diff --git a/02. python essentials/02.01 a tour of python.ipynb b/02. python essentials/02.01 a tour of python.ipynb
index ecdb7610..bb670b8c 100644
--- a/02. python essentials/02.01 a tour of python.ipynb	
+++ b/02. python essentials/02.01 a tour of python.ipynb	
@@ -411,7 +411,10 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "Python用`[]`来生成列表"
+    "Python用`[]`来生成列表，`[]`中的内容可迭代。\n",
+    "* 例如, list('abc') 返回 `['a', 'b', 'c']`\n",
+    "* list( (1, 2, 3) ) 返回 `[1, 2, 3]`\n",
+    "* 如果无参数, 返回一个空列表, `[]`."
    ]
   },
   {
@@ -1927,7 +1930,7 @@
    "name": "python",
    "nbconvert_exporter": "python",
    "pygments_lexer": "ipython2",
-   "version": "2.7.6"
+   "version": "2.7.10"
   }
  },
  "nbformat": 4,
diff --git a/02. python essentials/02.03 numbers.ipynb b/02. python essentials/02.03 numbers.ipynb
index 7c80e8da..cf93191c 100644
--- a/02. python essentials/02.03 numbers.ipynb	
+++ b/02. python essentials/02.03 numbers.ipynb	
@@ -1538,7 +1538,7 @@
    "name": "python",
    "nbconvert_exporter": "python",
    "pygments_lexer": "ipython2",
-   "version": "2.7.6"
+   "version": "2.7.10"
   }
  },
  "nbformat": 4,
diff --git a/02. python essentials/02.04 strings.ipynb b/02. python essentials/02.04 strings.ipynb
index ab859a1d..f2ad093b 100644
--- a/02. python essentials/02.04 strings.ipynb	
+++ b/02. python essentials/02.04 strings.ipynb	
@@ -1298,7 +1298,7 @@
    "name": "python",
    "nbconvert_exporter": "python",
    "pygments_lexer": "ipython2",
-   "version": "2.7.6"
+   "version": "2.7.10"
   }
  },
  "nbformat": 4,
diff --git a/02. python essentials/02.05 indexing and slicing.ipynb b/02. python essentials/02.05 indexing and slicing.ipynb
index e277d480..44fad26a 100644
--- a/02. python essentials/02.05 indexing and slicing.ipynb	
+++ b/02. python essentials/02.05 indexing and slicing.ipynb	
@@ -1,488 +1,488 @@
-{
- "cells": [
-  {
-   "cell_type": "markdown",
-   "metadata": {},
-   "source": [
-    "# 索引和分片"
-   ]
-  },
-  {
-   "cell_type": "markdown",
-   "metadata": {},
-   "source": [
-    "## 索引"
-   ]
-  },
-  {
-   "cell_type": "markdown",
-   "metadata": {},
-   "source": [
-    "对于一个有序序列，可以通过索引的方法来访问对应位置的值。字符串便是一个有序序列的例子，**Python**使用 `[]` 来对有序序列进行索引。"
-   ]
-  },
-  {
-   "cell_type": "code",
-   "execution_count": 1,
-   "metadata": {
-    "collapsed": false
-   },
-   "outputs": [
-    {
-     "data": {
-      "text/plain": [
-       "'h'"
-      ]
-     },
-     "execution_count": 1,
-     "metadata": {},
-     "output_type": "execute_result"
-    }
-   ],
-   "source": [
-    "s = \"hello world\"\n",
-    "s[0]"
-   ]
-  },
-  {
-   "cell_type": "markdown",
-   "metadata": {},
-   "source": [
-    "**Python**中索引是从 `0` 开始的，所以索引 `0` 对应与序列的第 `1` 个元素。为了得到第 `5` 个元素，需要使用索引值 `4` 。"
-   ]
-  },
-  {
-   "cell_type": "code",
-   "execution_count": 2,
-   "metadata": {
-    "collapsed": false
-   },
-   "outputs": [
-    {
-     "data": {
-      "text/plain": [
-       "'o'"
-      ]
-     },
-     "execution_count": 2,
-     "metadata": {},
-     "output_type": "execute_result"
-    }
-   ],
-   "source": [
-    "s[4]"
-   ]
-  },
-  {
-   "cell_type": "markdown",
-   "metadata": {},
-   "source": [
-    "除了正向索引，**Python**还引入了负索引值的用法，即从后向前开始计数，例如，索引 `-2` 表示倒数第 `2` 个元素："
-   ]
-  },
-  {
-   "cell_type": "code",
-   "execution_count": 3,
