updated lecture notes

2 files changed, 628 insertions, 161 deletions

diff --git a/LectureNotesOnPython.html b/LectureNotesOnPython.html
index 966a1b5..34d3462 100644
--- a/LectureNotesOnPython.html
+++ b/LectureNotesOnPython.html
@@ -377,41 +377,48 @@ ul.auto-toc {
 <div class="contents topic" id="id1">
 <p class="topic-title first">内容目录</p>
 <ul class="simple">
-<li><a class="reference internal" href="#python" id="id12">Python的发音纠正</a></li>
-<li><a class="reference internal" href="#id2" id="id13">Python源流</a></li>
-<li><a class="reference internal" href="#id3" id="id14">Python的关键词</a></li>
-<li><a class="reference internal" href="#id4" id="id15">值的类型</a></li>
-<li><a class="reference internal" href="#variable" id="id16">变量（Variable）</a></li>
-<li><a class="reference internal" href="#mutable" id="id17">可变（mutable）类型与不可变类型</a></li>
-<li><a class="reference internal" href="#id5" id="id18">数与格式化显示</a></li>
-<li><a class="reference internal" href="#strings" id="id19">字符串（Strings）</a></li>
-<li><a class="reference internal" href="#concatenation" id="id20">字符串相加（concatenation）</a></li>
-<li><a class="reference internal" href="#slice" id="id21">子串（slice）</a></li>
-<li><a class="reference internal" href="#id6" id="id22">搜索字符串</a></li>
-<li><a class="reference internal" href="#string" id="id23">String类（对象）方法</a></li>
-<li><a class="reference internal" href="#in" id="id24">in操作符</a></li>
-<li><a class="reference internal" href="#id7" id="id25">字符串比较</a></li>
-<li><a class="reference internal" href="#id8" id="id26">列表</a></li>
-<li><a class="reference internal" href="#id9" id="id27">列表作为参数</a></li>
-<li><a class="reference internal" href="#append" id="id28">注意区别 <tt class="docutils literal">append</tt> 与 <tt class="docutils literal">+</tt> 操作符</a><ul>
-<li><a class="reference internal" href="#tdd-test-driven-development" id="id29">TDD - Test-driven Development</a></li>
-<li><a class="reference internal" href="#id10" id="id30">计算复杂度</a></li>
+<li><a class="reference internal" href="#python" id="id16">Python的发音纠正</a></li>
+<li><a class="reference internal" href="#id2" id="id17">Python源流</a></li>
+<li><a class="reference internal" href="#id3" id="id18">Python的关键词</a></li>
+<li><a class="reference internal" href="#id4" id="id19">值的类型</a></li>
+<li><a class="reference internal" href="#variable" id="id20">变量（Variable）</a></li>
+<li><a class="reference internal" href="#mutable" id="id21">可变（mutable）类型与不可变类型</a></li>
+<li><a class="reference internal" href="#id5" id="id22">数与格式化显示</a></li>
+<li><a class="reference internal" href="#strings" id="id23">字符串（Strings）</a></li>
+<li><a class="reference internal" href="#concatenation" id="id24">字符串相加（concatenation）</a></li>
+<li><a class="reference internal" href="#slice" id="id25">子串（slice）</a></li>
+<li><a class="reference internal" href="#id6" id="id26">搜索字符串</a></li>
+<li><a class="reference internal" href="#string" id="id27">String类（对象）方法</a></li>
+<li><a class="reference internal" href="#in" id="id28">in操作符</a></li>
+<li><a class="reference internal" href="#id7" id="id29">字符串比较</a></li>
+<li><a class="reference internal" href="#id8" id="id30">列表</a></li>
+<li><a class="reference internal" href="#id9" id="id31">列表作为参数</a></li>
+<li><a class="reference internal" href="#append" id="id32">注意区别 <tt class="docutils literal">append</tt> 与 <tt class="docutils literal">+</tt> 操作符</a><ul>
+<li><a class="reference internal" href="#tdd-test-driven-development" id="id33">TDD - Test-driven Development</a></li>
+<li><a class="reference internal" href="#id10" id="id34">计算复杂度</a></li>
 </ul>
 </li>
-<li><a class="reference internal" href="#dictionary" id="id31">字典（Dictionary）</a><ul>
-<li><a class="reference internal" href="#incremental-development" id="id32">递增开发（Incremental Development）</a></li>
+<li><a class="reference internal" href="#dictionary" id="id35">字典（Dictionary）</a><ul>
+<li><a class="reference internal" href="#incremental-development" id="id36">递增开发（Incremental Development）</a></li>
+<li><a class="reference internal" href="#keyvalue" id="id37">key与value互换</a></li>
 </ul>
 </li>
-<li><a class="reference internal" href="#id11" id="id33">参考</a></li>
+<li><a class="reference internal" href="#id11" id="id38">函数</a><ul>
+<li><a class="reference internal" href="#id12" id="id39">局部变量</a></li>
+<li><a class="reference internal" href="#id13" id="id40">全局变量</a></li>
+<li><a class="reference internal" href="#id14" id="id41">函数执行顺序</a></li>
+</ul>
+</li>
+<li><a class="reference internal" href="#id15" id="id42">参考</a></li>
 </ul>
 </div>
 <div class="section" id="python">
-<h1><a class="toc-backref" href="#id12">Python的发音纠正</a></h1>
+<h1><a class="toc-backref" href="#id16">Python的发音纠正</a></h1>
 <p>国人普遍把th发作s。 Not quite correct。</p>
 <p>ˈpī-ˌthän , -thən <a class="reference external" href="https://cn.bing.com/search?q=define%20python&amp;tf=U2VydmljZT1EaWN0aW9uYXJ5QW5zd2VyVjIgU2NlbmFyaW89RGVmaW5pdGlvblNjZW5hcmlvIFBvc2l0aW9uPU5PUCBSYW5raW5nRGF0YT1UcnVlIEZvcmNlUGxhY2U9RmFsc2UgUGFpcnM9RGljdGlvbmFyeVdvcmQ6cHl0aG9uO3NjbjpEZWZpbml0aW9uU2NlbmFyaW87cDpRQVM7IHw%3d&amp;hs=hyRBF0mYq9hrfQUq66DIZnFVta1ZGRfBiBks25oUguk%3d">pronounciation</a></p>
 </div>
 <div class="section" id="id2">
-<h1><a class="toc-backref" href="#id13">Python源流</a></h1>
+<h1><a class="toc-backref" href="#id17">Python源流</a></h1>
 <p>Python之父Guido van Rossum，荷兰人，1956年生，1982年阿姆斯特丹大学获得
 数学与计算机科学硕士学位。有过ABC语言的工作经验。1989年设计了Python语
 言。</p>
@@ -461,7 +468,7 @@ ul.auto-toc {
 </blockquote>
 </div>
 <div class="section" id="id3">
-<h1><a class="toc-backref" href="#id14">Python的关键词</a></h1>
+<h1><a class="toc-backref" href="#id18">Python的关键词</a></h1>
 <div class="line-block">
 <div class="line">def pass</div>
 <div class="line">from import</div>
@@ -483,7 +490,7 @@ ul.auto-toc {
 <p>关键词被语言留用（reserved），无法作变量名。</p>
 </div>
 <div class="section" id="id4">
-<h1><a class="toc-backref" href="#id15">值的类型</a></h1>
+<h1><a class="toc-backref" href="#id19">值的类型</a></h1>
 <p>所有的值都是对象。a = 5, help(a)  a.bit_length()</p>
 <p>数字。1， 1.，1.1， .1, 1e1, 1e-1, 1E1, 1E-1</p>
 <dl class="docutils">
@@ -503,7 +510,7 @@ A list of objects</dd>
 <p>元组（tuple），字典（dict）。</p>
 </div>
 <div class="section" id="variable">
-<h1><a class="toc-backref" href="#id16">变量（Variable）</a></h1>
+<h1><a class="toc-backref" href="#id20">变量（Variable）</a></h1>
 <p>是一个名字（name），是指向一个值（value）的名字。</p>
 <p>值存放在内存（memory）中的某个地址。</p>
 <p>尽量选有意义的简短的名字。比如，代表个数用n，代表索引用i，j，k。</p>
@@ -592,7 +599,7 @@ A list of objects</dd>
 </ul>
 </div>
 <div class="section" id="mutable">
-<h1><a class="toc-backref" href="#id17">可变（mutable）类型与不可变类型</a></h1>
+<h1><a class="toc-backref" href="#id21">可变（mutable）类型与不可变类型</a></h1>
 <p>字符串是不可变的（immutable）类型，不能在原内存地址改变。</p>
 <p>a = 'hello'  不可以原地修改a[0] = 'H'。需要修改a的值时，需要对a进行重新赋值a = 'Hello'。</p>
 <p>列表是可变（mutable）类型，能在原内存地址改变。</p>
@@ -617,7 +624,8 @@ A list of objects</dd>
 </blockquote>
 </div>
 <div class="section" id="id5">
-<h1><a class="toc-backref" href="#id18">数与格式化显示</a></h1>
+<h1><a class="toc-backref" href="#id22">数与格式化显示</a></h1>
+<blockquote>
 <div class="line-block">
 <div class="line">x = 3.1415926</div>
 </div>
@@ -661,9 +669,10 @@ A list of objects</dd>
 <div class="line-block">
 <div class="line">print('%f' % (x))</div>
 </div>
+</blockquote>
 </div>
 <div class="section" id="strings">
-<h1><a class="toc-backref" href="#id19">字符串（Strings）</a></h1>
+<h1><a class="toc-backref" href="#id23">字符串（Strings）</a></h1>
 <p>由字符组成。</p>
 <div class="line-block">
 <div class="line">fruit = 'banana!'</div>
@@ -719,7 +728,7 @@ A list of objects</dd>
 <p>以上 <tt class="docutils literal"># [start,stop,step]</tt> 代表注释（comment），注释以 <tt class="docutils literal">#</tt> 号开头。</p>
 </div>
 <div class="section" id="concatenation">
-<h1><a class="toc-backref" href="#id20">字符串相加（concatenation）</a></h1>
+<h1><a class="toc-backref" href="#id24">字符串相加（concatenation）</a></h1>
 <p>输出Jack, Kack, Lack, Mack, Nack, Ouack, Pack, and Quack</p>
 <div class="line-block">
 <div class="line">prefixes = 'JKLMNOPQ'</div>
@@ -738,7 +747,7 @@ A list of objects</dd>
 </div>
 </div>
 <div class="section" id="slice">
-<h1><a class="toc-backref" href="#id21">子串（slice）</a></h1>
+<h1><a class="toc-backref" href="#id25">子串（slice）</a></h1>
 <p>s[n:m]，其中n或m可省略。
 包括第n个字符，不包括第m个字符。（索引自0开始）</p>
 <div class="line-block">
@@ -762,7 +771,7 @@ A list of objects</dd>
 </div>
 </div>
 <div class="section" id="id6">
-<h1><a class="toc-backref" href="#id22">搜索字符串</a></h1>
+<h1><a class="toc-backref" href="#id26">搜索字符串</a></h1>
 <div class="line-block">
 <div class="line">def find(word, c):</div>
 <div class="line-block">
@@ -788,7 +797,7 @@ A list of objects</dd>
 <p>练习：用上面三参数的find来做。</p>
 </div>
 <div class="section" id="string">
-<h1><a class="toc-backref" href="#id23">String类（对象）方法</a></h1>
+<h1><a class="toc-backref" href="#id27">String类（对象）方法</a></h1>
 <div class="line-block">
 <div class="line">upper()</div>
 <div class="line">lower()</div>
@@ -801,14 +810,14 @@ A list of objects</dd>
 </div>
 </div>
 <div class="section" id="in">
-<h1><a class="toc-backref" href="#id24">in操作符</a></h1>
+<h1><a class="toc-backref" href="#id28">in操作符</a></h1>
 <p>'a' in 'banana'
 'seed' in 'banana'</p>
 <p>练习：写出下面的函数，使得
 in_both('apples', 'oranges')返回'aes'</p>
 </div>
 <div class="section" id="id7">
