chapter15.ipynb

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   "source": [
    "# 上下文管理器和else块"
   ]
  },
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   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## else，不仅仅在if中\n",
    "- else语句不仅能在if语句中使用，也可以在while,for,try中使用\n",
    "- for:循环执行完，且没有被break打断，才运行else块\n",
    "- while:循环因为条件为假退出时才运行else块\n",
    "- try:仅当try中没有异常抛出时才运行else块，else子句抛出的异常不会由前面的except子句处理"
   ]
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   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## EAFP\n",
    "- Easier to Ask for Forgiveness than Permission\n",
    "- 取得原谅比获取许可容易\n",
    "- 先假定存在有效的键或树型，如果假设不成立，那么捕获异常"
   ]
  },
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   "source": [
    "## 上下文管理器和with块\n",
    "- with语句的目的时简化try/finally模式，保证一段代码运行完毕后，不管那段代码由于异常、return或者sys.exit()调用而中止，都会执行某项操作\n",
    "- 上下文管理器协议包含``__enter__``和``__exit__``方法，with语句开始执行时，在上下文管理器对象上调用``__enter__``，with语句执行结束后，在上下文管理器对象上调用``__exit__``方法"
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   "cell_type": "markdown",
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   "source": [
    "## 自定义一个上下文管理器类\n",
    "- 进入上下文后，调用``__enter__``方法，该方法替换标准的输出方法，将其改为逆序输出\n",
    "- 离开上下文后，调用``__exit__``方法，将输出方法还原，并且捕获异常"
   ]
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   "source": [
    "class LookingGlass:\n",
    "    def __enter__(self):\n",
    "        import sys\n",
    "        self.original_write = sys.stdout.write\n",
    "        sys.stdout.write = self.reverse_write\n",
    "        return 'JABBERWOCKY'\n",
    "\n",
    "    def reverse_write(self, text):\n",
    "        self.original_write(text[::-1])\n",
    "\n",
    "    def __exit__(self, exc_type, exc_value, traceback):\n",
    "        import sys\n",
    "        sys.stdout.write = self.original_write\n",
    "        if exc_type is ZeroDivisionError:\n",
    "            print('Please DO NOT divide by zero!')\n",
    "            return True"
   ]
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   "outputs": [
    {
     "name": "stdout",
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     "text": [
      "fedcba\n",
      "YKCOWREBBAJ\n"
     ]
    }
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   "source": [
    "with LookingGlass() as what:\n",
    "    print(\"abcdef\")\n",
    "    print(what)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 3,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "'JABBERWOCKY'"
      ]
     },
     "execution_count": 3,
     "metadata": {},
     "output_type": "execute_result"
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   "source": [
    "what"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 4,
   "metadata": {},
   "outputs": [
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     "name": "stdout",
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     "text": [
      "back to normal\n"
     ]
    }
   ],
   "source": [
    "print(\"back to normal\")"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## 可以在with块之外使用上下文管理器类\n",
    "- 手动调用``__enter__``和``__exit__``方法即可"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## 使用装饰器实现上下文管理器函数\n",
    "- 这时调用``__enter__``方法会调用生成i函数，保存生成器对象，调用next(gen)执行到yield关键字所在的位置，返回next(gen)产出的值，一遍吧产出的值绑定到with/as语句的目标变量上\n",
    "- 调用``__exit__``方法会检查是否有异常传给exc_type，如果有，调用gen.throw(exception)，在生成器函数定义体中包含yield关键字的那一行抛出异常，否则调用next(gen)，继续执行生成器函数定义体中yield语句之后的代码"
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   "cell_type": "code",
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   "metadata": {},
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   "source": [
    "import contextlib\n",
    "\n",
    "\n",
    "@contextlib.contextmanager\n",
    "def looking_glass():\n",
    "    import sys\n",
    "    original_write = sys.stdout.write\n",
    "\n",
    "    def reverse_write(text):\n",
    "        original_write(text[::-1])\n",
    "\n",
    "    sys.stdout.write = reverse_write\n",
    "    msg = ''\n",
    "    try:\n",
    "        yield 'JABBERWOCKY'\n",
    "    except ZeroDivisionError:\n",
    "        msg = 'Please DO NOT divide by zero!'\n",
    "    finally:\n",
    "        sys.stdout.write = original_write\n",
    "        if msg:\n",
    "            print(msg)"
   ]
  },
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   "cell_type": "code",
   "execution_count": 6,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "fedcba\n",
      "YKCOWREBBAJ\n"
     ]
    }
   ],
   "source": [
    "with looking_glass() as what:\n",
    "    print(\"abcdef\")\n",
    "    print(what)"
   ]
  }
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