-   "metadata": {
-    "collapsed": false
-   },
-   "outputs": [
-    {
-     "data": {
-      "text/plain": [
-       "'l'"
-      ]
-     },
-     "execution_count": 3,
-     "metadata": {},
-     "output_type": "execute_result"
-    }
-   ],
-   "source": [
-    "s[-2]"
-   ]
-  },
-  {
-   "cell_type": "markdown",
-   "metadata": {},
-   "source": [
-    "单个索引大于等于字符串的长度时，会报错："
-   ]
-  },
-  {
-   "cell_type": "code",
-   "execution_count": 4,
-   "metadata": {
-    "collapsed": false
-   },
-   "outputs": [
-    {
-     "ename": "IndexError",
-     "evalue": "string index out of range",
-     "output_type": "error",
-     "traceback": [
-      "\u001b[1;31m---------------------------------------------------------------------------\u001b[0m",
-      "\u001b[1;31mIndexError\u001b[0m                                Traceback (most recent call last)",
-      "\u001b[1;32m<ipython-input-4-79ffc22473a3>\u001b[0m in \u001b[0;36m<module>\u001b[1;34m()\u001b[0m\n\u001b[1;32m----> 1\u001b[1;33m \u001b[0ms\u001b[0m\u001b[1;33m[\u001b[0m\u001b[1;36m11\u001b[0m\u001b[1;33m]\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0m",
-      "\u001b[1;31mIndexError\u001b[0m: string index out of range"
-     ]
-    }
-   ],
-   "source": [
-    "s[11]"
-   ]
-  },
-  {
-   "cell_type": "markdown",
-   "metadata": {},
-   "source": [
-    "## 分片"
-   ]
-  },
-  {
-   "cell_type": "markdown",
-   "metadata": {},
-   "source": [
-    "分片用来从序列中提取出想要的子序列，其用法为：\n",
-    "\n",
-    "    var[lower:upper:step]\n",
-    "\n",
-    "其范围包括 `lower` ，但不包括 `upper` ，即 `[lower, upper)`， `step` 表示取值间隔大小，如果没有默认为`1`。"
-   ]
-  },
-  {
-   "cell_type": "code",
-   "execution_count": 5,
-   "metadata": {
-    "collapsed": false
-   },
-   "outputs": [
-    {
-     "data": {
-      "text/plain": [
-       "'hello world'"
-      ]
-     },
-     "execution_count": 5,
-     "metadata": {},
-     "output_type": "execute_result"
-    }
-   ],
-   "source": [
-    "s"
-   ]
-  },
-  {
-   "cell_type": "code",
-   "execution_count": 6,
-   "metadata": {
-    "collapsed": false
-   },
-   "outputs": [
-    {
-     "data": {
-      "text/plain": [
-       "'el'"
-      ]
-     },
-     "execution_count": 6,
-     "metadata": {},
-     "output_type": "execute_result"
-    }
-   ],
-   "source": [
-    "s[1:3]"
-   ]
-  },
-  {
-   "cell_type": "markdown",
-   "metadata": {},
-   "source": [
-    "分片中包含的元素的个数为 `3-1=2` 。"
-   ]
-  },
-  {
-   "cell_type": "markdown",
-   "metadata": {},
-   "source": [
-    "也可以使用负索引来指定分片的范围："
-   ]
-  },
-  {
-   "cell_type": "code",
-   "execution_count": 7,
-   "metadata": {
-    "collapsed": false
-   },
-   "outputs": [
-    {
-     "data": {
-      "text/plain": [
-       "'ello wor'"
-      ]
-     },
-     "execution_count": 7,
-     "metadata": {},
-     "output_type": "execute_result"
-    }
-   ],
-   "source": [
-    "s[1:-2]"
-   ]
-  },
-  {
-   "cell_type": "markdown",
-   "metadata": {},
-   "source": [
-    "包括索引 `1` 但是不包括索引 `-2` 。\n",
-    "\n",
-    "lower和upper可以省略，省略lower意味着从开头开始分片，省略upper意味着一直分片到结尾。"
-   ]
-  },
-  {
-   "cell_type": "code",
-   "execution_count": 8,
-   "metadata": {
-    "collapsed": false
-   },
-   "outputs": [
-    {
-     "data": {
-      "text/plain": [
-       "'hel'"
-      ]
-     },
-     "execution_count": 8,