-<h1><a class="toc-backref" href="#id25">字符串比较</a></h1>
+<h1><a class="toc-backref" href="#id29">字符串比较</a></h1>
 <p>字典序（alphabetical order）。大写字母排在小写字母前。</p>
 <p>字符串之间可以有以下对比操作:</p>
 <div class="line-block">
@@ -824,7 +833,7 @@ in_both('apples', 'oranges')返回'aes'</p>
 <p>即兴定义函数，制造一个长度不小于4的密码。</p>
 </div>
 <div class="section" id="id8">
-<h1><a class="toc-backref" href="#id26">列表</a></h1>
+<h1><a class="toc-backref" href="#id30">列表</a></h1>
 <p>语言的内置（built-in）类型。注意与String类比，index也是从0开始， in操作符， 求长度，获得字串，遍历操作类似。</p>
 <blockquote>
 <div class="line-block">
@@ -865,7 +874,7 @@ in_both('apples', 'oranges')返回'aes'</p>
 <dd>return 2*x</dd>
 </dl>
 <p>list(map(f, [1,2]]))</p>
-<p>filter方法，从几个值中选择符合条件的几个值</p>
+<p>filter方法，从几个值中选择符合条件的几个值。</p>
 <blockquote>
 <div class="line-block">
 <div class="line">def f(x):</div>
@@ -942,7 +951,7 @@ a与b是指向[1,2,3]的两个references。
 error-prone（易错）</p>
 </div>
 <div class="section" id="id9">
-<h1><a class="toc-backref" href="#id27">列表作为参数</a></h1>
+<h1><a class="toc-backref" href="#id31">列表作为参数</a></h1>
 <blockquote>
 <div class="line-block">
 <div class="line">def delete_head(t):</div>
@@ -958,7 +967,7 @@ error-prone（易错）</p>
 </blockquote>
 </div>
 <div class="section" id="append">
-<h1><a class="toc-backref" href="#id28">注意区别 <tt class="docutils literal">append</tt> 与 <tt class="docutils literal">+</tt> 操作符</a></h1>
+<h1><a class="toc-backref" href="#id32">注意区别 <tt class="docutils literal">append</tt> 与 <tt class="docutils literal">+</tt> 操作符</a></h1>
 <blockquote>
 <div class="line-block">
 <div class="line">t1 = [1, 2]</div>
@@ -985,18 +994,53 @@ error-prone（易错）</p>
 </dl>
 </blockquote>
 <div class="section" id="tdd-test-driven-development">
-<h2><a class="toc-backref" href="#id29">TDD - Test-driven Development</a></h2>
+<h2><a class="toc-backref" href="#id33">TDD - Test-driven Development</a></h2>
 <p>测试驱动开发。 My favourite。 刺激有挑战性。 帮助厘清需求。  帮助编写代码。</p>
-<p>推荐使用pytest。如何安装？ 使用命令 <tt class="docutils literal">pip install pytest</tt></p>
+<p>推荐使用pytest。如何安装？ 使用命令 <tt class="docutils literal">pip install pytest</tt>。</p>
+<p>在 <tt class="docutils literal">test_cases.py</tt> 写如下测试用例。然后在命令行运行： <tt class="docutils literal">python <span class="pre">-m</span> pytest test_cases.py</tt> 。</p>
+<pre class="code python literal-block">
+<span class="comment single"># Copyright (c) Hui Lan 2019</span>
+
+<span class="keyword namespace">import</span> <span class="name namespace">random</span>
+<span class="keyword namespace">import</span> <span class="name namespace">string</span>
+
+<span class="keyword">def</span> <span class="name function">make_password</span><span class="punctuation">(</span><span class="name">n</span><span class="punctuation">):</span>
+    <span class="literal string doc">'''
+    Return a string of length n consisting of a combination of
+    letters, digits and special characters.  Note that each password
+    must have at least one lower case letter, one upper case letter,
+    one digit and one special charater.  Return an empty string if n
+    is less than 4.
+    '''</span>
+
+    <span class="keyword">if</span> <span class="name">n</span> <span class="operator">&lt;</span> <span class="literal number integer">4</span><span class="punctuation">:</span>
+        <span class="keyword">return</span> <span class="literal string single">''</span>
+
+    <span class="name">password</span> <span class="operator">=</span> <span class="name">random</span><span class="operator">.</span><span class="name">choice</span><span class="punctuation">(</span><span class="name">string</span><span class="operator">.</span><span class="name">ascii_lowercase</span><span class="punctuation">)</span> <span class="operator">+</span> \
+        <span class="name">random</span><span class="operator">.</span><span class="name">choice</span><span class="punctuation">(</span><span class="name">string</span><span class="operator">.</span><span class="name">ascii_uppercase</span><span class="punctuation">)</span> <span class="operator">+</span> \
+        <span class="name">random</span><span class="operator">.</span><span class="name">choice</span><span class="punctuation">(</span><span class="name">string</span><span class="operator">.</span><span class="name">digits</span><span class="punctuation">)</span> <span class="operator">+</span> \
+        <span class="name">random</span><span class="operator">.</span><span class="name">choice</span><span class="punctuation">(</span><span class="name">string</span><span class="operator">.</span><span class="name">punctuation</span><span class="punctuation">)</span> <span class="operator">+</span> \
+        <span class="literal string single">''</span><span class="operator">.</span><span class="name">join</span><span class="punctuation">([</span><span class="name">random</span><span class="operator">.</span><span class="name">choice</span><span class="punctuation">(</span><span class="name">string</span><span class="operator">.</span><span class="name">ascii_letters</span> <span class="operator">+</span> <span class="name">string</span><span class="operator">.</span><span class="name">digits</span> <span class="operator">+</span> <span class="name">string</span><span class="operator">.</span><span class="name">punctuation</span><span class="punctuation">)</span> <span class="keyword">for</span> <span class="name">i</span> <span class="operator word">in</span> <span class="name builtin">range</span><span class="punctuation">(</span><span class="name">n</span><span class="operator">-</span><span class="literal number integer">4</span><span class="punctuation">)])</span>
+
+    <span class="keyword">return</span> <span class="literal string single">''</span><span class="operator">.</span><span class="name">join</span><span class="punctuation">(</span><span class="name">random</span><span class="operator">.</span><span class="name">sample</span><span class="punctuation">(</span><span class="name">password</span><span class="punctuation">,</span> <span class="name">n</span><span class="punctuation">))</span> <span class="comment single"># shuffle password then return</span>
+
+
+
+
+<span class="keyword">if</span> <span class="name variable magic">__name__</span> <span class="operator">==</span> <span class="literal string single">'__main__'</span><span class="punctuation">:</span>
+    <span class="keyword">for</span> <span class="name">n</span> <span class="operator word">in</span> <span class="name builtin">range</span><span class="punctuation">(</span><span class="literal number integer">0</span><span class="punctuation">,</span><span class="literal number integer">20</span><span class="punctuation">):</span>
+        <span class="name">pwd</span> <span class="operator">=</span> <span class="name">make_password</span><span class="punctuation">(</span><span class="name">n</span><span class="punctuation">)</span>
+        <span class="keyword">print</span><span class="punctuation">(</span><span class="name">pwd</span><span class="punctuation">)</span>
+</pre>
 </div>
 <div class="section" id="id10">
-<h2><a class="toc-backref" href="#id30">计算复杂度</a></h2>
+<h2><a class="toc-backref" href="#id34">计算复杂度</a></h2>
 <p>用Big O表述复杂度。O(n)， O(n^2), O(n^3)。</p>
 <p>密码实验回顾。</p>
 </div>
 </div>
 <div class="section" id="dictionary">
-<h1><a class="toc-backref" href="#id31">字典（Dictionary）</a></h1>
+<h1><a class="toc-backref" href="#id35">字典（Dictionary）</a></h1>
 <p>Mutable数据类型。</p>
 <p>实际开发中超级有用。</p>
 <blockquote>
@@ -1020,66 +1064,228 @@ error-prone（易错）</p>
 <p>key-value pair (item)</p>
 <p>item的顺序不可预测，不是按照创建时的顺序。</p>
 <div class="section" id="incremental-development">
-<h2><a class="toc-backref" href="#id32">递增开发（Incremental Development）</a></h2>
+<h2><a class="toc-backref" href="#id36">递增开发（Incremental Development）</a></h2>
 <p>每次完成一小点。从易到难。</p>
 <p>练习：给定一个字符串，数出每个字母出现的频率。</p>
-<blockquote>
-<div class="line-block">
-<div class="line">def histogram(s):</div>
-<div class="line-block">
-<div class="line">''' Cannot pass any test cases. '''</div>
-<div class="line">pass</div>
-</div>
-</div>
-<div class="line-block">
-<div class="line">def histogram(s):</div>
-<div class="line-block">
-<div class="line">''' Can pass the test case in which s is an empty string. '''</div>
-<div class="line">d = {}</div>
-<div class="line">return d</div>
-</div>
-</div>
-<div class="line-block">
-<div class="line">def histogram(s):</div>
-<div class="line-block">
-<div class="line">''' Can pass the test cases in which all characters in s are unique. '''</div>
-<div class="line">d = {}</div>
-<div class="line">for c in s:</div>
-<div class="line-block">
-<div class="line">d[c] = 1</div>
-</div>
-<div class="line">return d</div>
-</div>
-</div>
-<div class="line-block">
-<div class="line">def histogram(s):</div>
-<div class="line-block">
-<div class="line">''' Can pass all test cases. '''</div>
-<div class="line">d = {}</div>
-<div class="line">for c in s:</div>
-<div class="line-block">
-<div class="line">if c not in d:</div>
-<div class="line-block">
-<div class="line">d[c] = 1</div>
-</div>
-<div class="line">else:</div>
-<div class="line-block">
-<div class="line">d[c] += 1</div>
-</div>
+<pre class="code python literal-block">
