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<div class="document" id="lecture-notes-on-python">
<h1 class="title">Lecture Notes on Python</h1>
<table class="docinfo" frame="void" rules="none">
<col class="docinfo-name" />
<col class="docinfo-content" />
<tbody valign="top">
<tr><th class="docinfo-name">Authors:</th>
<td>蓝珲 (lanhui AT zjnu.edu.cn)</td></tr>
<tr><th class="docinfo-name">Version:</th>
<td>0.1 of 2019/03/30</td></tr>
</tbody>
</table>
<p>非学究写书，无空洞行文。Python语法简洁，库函数全面强大，编程速度快，运行速度也不慢。</p>
<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="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="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="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="#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&tf=U2VydmljZT1EaWN0aW9uYXJ5QW5zd2VyVjIgU2NlbmFyaW89RGVmaW5pdGlvblNjZW5hcmlvIFBvc2l0aW9uPU5PUCBSYW5raW5nRGF0YT1UcnVlIEZvcmNlUGxhY2U9RmFsc2UgUGFpcnM9RGljdGlvbmFyeVdvcmQ6cHl0aG9uO3NjbjpEZWZpbml0aW9uU2NlbmFyaW87cDpRQVM7IHw%3d&hs=hyRBF0mYq9hrfQUq66DIZnFVta1ZGRfBiBks25oUguk%3d">pronounciation</a></p>
</div>
<div class="section" id="id2">
<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>
<p>Python语法简洁，有大而全而有用的标准库。</p>
<p>自然（natural）语言。特点：歧义，重复。“The penny dropped。” “不要。”</p>
<p>正式（formal）语言。特点：只管字面意思。</p>
<p>计算机组成概要：CPU，总线，内存，硬盘。</p>
<p>Bit, byte, KB, MB, GB, TB换算。</p>
<p>变量的命名。如，层叠策略，用CDCL还是TiledStrategy?</p>
<p>教务管理系统，<a class="reference external" href="http://10.1.70.164/jwglxt">http://10.1.70.164/jwglxt</a>?</p>
<p>习语言、易语言等目前非主流语言。</p>
<p>最简单的类定义:</p>
<blockquote>
<dl class="docutils">
<dt>class A:</dt>
<dd>pass</dd>
</dl>
</blockquote>
<p>以上面的类为蓝本，创建一个实例：a = A()。 虽然这个a什么也做不了。</p>
<p>Python文件命令行执行。 python a.py。</p>
<p>函数头的三要素：def，函数名，参数列表:</p>
<blockquote>
<div class="line-block">
<div class="line">def add_number(a, b):</div>
<div class="line-block">
<div class="line">return a + b</div>
</div>
</div>
<div class="line-block">
<div class="line">def add_lst(a, b):</div>
<div class="line-block">
<div class="line">if len(a) != len(b):</div>
<div class="line-block">
<div class="line">return 'ERROR: a and b not in equal length.'</div>
</div>
<div class="line">n = len(a)</div>
<div class="line">result = []</div>
<div class="line">for i in range(n):</div>
<div class="line-block">
<div class="line">result.append(a[i] + b[i])</div>
</div>
<div class="line">return result</div>
<div class="line"><br /></div>
</div>
<div class="line">print( add_lst([1,2,3],[-1,-2,-3]) )</div>
</div>
</blockquote>
</div>
<div class="section" id="id3">
<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>
<div class="line">False True</div>
<div class="line">in</div>
<div class="line">None</div>
<div class="line">class</div>
<div class="line">return</div>
<div class="line">while for</div>
<div class="line">continue break</div>
<div class="line">and or not</div>
<div class="line">if else elif</div>
<div class="line">try except finally raise</div>
<div class="line">lambda nonlocal</div>
<div class="line">del global with</div>
<div class="line">yield assert</div>
<div class="line">as is</div>
</div>
<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="#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">
<dt>字符串（string）。'hello', 100 * 'hello', 'hello' * 100, 'Weight is %4.2f kg' % (70.2)</dt>
<dd>f = open('a.html')
s = f.read()
f.close()</dd>
<dt>列表（list）。['a', 'b', 'c', 'd']</dt>
<dd>['bob', 170, 'john', '180']
[1, 2, 3, 4]
range(10) 返回一个range对象。可以用list函数把这个对象变成列表。
等价的是range(0, 10, 1)，从0开始，步进1，不包括10。
A list of list
A list of tuples
A list of objects</dd>
</dl>
<p>元组（tuple），字典（dict）。</p>
</div>
<div class="section" id="variable">
<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>
<p>关键词不能用作变量名。</p>
<p>值存放在内存某处。值会记录指向它的变量个数。</p>
<p>为节省空间，如果几个变量的值相同，那么这些变量有时会指向这个值（而不是为每个变量单独分配内存空间单独存放该值）。</p>
<p>这叫做interning技术。但并非总是如此。</p>
<div class="line-block">
<div class="line">a = 10</div>
<div class="line">b = 10</div>
<div class="line">c = 10</div>
<div class="line">id(a), id(b), id(c)</div>
<div class="line">(8791229060416, 8791229060416, 8791229060416)</div>
</div>
<p>值10存在地址8791229060416，所有a，b，c三个变量都指向（point to）这个地址。</p>
<div class="line-block">
<div class="line">x = 257</div>
<div class="line">y = 257</div>
<div class="line">id(x), id(y)</div>
<div class="line">(46487024, 46487952)</div>
</div>
<p>以上虽然变量x与y的值都是一样，可是这两个值存放在不同的内存地址。</p>
