05-advanced-python/05.03-comma-separated-values.ipynb

{
 "cells": [
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "# CSV 文件和 csv 模块"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "标准库中有自带的 `csv` (逗号分隔值) 模块处理 `csv` 格式的文件："
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 1,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "import csv"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## 读 csv 文件"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "假设我们有这样的一个文件："
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 2,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "Writing data.csv\n"
     ]
    }
   ],
   "source": [
    "%%file data.csv\n",
    "\"alpha 1\",  100, -1.443\n",
    "\"beat  3\",   12, -0.0934\n",
    "\"gamma 3a\", 192, -0.6621\n",
    "\"delta 2a\",  15, -4.515"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "打开这个文件，并产生一个文件 reader："
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 3,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "fp = open(\"data.csv\")\n",
    "r = csv.reader(fp)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "可以按行迭代数据："
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 4,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "['alpha 1', '  100', ' -1.443']\n",
      "['beat  3', '   12', ' -0.0934']\n",
      "['gamma 3a', ' 192', ' -0.6621']\n",
      "['delta 2a', '  15', ' -4.515']\n"
     ]
    }
   ],
   "source": [
    "for row in r:\n",
    "    print row\n",
    "    \n",
    "fp.close()"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "默认数据内容都被当作字符串处理，不过可以自己进行处理："
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 5,
   "metadata": {
    "collapsed": false,
    "scrolled": true
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "[['alpha 1', 100, -1.443],\n",
       " ['beat  3', 12, -0.0934],\n",
       " ['gamma 3a', 192, -0.6621],\n",
       " ['delta 2a', 15, -4.515]]"
      ]
     },
     "execution_count": 5,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "data = []\n",
    "\n",
    "with open('data.csv') as fp:\n",
    "    r = csv.reader(fp)\n",
    "    for row in r:\n",
    "        data.append([row[0], int(row[1]), float(row[2])])\n",
    "    \n",
    "data"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 6,
   "metadata": {
    "collapsed": false
   },
   "outputs": [],
   "source": [
    "import os\n",
    "os.remove('data.csv')"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## 写 csv 文件"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "可以使用 `csv.writer` 写入文件，不过相应地，传入的应该是以写方式打开的文件，不过一般要用 `'wb'` 即二进制写入方式，防止出现换行不正确的问题："
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 7,
   "metadata": {
    "collapsed": false
   },
   "outputs": [],
   "source": [
    "data = [('one', 1, 1.5), ('two', 2, 8.0)]\n",
    "with open('out.csv', 'wb') as fp:\n",
    "    w = csv.writer(fp)\n",
    "    w.writerows(data)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "显示结果："
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 8,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "one,1,1.5\n",
      "two,2,8.0\n"
     ]
    }
   ],
   "source": [
    "!cat 'out.csv'"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## 更换分隔符"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "默认情况下，`csv` 模块默认 `csv` 文件都是由 `excel` 产生的，实际中可能会遇到这样的问题："
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 9,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "data = [('one, \\\"real\\\" string', 1, 1.5), ('two', 2, 8.0)]\n",
    "with open('out.csv', 'wb') as fp:\n",
    "    w = csv.writer(fp)\n",
    "    w.writerows(data)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 10,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "\"one, \"\"real\"\" string\",1,1.5\n",
      "two,2,8.0\n"
     ]
    }
   ],
   "source": [
    "!cat 'out.csv'"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "可以修改分隔符来处理这组数据："
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 11,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "data = [('one, \\\"real\\\" string', 1, 1.5), ('two', 2, 8.0)]\n",
    "with open('out.psv', 'wb') as fp:\n",
    "    w = csv.writer(fp, delimiter=\"|\")\n",
    "    w.writerows(data)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 12,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "\"one, \"\"real\"\" string\"|1|1.5\n",
      "two|2|8.0\n"
     ]
    }
   ],
   "source": [
    "!cat 'out.psv'"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 13,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "import os\n",
    "os.remove('out.psv')\n",
    "os.remove('out.csv')"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## 其他选项"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "`numpy.loadtxt()` 和 `pandas.read_csv()` 可以用来读写包含很多数值数据的 `csv` 文件："
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 14,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "Writing trades.csv\n"
     ]
    }
   ],
   "source": [
    "%%file trades.csv\n",
    "Order,Date,Stock,Quantity,Price\n",
    "A0001,2013-12-01,AAPL,1000,203.4\n",
    "A0002,2013-12-01,MSFT,1500,167.5\n",
    "A0003,2013-12-02,GOOG,1500,167.5"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "使用 `pandas` 进行处理，生成一个 `DataFrame` 对象："
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 15,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "             Date Stock  Quantity  Price\n",
      "Order                                   \n",
      "A0001  2013-12-01  AAPL      1000  203.4\n",
      "A0002  2013-12-01  MSFT      1500  167.5\n",
      "A0003  2013-12-02  GOOG      1500  167.5\n"
     ]
    }
   ],
   "source": [
    "import pandas\n",
    "df = pandas.read_csv('trades.csv', index_col=0)\n",
    "print df"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "通过名字进行索引："
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 16,
   "metadata": {
    "collapsed": false,
    "scrolled": true
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "Order\n",
       "A0001    203400\n",
       "A0002    251250\n",
       "A0003    251250\n",
       "dtype: float64"
      ]
     },
     "execution_count": 16,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "df['Quantity'] * df['Price']"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 17,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "import os\n",
    "os.remove('trades.csv')"
   ]
  }
 ],
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