+<span class="keyword">def</span> <span class="name function">histogram</span><span class="punctuation">(</span><span class="name">s</span><span class="punctuation">):</span>
+    <span class="literal string doc">''' Cannot pass any test cases. '''</span>
+    <span class="keyword">pass</span>
+
+<span class="keyword">def</span> <span class="name function">histogram</span><span class="punctuation">(</span><span class="name">s</span><span class="punctuation">):</span>
+    <span class="literal string doc">''' Can pass the test case in which s is an empty string. '''</span>
+    <span class="name">d</span> <span class="operator">=</span> <span class="punctuation">{}</span>
+    <span class="keyword">return</span> <span class="name">d</span>
+
+<span class="keyword">def</span> <span class="name function">histogram</span><span class="punctuation">(</span><span class="name">s</span><span class="punctuation">):</span>
+    <span class="literal string doc">''' Can pass the test cases in which all characters in s are unique. '''</span>
+    <span class="name">d</span> <span class="operator">=</span> <span class="punctuation">{}</span>
+    <span class="keyword">for</span> <span class="name">c</span> <span class="operator word">in</span> <span class="name">s</span><span class="punctuation">:</span>
+        <span class="name">d</span><span class="punctuation">[</span><span class="name">c</span><span class="punctuation">]</span> <span class="operator">=</span> <span class="literal number integer">1</span>
+    <span class="keyword">return</span> <span class="name">d</span>
+
+<span class="keyword">def</span> <span class="name function">histogram</span><span class="punctuation">(</span><span class="name">s</span><span class="punctuation">):</span>
+    <span class="literal string doc">''' Can pass all test cases. '''</span>
+    <span class="name">d</span> <span class="operator">=</span> <span class="punctuation">{}</span>
+    <span class="keyword">for</span> <span class="name">c</span> <span class="operator word">in</span> <span class="name">s</span><span class="punctuation">:</span>
+        <span class="keyword">if</span> <span class="name">c</span> <span class="operator word">not</span> <span class="operator word">in</span> <span class="name">d</span><span class="punctuation">:</span>
+            <span class="name">d</span><span class="punctuation">[</span><span class="name">c</span><span class="punctuation">]</span> <span class="operator">=</span> <span class="literal number integer">1</span>
+        <span class="keyword">else</span><span class="punctuation">:</span>
+            <span class="name">d</span><span class="punctuation">[</span><span class="name">c</span><span class="punctuation">]</span> <span class="operator">+=</span> <span class="literal number integer">1</span>
+    <span class="keyword">return</span> <span class="name">d</span>
+
+
+<span class="name">h</span> <span class="operator">=</span> <span class="name">histogram</span><span class="punctuation">(</span><span class="literal string single">'good'</span><span class="punctuation">)</span>
+<span class="keyword">print</span><span class="punctuation">(</span><span class="name">h</span><span class="punctuation">)</span>
+</pre>
+<p>练习：给定一个字符串，数出每个单词出现的频率。</p>
+<p>练习：给定一个新闻文本，数出每个单词出现的频率。考虑以下方面，（1）只考虑字典里有的单词。（2）单词周围如有标点符号，要先移除。</p>
+<pre class="code python literal-block">
+<span class="comment single"># Copyright (C) 2019 Hui Lan</span>
+<span class="comment single"># The following line fixes SyntaxError: Non-UTF-8 code starting with ...</span>
+<span class="comment single"># coding=utf8</span>
+
+<span class="keyword">def</span> <span class="name function">file2lst</span><span class="punctuation">(</span><span class="name">fname</span><span class="punctuation">):</span>
+    <span class="literal string doc">''' Return a list where each element is a word from fname. '''</span>
+    <span class="name">L</span> <span class="operator">=</span> <span class="punctuation">[]</span>
+    <span class="name">f</span> <span class="operator">=</span> <span class="name builtin">open</span><span class="punctuation">(</span><span class="name">fname</span><span class="punctuation">)</span>
+    <span class="keyword">for</span> <span class="name">line</span> <span class="operator word">in</span> <span class="name">f</span><span class="punctuation">:</span>
+        <span class="name">line</span> <span class="operator">=</span> <span class="name">line</span><span class="operator">.</span><span class="name">strip</span><span class="punctuation">()</span>
+        <span class="name">lst</span> <span class="operator">=</span> <span class="name">line</span><span class="operator">.</span><span class="name">split</span><span class="punctuation">()</span>
+        <span class="keyword">for</span> <span class="name">x</span> <span class="operator word">in</span> <span class="name">lst</span><span class="punctuation">:</span>
+            <span class="name">L</span><span class="operator">.</span><span class="name">append</span><span class="punctuation">(</span><span class="name">x</span><span class="punctuation">)</span>
+    <span class="name">f</span><span class="operator">.</span><span class="name">close</span><span class="punctuation">()</span>
+    <span class="keyword">return</span> <span class="name">L</span>
+
+
+<span class="keyword">def</span> <span class="name function">lst2dict</span><span class="punctuation">(</span><span class="name">lst</span><span class="punctuation">):</span>
+    <span class="literal string doc">''' Return a dictionary given list lst.  Each key is an element in the lst.
+    The value is always 1.'''</span>
+    <span class="name">d</span> <span class="operator">=</span> <span class="punctuation">{}</span>
+    <span class="keyword">for</span> <span class="name">w</span> <span class="operator word">in</span> <span class="name">lst</span><span class="punctuation">:</span>
+        <span class="name">d</span><span class="punctuation">[</span><span class="name">w</span><span class="punctuation">]</span> <span class="operator">=</span> <span class="literal number integer">1</span>
+    <span class="keyword">return</span> <span class="name">d</span>
+
+
+<span class="keyword namespace">import</span> <span class="name namespace">string</span>
+<span class="keyword">def</span> <span class="name function">remove_punctuation</span><span class="punctuation">(</span><span class="name">s</span><span class="punctuation">):</span>
+    <span class="name">p</span> <span class="operator">=</span> <span class="literal string single">',.:’“”'</span> <span class="operator">+</span> <span class="name">string</span><span class="operator">.</span><span class="name">punctuation</span>
+    <span class="name">t</span> <span class="operator">=</span> <span class="literal string single">''</span>
+    <span class="keyword">for</span> <span class="name">c</span> <span class="operator word">in</span> <span class="name">s</span><span class="punctuation">:</span>
+        <span class="keyword">if</span> <span class="operator word">not</span> <span class="name">c</span> <span class="operator word">in</span> <span class="name">p</span><span class="punctuation">:</span>
+            <span class="name">t</span> <span class="operator">+=</span> <span class="name">c</span>
+        <span class="keyword">elif</span> <span class="name">c</span> <span class="operator">==</span> <span class="literal string single">'’'</span><span class="punctuation">:</span> <span class="comment single"># handle the case such as May’s</span>
+            <span class="keyword">return</span> <span class="name">t</span>
+    <span class="keyword">return</span> <span class="name">t</span>
+
+<span class="keyword">def</span> <span class="name function">word_frequency</span><span class="punctuation">(</span><span class="name">fname</span><span class="punctuation">,</span> <span class="name">english_dictionary</span><span class="punctuation">):</span>
+    <span class="literal string doc">''' Return a dictionary where each key is a word both in the file fname and in
+    the dictionary english_dictionary, and the corresponding value is the frequency
+    of that word.'''</span>
+    <span class="name">d</span> <span class="operator">=</span> <span class="punctuation">{}</span>
+    <span class="name">L</span> <span class="operator">=</span> <span class="name">file2lst</span><span class="punctuation">(</span><span class="name">fname</span><span class="punctuation">)</span>
+    <span class="keyword">for</span> <span class="name">x</span> <span class="operator word">in</span> <span class="name">L</span><span class="punctuation">:</span>
+        <span class="name">x</span> <span class="operator">=</span> <span class="name">remove_punctuation</span><span class="punctuation">(</span><span class="name">x</span><span class="operator">.</span><span class="name">lower</span><span class="punctuation">())</span>
+        <span class="keyword">if</span> <span class="name">x</span> <span class="operator word">in</span> <span class="name">english_dictionary</span><span class="punctuation">:</span>