-     "metadata": {},
-     "output_type": "execute_result"
-    }
-   ],
-   "source": [
-    "s[:3]"
-   ]
-  },
-  {
-   "cell_type": "code",
-   "execution_count": 9,
-   "metadata": {
-    "collapsed": false
-   },
-   "outputs": [
-    {
-     "data": {
-      "text/plain": [
-       "'rld'"
-      ]
-     },
-     "execution_count": 9,
-     "metadata": {},
-     "output_type": "execute_result"
-    }
-   ],
-   "source": [
-    "s[-3:]"
-   ]
-  },
-  {
-   "cell_type": "code",
-   "execution_count": 10,
-   "metadata": {
-    "collapsed": false
-   },
-   "outputs": [
-    {
-     "data": {
-      "text/plain": [
-       "'hello world'"
-      ]
-     },
-     "execution_count": 10,
-     "metadata": {},
-     "output_type": "execute_result"
-    }
-   ],
-   "source": [
-    "s[:]"
-   ]
-  },
-  {
-   "cell_type": "markdown",
-   "metadata": {},
-   "source": [
-    "每隔两个取一个值："
-   ]
-  },
-  {
-   "cell_type": "code",
-   "execution_count": 11,
-   "metadata": {
-    "collapsed": false
-   },
-   "outputs": [
-    {
-     "data": {
-      "text/plain": [
-       "'hlowrd'"
-      ]
-     },
-     "execution_count": 11,
-     "metadata": {},
-     "output_type": "execute_result"
-    }
-   ],
-   "source": [
-    "s[::2]"
-   ]
-  },
-  {
-   "cell_type": "markdown",
-   "metadata": {},
-   "source": [
-    "当step的值为负时，省略lower意味着从结尾开始分片，省略upper意味着一直分片到开头。"
-   ]
-  },
-  {
-   "cell_type": "code",
-   "execution_count": 12,
-   "metadata": {
-    "collapsed": false
-   },
-   "outputs": [
-    {
-     "data": {
-      "text/plain": [
-       "'dlrow olleh'"
-      ]
-     },
-     "execution_count": 12,
-     "metadata": {},
-     "output_type": "execute_result"
-    }
-   ],
-   "source": [
-    "s[::-1]"
-   ]
-  },
-  {
-   "cell_type": "markdown",
-   "metadata": {},
-   "source": [
-    "当给定的upper超出字符串的长度（注意：因为不包含upper，所以可以等于）时，Python并不会报错，不过只会计算到结尾。"
-   ]
-  },
-  {
-   "cell_type": "code",
-   "execution_count": 13,
-   "metadata": {
-    "collapsed": false
-   },
-   "outputs": [
-    {
-     "data": {
-      "text/plain": [
-       "'hello world'"
-      ]
-     },
-     "execution_count": 13,
-     "metadata": {},
-     "output_type": "execute_result"
-    }
-   ],
-   "source": [
-    "s[:100]"
-   ]
-  },
-  {
-   "cell_type": "markdown",
-   "metadata": {},
-   "source": [
-    "## 使用“0”作为索引开头的原因"
-   ]
-  },
-  {
-   "cell_type": "markdown",
-   "metadata": {},
-   "source": [
-    "### 使用`[low, up)`形式的原因"
-   ]
-  },
-  {
-   "cell_type": "markdown",
-   "metadata": {},
-   "source": [
-    "假设需要表示字符串 `hello` 中的内部子串 `el` ：\n",
-    "\n",
-    "|方式|`[low, up)`|`(low, up]`|`(lower, upper)`|`[lower, upper]`\n",
-    "|--|--|--|--|--|\n",
-    "|表示|`[1,3)`|`(0,2]`|`(0,3)`|`[1,2]`\n",
-    "|序列长度|`up - low`|`up - low`|`up - low - 1`|`up - low + 1`\n",
-    "\n",
-    "对长度来说，前两种方式比较好，因为不需要烦人的加一减一。\n",
-    "\n",
-    "现在只考虑前两种方法，假设要表示字符串`hello`中的从头开始的子串`hel`：\n",
-    "\n",
-    "|方式|`[low, up)`|`(low, up]`\n",
-    "|--|--|\n",
-    "|表示|`[0,3)`|`(-1,2]`|\n",
-    "|序列长度|`up - low`|`up - low`|\n",
-    "\n",
-    "第二种表示方法从`-1`开始，不是很好，所以选择使用第一种`[low, up)`的形式。"
-   ]
-  },
-  {
-   "cell_type": "markdown",
-   "metadata": {},
-   "source": [
-    "### 使用0-base的形式"
-   ]
-  },
-  {
-   "cell_type": "markdown",
-   "metadata": {},
