false
   },
   "outputs": [],
   "source": [
    "loudlen = loud(len)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 12,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "calling with ([10, 20, 30],) {}\n",
      "return value is 3\n"
     ]
    },
    {
     "data": {
      "text/plain": [
       "3"
      ]
     },
     "execution_count": 12,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "loudlen([10, 20, 30])"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## 用 @ 来使用修饰符"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "`Python` 使用 `@` 符号来将某个函数替换为修饰符之后的函数： "
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "例如这个函数："
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 13,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "I am decorating function 64021672\n"
     ]
    }
   ],
   "source": [
    "def foo(x):\n",
    "    print x\n",
    "    \n",
    "foo = dec(foo)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "可以替换为："
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 14,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "I am decorating function 64021112\n"
     ]
    }
   ],
   "source": [
    "@dec\n",
    "def foo(x):\n",
    "    print x"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "事实上，如果修饰符返回的是一个函数，那么可以链式的使用修饰符：\n",
    "\n",
    "```python\n",
    "@dec1\n",
    "@dec2\n",
    "def foo(x):\n",
    "    print x\n",
    "```\n",
    "\n",
    "使用修饰符 `loud` 来定义这个函数："
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 15,
   "metadata": {
    "collapsed": false
   },
   "outputs": [],
   "source": [
    "@loud\n",
    "def foo(x):\n",
    "    print x"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 16,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "calling with (42,) {}\n",
      "42\n",
      "return value is None\n"
     ]
    }
   ],
   "source": [
    "foo(42)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## 例子"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "定义两个修饰器函数，一个将原来的函数值加一，另一个乘二："
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 17,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "def plus_one(f):\n",
    "    def new_func(x):\n",
    "        return f(x) + 1\n",
    "    return new_func\n",
    "\n",
    "def times_two(f):\n",
    "    def new_func(x):\n",
    "        return f(x) * 2\n",
    "    return new_func"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "定义函数，先乘二再加一："
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 18,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "@plus_one\n",
    "@times_two\n",
    "def foo(x):\n",
    "    return int(x)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 19,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "27"
      ]
     },
     "execution_count": 19,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "foo(13)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## 修饰器工厂"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "`decorators factories` 是返回修饰器的函数，例如："
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 20,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "def super_dec(x, y, z):\n",
    "    def dec(f):\n",
    "        def new_func(*args, **kw):\n",
    "            print x + y + z\n",
    "            return f(*args, **kw)\n",
    "        return new_func\n",
    "    return dec"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "它的作用在于产生一个可以接受参数的修饰器，例如我们想将 `loud` 输出的内容写入一个文件去，可以这样做："
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 21,
   "metadata": {
    "collapsed": false
   },
   "outputs": [],
   "source": [
    "def super_loud(filename):\n",
    "    fp = open(filename, 'w')\n",
    "    def loud(f):\n",
    "        def new_func(*args, **kw):\n",
    "            fp.write('calling with' + str(args) + str(kw))\n",
    "            # 确保内容被写入\n",
    "            fp.flush()\n",
    "            fp.close()\n",
    "            rtn = f(*args, **kw)\n",
    "            return rtn\n",
    "        return new_func\n",
    "    return loud"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "可以这样使用这个修饰器工厂："
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 22,
   "metadata": {
    "collapsed": false
   },
   "outputs": [],
   "source": [
    "@super_loud('test.txt')\n",
    "def foo(x):\n",
    "    print x"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "调用 `foo` 就会在文件中写入内容："
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 23,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "12\n"
     ]
    }
   ],
   "source": [
    "foo(12)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "查看文件内容："
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 24,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "calling with(12,){}\n"
     ]
    }
   ],
   "source": [
    "with open('test.txt') as fp:\n",
    "    print fp.read()"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 25,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "import os\n",
    "os.remove('test.txt')"
   ]
  }
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