+            <span class="keyword">if</span> <span class="operator word">not</span> <span class="name">x</span> <span class="operator word">in</span> <span class="name">d</span><span class="punctuation">:</span>
+                <span class="name">d</span><span class="punctuation">[</span><span class="name">x</span><span class="punctuation">]</span> <span class="operator">=</span> <span class="literal number integer">1</span>
+            <span class="keyword">else</span><span class="punctuation">:</span>
+                <span class="name">d</span><span class="punctuation">[</span><span class="name">x</span><span class="punctuation">]</span> <span class="operator">+=</span> <span class="literal number integer">1</span>
+    <span class="keyword">return</span> <span class="name">d</span>
+
+
+<span class="keyword">def</span> <span class="name function">sort_by_value</span><span class="punctuation">(</span><span class="name">d</span><span class="punctuation">):</span>
+    <span class="literal string doc">''' Return a sorted list of tuples, each tuple containing a key and a value.
+        Note that the tuples are order in descending order of the value.'''</span>
+    <span class="keyword namespace">import</span> <span class="name namespace">operator</span>
+    <span class="name">lst</span> <span class="operator">=</span> <span class="name builtin">sorted</span><span class="punctuation">(</span><span class="name">d</span><span class="operator">.</span><span class="name">items</span><span class="punctuation">(),</span> <span class="name">key</span><span class="operator">=</span><span class="name">operator</span><span class="operator">.</span><span class="name">itemgetter</span><span class="punctuation">(</span><span class="literal number integer">1</span><span class="punctuation">),</span> <span class="name">reverse</span><span class="operator">=</span><span class="name builtin pseudo">True</span><span class="punctuation">)</span>
+    <span class="keyword">return</span> <span class="name">lst</span>
+
+
+<span class="keyword">if</span> <span class="name variable magic">__name__</span> <span class="operator">==</span> <span class="literal string single">'__main__'</span><span class="punctuation">:</span>
+    <span class="name">ed</span> <span class="operator">=</span> <span class="name">lst2dict</span><span class="punctuation">(</span><span class="name">file2lst</span><span class="punctuation">(</span><span class="literal string single">'words.txt'</span><span class="punctuation">))</span> <span class="comment single"># from http://greenteapress.com/thinkpython2/code/words.txt</span>
+    <span class="name">d</span> <span class="operator">=</span> <span class="name">word_frequency</span><span class="punctuation">(</span><span class="literal string single">'brexit-news.txt'</span><span class="punctuation">,</span> <span class="name">ed</span><span class="punctuation">)</span>
+    <span class="name">lst</span> <span class="operator">=</span> <span class="name">sort_by_value</span><span class="punctuation">(</span><span class="name">d</span><span class="punctuation">)</span>
+    <span class="keyword">for</span> <span class="name">x</span> <span class="operator word">in</span> <span class="name">lst</span><span class="punctuation">:</span>
+        <span class="keyword">print</span><span class="punctuation">(</span><span class="literal string single">'</span><span class="literal string interpol">%s</span><span class="literal string single"> (</span><span class="literal string interpol">%d</span><span class="literal string single">)'</span> <span class="operator">%</span> <span class="punctuation">(</span><span class="name">x</span><span class="punctuation">[</span><span class="literal number integer">0</span><span class="punctuation">],</span> <span class="name">x</span><span class="punctuation">[</span><span class="literal number integer">1</span><span class="punctuation">]))</span>
+</pre>
+</div>
+<div class="section" id="keyvalue">
+<h2><a class="toc-backref" href="#id37">key与value互换</a></h2>
+<p>注意到在原来的字典中一个value可能对应多个key的值。比如 <tt class="docutils literal">d = <span class="pre">{'a':1,</span> <span class="pre">'b':2,</span> <span class="pre">'c':2}</span></tt> 中，2就对应两个key，'b'与'c'。</p>
+<pre class="code python literal-block">
+<span class="keyword">def</span> <span class="name function">inverse_dictionary</span><span class="punctuation">(</span><span class="name">d</span><span class="punctuation">):</span>
+    <span class="name">d2</span> <span class="operator">=</span> <span class="punctuation">{}</span>
+    <span class="keyword">for</span> <span class="name">k</span> <span class="operator word">in</span> <span class="name">d</span><span class="punctuation">:</span>
+        <span class="name">v</span> <span class="operator">=</span> <span class="name">d</span><span class="punctuation">[</span><span class="name">k</span><span class="punctuation">]</span>
+        <span class="keyword">if</span> <span class="operator word">not</span> <span class="name">v</span> <span class="operator word">in</span> <span class="name">d2</span><span class="punctuation">:</span>
+            <span class="name">d2</span><span class="punctuation">[</span><span class="name">v</span><span class="punctuation">]</span> <span class="operator">=</span> <span class="punctuation">[</span><span class="name">k</span><span class="punctuation">]</span>
+        <span class="keyword">else</span><span class="punctuation">:</span>
+            <span class="name">d2</span><span class="punctuation">[</span><span class="name">v</span><span class="punctuation">]</span><span class="operator">.</span><span class="name">append</span><span class="punctuation">(</span><span class="name">k</span><span class="punctuation">)</span>
+    <span class="keyword">return</span> <span class="name">d2</span>
+
+
+
+<span class="name">d</span> <span class="operator">=</span> <span class="punctuation">{</span><span class="literal string single">'a'</span><span class="punctuation">:</span><span class="literal number integer">1</span><span class="punctuation">,</span> <span class="literal string single">'b'</span><span class="punctuation">:</span><span class="literal number integer">2</span><span class="punctuation">,</span> <span class="literal string single">'c'</span><span class="punctuation">:</span><span class="literal number integer">2</span><span class="punctuation">}</span>
+<span class="name">d2</span> <span class="operator">=</span> <span class="name">inverse_dictionary</span><span class="punctuation">(</span><span class="name">d</span><span class="punctuation">)</span>
+<span class="keyword">print</span><span class="punctuation">(</span><span class="name">d2</span><span class="punctuation">)</span>
+</pre>
+<p>练习： 用 <tt class="docutils literal">inverse_dictionary</tt> 对上面 <tt class="docutils literal">d = <span class="pre">word_frequency('brexit-news.txt',</span> ed)</tt> 产生的 <tt class="docutils literal">d</tt> 进行转化。然后按照单词出现频率从高到低把所有单词都显示出来。每行显示一个频率内的所有单词。</p>
+<pre class="code python literal-block">
+<span class="name">d2</span> <span class="operator">=</span> <span class="name">inverse_dictionary</span><span class="punctuation">(</span><span class="name">d</span><span class="punctuation">)</span>
+<span class="keyword">for</span> <span class="name">k</span> <span class="operator word">in</span> <span class="name builtin">sorted</span><span class="punctuation">(</span><span class="name">d2</span><span class="operator">.</span><span class="name">keys</span><span class="punctuation">(),</span> <span class="name">reverse</span><span class="operator">=</span><span class="name builtin pseudo">True</span><span class="punctuation">):</span>
+    <span class="keyword">print</span><span class="punctuation">(</span><span class="literal string single">'</span><span class="literal string interpol">%d</span><span class="literal string single"> </span><span class="literal string interpol">%s</span><span class="literal string single">'</span> <span class="operator">%</span> <span class="punctuation">(</span><span class="name">k</span><span class="punctuation">,</span> <span class="literal string single">' '</span><span class="operator">.</span><span class="name">join</span><span class="punctuation">(</span><span class="name">d2</span><span class="punctuation">[</span><span class="name">k</span><span class="punctuation">])))</span>
+</pre>
+<p>练习： 使用 <tt class="docutils literal">setdefault</tt> 方法对上面的 <tt class="docutils literal">inverse_dictionary</tt> 进行简化 （减少行数）。</p>
+<pre class="code python literal-block">
+<span class="keyword">def</span> <span class="name function">inverse_dictionary</span><span class="punctuation">(</span><span class="name">d</span><span class="punctuation">):</span>
+    <span class="name">d2</span> <span class="operator">=</span> <span class="punctuation">{}</span>
+    <span class="keyword">for</span> <span class="name">k</span> <span class="operator word">in</span> <span class="name">d</span><span class="punctuation">:</span>
+        <span class="name">v</span> <span class="operator">=</span> <span class="name">d</span><span class="punctuation">[</span><span class="name">k</span><span class="punctuation">]</span>