-   "source": [
-    "> Just to beautiful to ignore.  \n",
-    "----Guido van Rossum"
-   ]
-  },
-  {
-   "cell_type": "markdown",
-   "metadata": {},
-   "source": [
-    "两种简单的情况：\n",
-    "\n",
-    "- 从头开始的n个元素；\n",
-    "    - 使用0-base：`[0, n)`\n",
-    "    - 使用1-base：`[1, n+1)`\n",
-    "\n",
-    "- 第`i+1`个元素到第`i+n`个元素。\n",
-    "    - 使用0-base：`[i, n+i)`\n",
-    "    - 使用1-base：`[i+1, n+i+1)`\n",
-    "\n",
-    "1-base有个`+1`部分，所以不推荐。\n",
-    "\n",
-    "综合这两种原因，**Python**使用0-base的方法来进行索引。"
-   ]
-  }
- ],
- "metadata": {
-  "kernelspec": {
-   "display_name": "Python 2",
-   "language": "python",
-   "name": "python2"
-  },
-  "language_info": {
-   "codemirror_mode": {
-    "name": "ipython",
-    "version": 2
-   },
-   "file_extension": ".py",
-   "mimetype": "text/x-python",
-   "name": "python",
-   "nbconvert_exporter": "python",
-   "pygments_lexer": "ipython2",
-   "version": "2.7.10"
-  }
- },
- "nbformat": 4,
- "nbformat_minor": 0
-}
+{
+ "cells": [
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "# 索引和分片"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## 索引"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "对于一个有序序列，可以通过索引的方法来访问对应位置的值。字符串便是一个有序序列的例子，**Python**使用 `[]` 来对有序序列进行索引。"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 1,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [
+    {
+     "data": {
+      "text/plain": [
+       "'h'"
+      ]
+     },
+     "execution_count": 1,
+     "metadata": {},
+     "output_type": "execute_result"
+    }
+   ],
+   "source": [
+    "s = \"hello world\"\n",
+    "s[0]"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "**Python**中索引是从 `0` 开始的，所以索引 `0` 对应与序列的第 `1` 个元素。为了得到第 `5` 个元素，需要使用索引值 `4` 。"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 2,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [
+    {
+     "data": {
+      "text/plain": [
+       "'o'"
+      ]
+     },
+     "execution_count": 2,
+     "metadata": {},
+     "output_type": "execute_result"
+    }
+   ],
+   "source": [
+    "s[4]"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "除了正向索引，**Python**还引入了负索引值的用法，即从后向前开始计数，例如，索引 `-2` 表示倒数第 `2` 个元素："
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 3,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [
+    {
+     "data": {
+      "text/plain": [
+       "'l'"
+      ]
+     },
+     "execution_count": 3,
+     "metadata": {},
+     "output_type": "execute_result"
+    }
+   ],
+   "source": [
+    "s[-2]"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "单个索引大于等于字符串的长度时，会报错："
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 4,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [
+    {
+     "ename": "IndexError",
+     "evalue": "string index out of range",
+     "output_type": "error",
+     "traceback": [
+      "\u001b[1;31m---------------------------------------------------------------------------\u001b[0m",
+      "\u001b[1;31mIndexError\u001b[0m                                Traceback (most recent call last)",
+      "\u001b[1;32m<ipython-input-4-79ffc22473a3>\u001b[0m in \u001b[0;36m<module>\u001b[1;34m()\u001b[0m\n\u001b[1;32m----> 1\u001b[1;33m \u001b[0ms\u001b[0m\u001b[1;33m[\u001b[0m\u001b[1;36m11\u001b[0m\u001b[1;33m]\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0m",
+      "\u001b[1;31mIndexError\u001b[0m: string index out of range"
