第四章.ipynb

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   "source": [
    "# 第四章：迭代器与生成器\n",
    "\n",
    "迭代是 Python 最强大的功能之一。初看起来，你可能会简单的认为迭代只不过是处理序列中元素的一种方法。 然而，绝非仅仅就是如此，还有很多你可能不知道的， 比如创建你自己的迭代器对象，在itertools模块中使用有用的迭代模式，构造生成器函数等等。 这一章目的就是向你展示跟迭代有关的各种常见问题"
   ]
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   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## 4.1 手动遍历迭代器\n",
    "\n",
    "**问题** \n",
    "\n",
    "    你想遍历一个可迭代对象中的所有元素，但是却不想使用 for 循环\n",
    "    \n",
    "**解决方案**\n",
    "\n",
    "    为了手动的遍历可迭代对象，使用 next() 函数并在代码中捕获 StopIteration 异常。比如，下面的列子手动读取一个文件中的所有行："
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 1,
   "metadata": {},
   "outputs": [],
   "source": [
    "def manual_iter():\n",
    "    with open('/etc/passed') as f:\n",
    "        try:\n",
    "            while True:\n",
    "                line = next(f)\n",
    "                print(line, end='')\n",
    "        except StopIteration:\n",
    "            pass"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "    通常来讲， StopIteration 用来指示迭代的结尾。 然而，如果你手动使用上面演示的 next() 函数的话，你还可以通过返回一个指定值来标记结尾，比如 None 。 下面是示例："
   ]
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    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "root:x:0:0:root:/root:/bin/bash\n",
      "daemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin\n",
      "bin:x:2:2:bin:/bin:/usr/sbin/nologin\n",
      "sys:x:3:3:sys:/dev:/usr/sbin/nologin\n",
      "sync:x:4:65534:sync:/bin:/bin/sync\n",
      "games:x:5:60:games:/usr/games:/usr/sbin/nologin\n",
      "man:x:6:12:man:/var/cache/man:/usr/sbin/nologin\n",
      "lp:x:7:7:lp:/var/spool/lpd:/usr/sbin/nologin\n",
      "mail:x:8:8:mail:/var/mail:/usr/sbin/nologin\n",
      "news:x:9:9:news:/var/spool/news:/usr/sbin/nologin\n",
      "uucp:x:10:10:uucp:/var/spool/uucp:/usr/sbin/nologin\n",
      "proxy:x:13:13:proxy:/bin:/usr/sbin/nologin\n",
      "www-data:x:33:33:www-data:/var/www:/usr/sbin/nologin\n",
      "backup:x:34:34:backup:/var/backups:/usr/sbin/nologin\n",
      "list:x:38:38:Mailing List Manager:/var/list:/usr/sbin/nologin\n",
      "irc:x:39:39:ircd:/var/run/ircd:/usr/sbin/nologin\n",
      "gnats:x:41:41:Gnats Bug-Reporting System (admin):/var/lib/gnats:/usr/sbin/nologin\n",
      "nobody:x:65534:65534:nobody:/nonexistent:/usr/sbin/nologin\n",
      "systemd-network:x:100:102:systemd Network Management,,,:/run/systemd:/usr/sbin/nologin\n",
      "systemd-resolve:x:101:103:systemd Resolver,,,:/run/systemd:/usr/sbin/nologin\n",
      "systemd-timesync:x:102:104:systemd Time Synchronization,,,:/run/systemd:/usr/sbin/nologin\n",
      "messagebus:x:103:106::/nonexistent:/usr/sbin/nologin\n",
      "syslog:x:104:110::/home/syslog:/usr/sbin/nologin\n",
      "_apt:x:105:65534::/nonexistent:/usr/sbin/nologin\n",
      "tss:x:106:111:TPM software stack,,,:/var/lib/tpm:/bin/false\n",
      "uuidd:x:107:112::/run/uuidd:/usr/sbin/nologin\n",
      "tcpdump:x:108:113::/nonexistent:/usr/sbin/nologin\n",
      "sshd:x:109:65534::/run/sshd:/usr/sbin/nologin\n",
      "landscape:x:110:115::/var/lib/landscape:/usr/sbin/nologin\n",
      "pollinate:x:111:1::/var/cache/pollinate:/bin/false\n",
      "xia:x:1000:1000:,,,:/home/xia:/bin/bash\n"
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   "source": [
    "with open('/etc/passwd') as f:\n",
    "    while True:\n",
    "        line = next(f, None)\n",
    "        if line is None:\n",
    "            break\n",
    "        print(line, end='')"
   ]
  },
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   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "**讨论**\n",
    "\n",
    "    大多数情况下，我们会使用 for 循环语句用来遍历一个可迭代对象。但是，偶尔也需要对迭代做更加精确的控制，这时候了解底层机制就显得尤为重要了。\n",
    "    \n",
    "    下面的交互示例向我们演示了迭代期间所发生的基本细节："
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 3,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "1"
      ]
     },
     "execution_count": 3,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    ">>> items = [1, 2, 3]\n",
    ">>> # Get the iterator\n",
    ">>> it = iter(items) # Invokes items.__iter__()\n",
    ">>> # Run the iterator\n",
    ">>> next(it) # Invokes it.__next__()"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 4,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "2"
      ]
     },
     "execution_count": 4,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    ">>> next(it)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 5,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "3"
      ]
     },
     "execution_count": 5,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    ">>> next(it)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 6,
   "metadata": {},
   "outputs": [
    {
     "ename": "StopIteration",
     "evalue": "",
     "output_type": "error",
     "traceback": [
      "\u001b[0;31m---------------------------------------------------------------------------\u001b[0m",
      "\u001b[0;31mStopIteration\u001b[0m                             Traceback (most recent call last)",
      "\u001b[0;32m<ipython-input-6-bc1ab118995a>\u001b[0m in \u001b[0;36m<module>\u001b[0;34m\u001b[0m\n\u001b[0;32m----> 1\u001b[0;31m \u001b[0mnext\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0mit\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0m",
      "\u001b[0;31mStopIteration\u001b[0m: "
     ]
    }
   ],
   "source": [
    ">>> next(it)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "    本章接下来几小节会更深入的讲解迭代相关技术，前提是你先要理解基本的迭代协议机制。 所以确保你已经把这章的内容牢牢记在心中。"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": []
  }
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