+        <span class="name">d2</span><span class="operator">.</span><span class="name">setdefault</span><span class="punctuation">(</span><span class="name">v</span><span class="punctuation">,</span> <span class="punctuation">[])</span><span class="operator">.</span><span class="name">append</span><span class="punctuation">(</span><span class="name">k</span><span class="punctuation">)</span>
+
+    <span class="keyword">return</span> <span class="name">d2</span>
+</pre>
 </div>
-<div class="line">return d</div>
 </div>
+<div class="section" id="id11">
+<h1><a class="toc-backref" href="#id38">函数</a></h1>
+<p>函数 <tt class="docutils literal">unique_words</tt> 与 <tt class="docutils literal">unique_words2</tt> 哪个运行速度快？</p>
+<pre class="code python literal-block">
+<span class="keyword">def</span> <span class="name function">unique_words</span><span class="punctuation">(</span><span class="name">lst</span><span class="punctuation">):</span>
+    <span class="name">d</span> <span class="operator">=</span> <span class="punctuation">{}</span>
+    <span class="keyword">for</span> <span class="name">x</span> <span class="operator word">in</span> <span class="name">lst</span><span class="punctuation">:</span>
+        <span class="name">d</span><span class="punctuation">[</span><span class="name">x</span><span class="punctuation">]</span> <span class="operator">=</span> <span class="literal number integer">1</span>
+    <span class="keyword">return</span> <span class="name builtin">sorted</span><span class="punctuation">(</span><span class="name">d</span><span class="operator">.</span><span class="name">keys</span><span class="punctuation">())</span>
+
+<span class="keyword">def</span> <span class="name function">unique_words2</span><span class="punctuation">(</span><span class="name">lst</span><span class="punctuation">):</span>
+    <span class="keyword">return</span> <span class="name builtin">sorted</span><span class="punctuation">(</span><span class="name builtin">list</span><span class="punctuation">(</span><span class="name builtin">set</span><span class="punctuation">(</span><span class="name">lst</span><span class="punctuation">)))</span>
+
+
+<span class="name">N</span> <span class="operator">=</span> <span class="literal number integer">10000000</span>
+<span class="keyword">print</span><span class="punctuation">(</span><span class="name">unique_words</span><span class="punctuation">([</span><span class="literal string single">'hello'</span><span class="punctuation">,</span> <span class="literal string single">'world'</span><span class="punctuation">,</span> <span class="literal string single">'am'</span><span class="punctuation">,</span> <span class="literal string single">'he'</span><span class="punctuation">]</span> <span class="operator">*</span> <span class="name">N</span><span class="punctuation">))</span>
+<span class="keyword">print</span><span class="punctuation">(</span><span class="name">unique_words2</span><span class="punctuation">([</span><span class="literal string single">'hello'</span><span class="punctuation">,</span> <span class="literal string single">'world'</span><span class="punctuation">,</span> <span class="literal string single">'am'</span><span class="punctuation">,</span> <span class="literal string single">'he'</span><span class="punctuation">]</span> <span class="operator">*</span> <span class="name">N</span><span class="punctuation">))</span>
+</pre>
+<div class="section" id="id12">
+<h2><a class="toc-backref" href="#id39">局部变量</a></h2>
+<p>在函数之内。函数执行结束，局部变量消失。</p>
 </div>
-<div class="line-block">
-<div class="line">h = histogram('good')</div>
-<div class="line">print(h)</div>
+<div class="section" id="id13">
+<h2><a class="toc-backref" href="#id40">全局变量</a></h2>
+<p>全局变量位于函数之外，模块之内。全局变量对所有模块内的函数可见（可读）。如果在函数内要对全局变量重新赋值，那么要先用 <tt class="docutils literal">global</tt> 声明之 （declare）。</p>
+<pre class="code python literal-block">
+<span class="name">verbose</span> <span class="operator">=</span> <span class="name builtin pseudo">True</span>
+
+<span class="keyword">def</span> <span class="name function">example1</span><span class="punctuation">():</span>
+    <span class="keyword">if</span> <span class="name">verbose</span><span class="punctuation">:</span>
+        <span class="keyword">print</span><span class="punctuation">(</span><span class="literal string single">'Running example1'</span><span class="punctuation">)</span>
+
+<span class="keyword">def</span> <span class="name function">example2</span><span class="punctuation">():</span>
+    <span class="name">verbose</span> <span class="operator">=</span> <span class="name builtin pseudo">False</span>  <span class="comment single"># a NEW local variable verbose</span>
+    <span class="keyword">if</span> <span class="name">verbose</span><span class="punctuation">:</span>
+        <span class="keyword">print</span><span class="punctuation">(</span><span class="literal string single">'Running example2'</span><span class="punctuation">)</span>
+
+<span class="keyword">def</span> <span class="name function">example3</span><span class="punctuation">():</span>
+    <span class="keyword">global</span> <span class="name">verbose</span> <span class="comment single"># I am actually going to use the global variable verbose; don't create a local one.</span>
+    <span class="name">verbose</span> <span class="operator">=</span> <span class="name builtin pseudo">False</span>
+    <span class="keyword">if</span> <span class="name">verbose</span><span class="punctuation">:</span>
+        <span class="keyword">print</span><span class="punctuation">(</span><span class="literal string single">'Running example3'</span><span class="punctuation">)</span>
+
+
+
+<span class="keyword">print</span><span class="punctuation">(</span><span class="name">verbose</span><span class="punctuation">)</span>
+<span class="name">example1</span><span class="punctuation">()</span>
+
+<span class="keyword">print</span><span class="punctuation">(</span><span class="name">verbose</span><span class="punctuation">)</span>
+<span class="name">example2</span><span class="punctuation">()</span>
+<span class="name">example1</span><span class="punctuation">()</span>
+
+<span class="keyword">print</span><span class="punctuation">(</span><span class="name">verbose</span><span class="punctuation">)</span>
+<span class="name">example3</span><span class="punctuation">()</span>
+<span class="name">example1</span><span class="punctuation">()</span>
+
+<span class="keyword">print</span><span class="punctuation">(</span><span class="name">verbose</span><span class="punctuation">)</span>
+</pre>
+<p>全局的列表与字典，如果只需改变其内容，而不是重新赋值，则不需要用 <tt class="docutils literal">global</tt> 声明。</p>
+<pre class="code python literal-block">
+<span class="name">record</span> <span class="operator">=</span> <span class="punctuation">{</span><span class="literal string single">'s1'</span><span class="punctuation">:</span><span class="literal number integer">65</span><span class="punctuation">,</span> <span class="literal string single">'s2'</span><span class="punctuation">:</span><span class="literal number integer">60</span><span class="punctuation">}</span>
+
+<span class="keyword">def</span> <span class="name function">add_score</span><span class="punctuation">(</span><span class="name">student</span><span class="punctuation">,</span> <span class="name">score</span><span class="punctuation">):</span>
+    <span class="name">record</span><span class="punctuation">[</span><span class="name">student</span><span class="punctuation">]</span> <span class="operator">=</span> <span class="name">score</span>
+
+
+<span class="keyword">print</span><span class="punctuation">(</span><span class="name">record</span><span class="punctuation">)</span>
+<span class="name">add_score</span><span class="punctuation">(</span><span class="literal string single">'s3'</span><span class="punctuation">,</span> <span class="literal number integer">75</span><span class="punctuation">)</span>
+<span class="keyword">print</span><span class="punctuation">(</span><span class="name">record</span><span class="punctuation">)</span>
+</pre>
+<p>练习： 定义一个函数 <tt class="docutils literal">empty_dict</tt> 清空字典 <tt class="docutils literal">record</tt>。 要求: 不能用 <tt class="docutils literal">return</tt> 语句。 提示： 可以用 <tt class="docutils literal">pop</tt> 方法， 或者直接给 <tt class="docutils literal">record</tt> 赋值 <tt class="docutils literal">{}</tt> 。</p>
 </div>
-</blockquote>
-<p>练习：给定一个字符串，数出每个单词出现的频率。</p>
+<div class="section" id="id14">
+<h2><a class="toc-backref" href="#id41">函数执行顺序</a></h2>
 </div>
 </div>
-<div class="section" id="id11">
-<h1><a class="toc-backref" href="#id33">参考</a></h1>
+<div class="section" id="id15">
+<h1><a class="toc-backref" href="#id42">参考</a></h1>
 <ul class="simple">
 <li>Think Python 2e – Green Tea Press.  <a class="reference external" href="http://greenteapress.com/thinkpython2/thinkpython2.pdf">http://greenteapress.com/thinkpython2/thinkpython2.pdf</a>.</li>
 </ul>
diff --git a/LectureNotesOnPython.rst b/LectureNotesOnPython.rst
index 8505e95..80e9d5e 100644
--- a/LectureNotesOnPython.rst
+++ b/LectureNotesOnPython.rst
@@ -263,45 +263,45 @@ a = [1, 2]   可以原地修改a[0] = 2
 数与格式化显示
 -------------------------
 