+     ]
+    }
+   ],
+   "source": [
+    "s[11]"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## 分片"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "分片用来从序列中提取出想要的子序列，其用法为：\n",
+    "\n",
+    "    var[lower:upper:step]\n",
+    "\n",
+    "其范围包括 `lower` ，但不包括 `upper` ，即 `[lower, upper)`， `step` 表示取值间隔大小，如果没有默认为`1`。"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 5,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [
+    {
+     "data": {
+      "text/plain": [
+       "'hello world'"
+      ]
+     },
+     "execution_count": 5,
+     "metadata": {},
+     "output_type": "execute_result"
+    }
+   ],
+   "source": [
+    "s"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 6,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [
+    {
+     "data": {
+      "text/plain": [
+       "'el'"
+      ]
+     },
+     "execution_count": 6,
+     "metadata": {},
+     "output_type": "execute_result"
+    }
+   ],
+   "source": [
+    "s[1:3]"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "分片中包含的元素的个数为 `3-1=2` 。"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "也可以使用负索引来指定分片的范围："
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 7,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [
+    {
+     "data": {
+      "text/plain": [
+       "'ello wor'"
+      ]
+     },
+     "execution_count": 7,
+     "metadata": {},
+     "output_type": "execute_result"
+    }
+   ],
+   "source": [
+    "s[1:-2]"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "包括索引 `1` 但是不包括索引 `-2` 。\n",
+    "\n",
+    "lower和upper可以省略，省略lower意味着从开头开始分片，省略upper意味着一直分片到结尾。"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 8,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [
+    {
+     "data": {
+      "text/plain": [
+       "'hel'"
+      ]
+     },
+     "execution_count": 8,
+     "metadata": {},
+     "output_type": "execute_result"
+    }
+   ],
+   "source": [
+    "s[:3]"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 9,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [
+    {
+     "data": {
+      "text/plain": [
+       "'rld'"
+      ]
+     },
+     "execution_count": 9,
+     "metadata": {},
+     "output_type": "execute_result"
+    }
+   ],
+   "source": [
+    "s[-3:]"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 10,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [
+    {
+     "data": {
+      "text/plain": [
+       "'hello world'"
+      ]
+     },
+     "execution_count": 10,
+     "metadata": {},
+     "output_type": "execute_result"
+    }
+   ],
+   "source": [
+    "s[:]"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "每隔两个取一个值："
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 11,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [
+    {
+     "data": {
+      "text/plain": [
+       "'hlowrd'"
+      ]
+     },
+     "execution_count": 11,
+     "metadata": {},
+     "output_type": "execute_result"
+    }
+   ],
+   "source": [
+    "s[::2]"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "当step的值为负时，省略lower意味着从结尾开始分片，省略upper意味着一直分片到开头。"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 12,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [
+    {
+     "data": {
+      "text/plain": [
+       "'dlrow olleh'"
+      ]
+     },