-| x = 3.1415926
-
-| print('%4.0f' % (x))
-| print('%4.1f' % (x))
-| print('%4.2f' % (x))
-| print('%4.3f' % (x))
-| print('%4.4f' % (x))
-
-
-| print('%6.0f' % (x))
-| print('%6.1f' % (x))
-| print('%6.2f' % (x))
-| print('%6.3f' % (x))
-| print('%6.4f' % (x))
-
-
-| print('%.0f' % (x))
-| print('%.1f' % (x))
-| print('%.2f' % (x))
-| print('%.3f' % (x))
-| print('%.4f' % (x))
-| print('%.5f' % (x))
-| print('%.6f' % (x))
-| print('%.7f' % (x))
-| print('%.8f' % (x))
-| print('%.9f' % (x))
-| print('%.15f' % (x))
-| print('%.16f' % (x))
-| print('%.17f' % (x))
-| print('%.18f' % (x))
-
-| print('%4.f' % (x))
-| print('%5.f' % (x))
-| print('%6.f' % (x))
-| print('%7.f' % (x))
-| print('%8.f' % (x))
-
-| print('%f' % (x))
-
+    | x = 3.1415926
+    
+    | print('%4.0f' % (x))
+    | print('%4.1f' % (x))
+    | print('%4.2f' % (x))
+    | print('%4.3f' % (x))
+    | print('%4.4f' % (x))
+    
+    
+    | print('%6.0f' % (x))
+    | print('%6.1f' % (x))
+    | print('%6.2f' % (x))
+    | print('%6.3f' % (x))
+    | print('%6.4f' % (x))
+    
+    
+    | print('%.0f' % (x))
+    | print('%.1f' % (x))
+    | print('%.2f' % (x))
+    | print('%.3f' % (x))
+    | print('%.4f' % (x))
+    | print('%.5f' % (x))
+    | print('%.6f' % (x))
+    | print('%.7f' % (x))
+    | print('%.8f' % (x))
+    | print('%.9f' % (x))
+    | print('%.15f' % (x))
+    | print('%.16f' % (x))
+    | print('%.17f' % (x))
+    | print('%.18f' % (x))
+    
+    | print('%4.f' % (x))
+    | print('%5.f' % (x))
+    | print('%6.f' % (x))
+    | print('%7.f' % (x))
+    | print('%8.f' % (x))
+    
+    | print('%f' % (x))
+    
 