+     "execution_count": 12,
+     "metadata": {},
+     "output_type": "execute_result"
+    }
+   ],
+   "source": [
+    "s[::-1]"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "当给定的upper超出字符串的长度（注意：因为不包含upper，所以可以等于）时，Python并不会报错，不过只会计算到结尾。"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 13,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [
+    {
+     "data": {
+      "text/plain": [
+       "'hello world'"
+      ]
+     },
+     "execution_count": 13,
+     "metadata": {},
+     "output_type": "execute_result"
+    }
+   ],
+   "source": [
+    "s[:100]"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## 使用“0”作为索引开头的原因"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "### 使用`[low, up)`形式的原因"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "假设需要表示字符串 `hello` 中的内部子串 `el` ：\n",
+    "\n",
+    "|方式|`[low, up)`|`(low, up]`|`(lower, upper)`|`[lower, upper]`\n",
+    "|--|--|--|--|--|\n",
+    "|表示|`[1,3)`|`(0,2]`|`(0,3)`|`[1,2]`\n",
+    "|序列长度|`up - low`|`up - low`|`up - low - 1`|`up - low + 1`\n",
+    "\n",
+    "对长度来说，前两种方式比较好，因为不需要烦人的加一减一。\n",
+    "\n",
+    "现在只考虑前两种方法，假设要表示字符串`hello`中的从头开始的子串`hel`：\n",
+    "\n",
+    "|方式|`[low, up)`|`(low, up]`\n",
+    "|--|--|\n",
+    "|表示|`[0,3)`|`(-1,2]`|\n",
+    "|序列长度|`up - low`|`up - low`|\n",
+    "\n",
+    "第二种表示方法从`-1`开始，不是很好，所以选择使用第一种`[low, up)`的形式。"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "### 使用0-base的形式"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "> Just too beautiful to ignore.  \n",
+    "----Guido van Rossum"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "两种简单的情况：\n",
+    "\n",
+    "- 从头开始的n个元素；\n",
+    "    - 使用0-base：`[0, n)`\n",
+    "    - 使用1-base：`[1, n+1)`\n",
+    "\n",
+    "- 第`i+1`个元素到第`i+n`个元素。\n",
+    "    - 使用0-base：`[i, n+i)`\n",
+    "    - 使用1-base：`[i+1, n+i+1)`\n",
+    "\n",
+    "1-base有个`+1`部分，所以不推荐。\n",
+    "\n",
+    "综合这两种原因，**Python**使用0-base的方法来进行索引。"
+   ]
+  }
+ ],
+ "metadata": {
+  "kernelspec": {
+   "display_name": "Python 2",
+   "language": "python",
+   "name": "python2"
+  },
+  "language_info": {
+   "codemirror_mode": {
+    "name": "ipython",
+    "version": 2
+   },
+   "file_extension": ".py",
+   "mimetype": "text/x-python",
+   "name": "python",
+   "nbconvert_exporter": "python",
+   "pygments_lexer": "ipython2",
+   "version": "2.7.10"
+  }
+ },
+ "nbformat": 4,
+ "nbformat_minor": 0
+}
diff --git a/02. python essentials/02.06 lists.ipynb b/02. python essentials/02.06 lists.ipynb
index 84336636..3ee6c57a 100644
--- a/02. python essentials/02.06 lists.ipynb	
+++ b/02. python essentials/02.06 lists.ipynb	
@@ -1240,7 +1240,7 @@
    "name": "python",
    "nbconvert_exporter": "python",
    "pygments_lexer": "ipython2",
-   "version": "2.7.6"
+   "version": "2.7.10"
   }
  },
  "nbformat": 4,
diff --git a/02. python essentials/02.07 mutable and immutable data types.ipynb b/02. python essentials/02.07 mutable and immutable data types.ipynb
index cc103892..8d6e1623 100644
--- a/02. python essentials/02.07 mutable and immutable data types.ipynb	
+++ b/02. python essentials/02.07 mutable and immutable data types.ipynb	
@@ -361,7 +361,7 @@
    "name": "python",
    "nbconvert_exporter": "python",
    "pygments_lexer": "ipython2",
-   "version": "2.7.6"
+   "version": "2.7.10"
   }
  },
  "nbformat": 4,