 
 字符串（Strings）
@@ -535,8 +535,8 @@ def f(x):
 list(map(f, [1,2]]))
 
 
+filter方法，从几个值中选择符合条件的几个值。
 
-filter方法，从几个值中选择符合条件的几个值
 
     | def f(x):
     |     if x % 2 == 0:
@@ -647,7 +647,45 @@ TDD - Test-driven Development
 
 测试驱动开发。 My favourite。 刺激有挑战性。 帮助厘清需求。  帮助编写代码。
 
-推荐使用pytest。如何安装？ 使用命令 ``pip install pytest`` 
+推荐使用pytest。如何安装？ 使用命令 ``pip install pytest``。
+
+在 ``test_cases.py`` 写如下测试用例。然后在命令行运行： ``python -m pytest test_cases.py`` 。
+
+.. code:: python
+
+	  # Copyright (c) Hui Lan 2019
+
+          import random
+          import string
+          
+          def make_password(n):
+              '''
+              Return a string of length n consisting of a combination of
+              letters, digits and special characters.  Note that each password
+              must have at least one lower case letter, one upper case letter,
+              one digit and one special charater.  Return an empty string if n
+              is less than 4.
+              '''
+          
+              if n < 4:
+                  return ''
+              
+              password = random.choice(string.ascii_lowercase) + \
+                  random.choice(string.ascii_uppercase) + \
+                  random.choice(string.digits) + \
+                  random.choice(string.punctuation) + \
+                  ''.join([random.choice(string.ascii_letters + string.digits + string.punctuation) for i in range(n-4)])
+          
+              return ''.join(random.sample(password, n)) # shuffle password then return
+          
+          
+          
+          
+          if __name__ == '__main__':
+              for n in range(0,20):
+                  pwd = make_password(n)
+                  print(pwd)
+            
 
 
 
@@ -697,39 +735,262 @@ item的顺序不可预测，不是按照创建时的顺序。
 
 练习：给定一个字符串，数出每个字母出现的频率。
 
-    | def histogram(s):
-    |     ''' Cannot pass any test cases. '''
-    |     pass
+.. code:: python
+	  
+     def histogram(s):
+         ''' Cannot pass any test cases. '''
+         pass
     
-    | def histogram(s):
-    |     ''' Can pass the test case in which s is an empty string. '''
-    |     d = {}
-    |     return d
+     def histogram(s):
+         ''' Can pass the test case in which s is an empty string. '''
+         d = {}
+         return d
     
-    | def histogram(s):
-    |     ''' Can pass the test cases in which all characters in s are unique. '''
-    |     d = {}
-    |     for c in s:
-    |         d[c] = 1
-    |     return d
+     def histogram(s):
+         ''' Can pass the test cases in which all characters in s are unique. '''
+         d = {}
+         for c in s:
+             d[c] = 1
+         return d
     
-    | def histogram(s):
-    |     ''' Can pass all test cases. '''
-    |     d = {}
-    |     for c in s:
-    |         if c not in d:
-    |             d[c] = 1
-    |         else:
-    |             d[c] += 1
-    |     return d
+     def histogram(s):
+         ''' Can pass all test cases. '''
+         d = {}
+         for c in s:
+             if c not in d:
+                 d[c] = 1
+             else:
+                 d[c] += 1
+         return d
     
     
-    | h = histogram('good')
-    | print(h)
+     h = histogram('good')
+     print(h)
     
 
 练习：给定一个字符串，数出每个单词出现的频率。
 
+练习：给定一个新闻文本，数出每个单词出现的频率。考虑以下方面，（1）只考虑字典里有的单词。（2）单词周围如有标点符号，要先移除。
+
+.. code:: python
+	  
+	  # Copyright (C) 2019 Hui Lan
+          # The following line fixes SyntaxError: Non-UTF-8 code starting with ...
+          # coding=utf8
+          
+          def file2lst(fname):
+              ''' Return a list where each element is a word from fname. '''
+              L = []
+              f = open(fname)
+              for line in f:
+                  line = line.strip()
+                  lst = line.split()
+                  for x in lst:
+                      L.append(x)
+              f.close()
+              return L
+          
+          
+          def lst2dict(lst):
+              ''' Return a dictionary given list lst.  Each key is an element in the lst.
+              The value is always 1.'''
+              d = {}
+              for w in lst:
+                  d[w] = 1 
+              return d
+          
+          
+          import string
+          def remove_punctuation(s):
+              p = ',.:’“”' + string.punctuation  
+              t = ''
+              for c in s:
+                  if not c in p:
+                      t += c
+                  elif c == '’': # handle the case such as May’s
+                      return t
+              return t
+          
+          def word_frequency(fname, english_dictionary):
+              ''' Return a dictionary where each key is a word both in the file fname and in 
+              the dictionary english_dictionary, and the corresponding value is the frequency
+              of that word.'''
+              d = {}
+              L = file2lst(fname)
+              for x in L:
+                  x = remove_punctuation(x.lower())
+                  if x in english_dictionary:
+                      if not x in d:
+                          d[x] = 1
+                      else:
+                          d[x] += 1
+              return d
+          
+          
+          def sort_by_value(d):
+              ''' Return a sorted list of tuples, each tuple containing a key and a value.
+                  Note that the tuples are order in descending order of the value.'''
+              import operator
+              lst = sorted(d.items(), key=operator.itemgetter(1), reverse=True)    
+              return lst
+          
+          
+          if __name__ == '__main__':    
+              ed = lst2dict(file2lst('words.txt')) # from http://greenteapress.com/thinkpython2/code/words.txt
+              d = word_frequency('brexit-news.txt', ed)
+              lst = sort_by_value(d)
+              for x in lst:
+                  print('%s (%d)' % (x[0], x[1]))
+          
+    
+
+key与value互换
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+注意到在原来的字典中一个value可能对应多个key的值。比如 ``d = {'a':1, 'b':2, 'c':2}`` 中，2就对应两个key，'b'与'c'。
+
+
+.. code:: python
+	  
+          def inverse_dictionary(d):
+              d2 = {}
+              for k in d:
+                  v = d[k]
+                  if not v in d2:
+                      d2[v] = [k]
+                  else:
+                      d2[v].append(k)
+              return d2
+          
+          
+          
+          d = {'a':1, 'b':2, 'c':2}
+          d2 = inverse_dictionary(d)
+          print(d2)
+          
+
+练习： 用 ``inverse_dictionary`` 对上面 ``d = word_frequency('brexit-news.txt', ed)`` 产生的 ``d`` 进行转化。然后按照单词出现频率从高到低把所有单词都显示出来。每行显示一个频率内的所有单词。
+
+
+.. code:: python
+	  
+          d2 = inverse_dictionary(d)
+          for k in sorted(d2.keys(), reverse=True):
+              print('%d %s' % (k, ' '.join(d2[k])))
+              
+
+练习： 使用 ``setdefault`` 方法对上面的 ``inverse_dictionary`` 进行简化 （减少行数）。
+
+
+.. code:: python
+
+
+
+          def inverse_dictionary(d):
+              d2 = {}
+              for k in d:
+                  v = d[k]
+                  d2.setdefault(v, []).append(k)
+          
+              return d2
+          
+
+
+
+函数
+------
+
+函数 ``unique_words`` 与 ``unique_words2`` 哪个运行速度快？
+
+.. code:: python
+
+          def unique_words(lst):
+              d = {}
+              for x in lst:
+                  d[x] = 1
+              return sorted(d.keys())
+          
+          def unique_words2(lst):
+              return sorted(list(set(lst)))
+          
+          
+          N = 10000000
+          print(unique_words(['hello', 'world', 'am', 'he'] * N))
+          print(unique_words2(['hello', 'world', 'am', 'he'] * N))
+          
+
+
+局部变量
+~~~~~~~~~~~~~~~~
+
+在函数之内。函数执行结束，局部变量消失。
+
+
+全局变量
+~~~~~~~~~~~~~~~~
+
+全局变量位于函数之外，模块之内。全局变量对所有模块内的函数可见（可读）。如果在函数内要对全局变量重新赋值，那么要先用 ``global`` 声明之 （declare）。
+
+
+.. code:: python
+	  
+          verbose = True
+          
+          def example1():
+              if verbose:
+                  print('Running example1')
+                  
+          def example2():
+              verbose = False  # a NEW local variable verbose
+              if verbose:
+                  print('Running example2')
+                  
+          def example3():
+              global verbose # I am actually going to use the global variable verbose; don't create a local one.
+              verbose = False
+              if verbose:
+                  print('Running example3')
+          
+          
+          
+          print(verbose)     
+          example1()
+          
+          print(verbose) 
+          example2()
+          example1()
+          
+          print(verbose) 
+          example3()
+          example1()
+          
+          print(verbose) 
+          
+
+
+全局的列表与字典，如果只需改变其内容，而不是重新赋值，则不需要用 ``global`` 声明。
+
+
+.. code:: python
+	  
+          record = {'s1':65, 's2':60}
+          
+          def add_score(student, score):
+              record[student] = score
+              
+              
+          print(record)
+          add_score('s3', 75)
+          print(record)
+
+
+练习： 定义一个函数 ``empty_dict`` 清空字典 ``record``。 要求: 不能用 ``return`` 语句。 提示： 可以用 ``pop`` 方法， 或者直接给 ``record`` 赋值 ``{}`` 。
+
+
+函数执行顺序
+~~~~~~~~~~~~~~~~~~~~~~~~~
+
+
 
 参考
 ------