<div class="line-block">
<div class="line">s1 = 'hello'</div>
<div class="line">s2 = 'hello'</div>
<div class="line">id(s1), id(s2)</div>
<div class="line">s1 == s2</div>
<div class="line">s1 is s2</div>
</div>
<div class="line-block">
<div class="line">s1 = 'h' * 100</div>
<div class="line">s2 = 'h' * 100</div>
<div class="line">id(s1), id(s2)</div>
</div>
<div class="line-block">
<div class="line">s3 = 'hello, world!'</div>
<div class="line">s4 = 'hello, world!'</div>
<div class="line">id(s3), id(s4)</div>
<div class="line">(46703536, 46705136)</div>
</div>
<div class="line-block">
<div class="line">class A:</div>
<div class="line-block">
<div class="line">pass</div>
</div>
</div>
<div class="line-block">
<div class="line">a = A()</div>
<div class="line">b = A()</div>
<div class="line">a</div>
<div class="line"><__main__.A object at 0x0000000002CD92E8></div>
<div class="line">b</div>
<div class="line"><__main__.A object at 0x0000000002CD9240></div>
</div>
<div class="line-block">
<div class="line">x = [1,2,3]</div>
<div class="line">id(x)</div>
<div class="line">46869512</div>
<div class="line">y = x</div>
<div class="line">id(y)</div>
<div class="line">46869512</div>
<div class="line">x.append(4)</div>
<div class="line">x</div>
<div class="line">[1, 2, 3, 4]</div>
<div class="line">y</div>
<div class="line">[1, 2, 3, 4]</div>
</div>
<div class="line-block">
<div class="line">x = []</div>
<div class="line">id(x)</div>
<div class="line">46869640</div>
</div>
<div class="line-block">
<div class="line">x = [1,2,3,4]</div>
<div class="line">y = [1,2,3,4]</div>
<div class="line">id(x)</div>
<div class="line">46869768</div>
<div class="line">id(y)</div>
<div class="line">46868808</div>
</div>
<p>一个没有名字与之对应的值将会被清出内存。</p>
<p>参考资料：</p>
<ul class="simple">
<li><a class="reference external" href="http://foobarnbaz.com/2012/07/08/understanding-python-variables/">http://foobarnbaz.com/2012/07/08/understanding-python-variables/</a></li>
<li><a class="reference external" href="https://stackoverflow.com/questions/19721002/is-a-variable-the-name-the-value-or-the-memory-location">https://stackoverflow.com/questions/19721002/is-a-variable-the-name-the-value-or-the-memory-location</a></li>
</ul>
</div>
<div class="section" id="mutable">
<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>
<p>a = [1, 2] 可以原地修改a[0] = 2</p>
<p>参考资料：</p>
<ul class="simple">
<li><a class="reference external" href="https://stackoverflow.com/questions/8056130/immutable-vs-mutable-types">https://stackoverflow.com/questions/8056130/immutable-vs-mutable-types</a></li>
</ul>
<p>表达式（expression）：值，变量或操作符的组合。</p>
<blockquote>
<div class="line-block">
<div class="line">17</div>
<div class="line">n + 2</div>
</div>
</blockquote>
<p>语句（statement）：能够制造一个变量或者显示信息的代码。</p>
<blockquote>
<div class="line-block">
<div class="line">n = 17</div>
<div class="line">print(n)</div>
</div>
</blockquote>
</div>
<div class="section" id="id5">
<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>
<div class="line-block">
<div class="line">print('%4.0f' % (x))</div>
<div class="line">print('%4.1f' % (x))</div>
<div class="line">print('%4.2f' % (x))</div>
<div class="line">print('%4.3f' % (x))</div>
<div class="line">print('%4.4f' % (x))</div>
</div>
<div class="line-block">
<div class="line">print('%6.0f' % (x))</div>
<div class="line">print('%6.1f' % (x))</div>
<div class="line">print('%6.2f' % (x))</div>
<div class="line">print('%6.3f' % (x))</div>
<div class="line">print('%6.4f' % (x))</div>
</div>
<div class="line-block">
<div class="line">print('%.0f' % (x))</div>
<div class="line">print('%.1f' % (x))</div>
<div class="line">print('%.2f' % (x))</div>
<div class="line">print('%.3f' % (x))</div>
<div class="line">print('%.4f' % (x))</div>
<div class="line">print('%.5f' % (x))</div>
<div class="line">print('%.6f' % (x))</div>
<div class="line">print('%.7f' % (x))</div>
<div class="line">print('%.8f' % (x))</div>
<div class="line">print('%.9f' % (x))</div>
<div class="line">print('%.15f' % (x))</div>
<div class="line">print('%.16f' % (x))</div>
<div class="line">print('%.17f' % (x))</div>
<div class="line">print('%.18f' % (x))</div>
</div>
<div class="line-block">
<div class="line">print('%4.f' % (x))</div>
<div class="line">print('%5.f' % (x))</div>
<div class="line">print('%6.f' % (x))</div>
<div class="line">print('%7.f' % (x))</div>
<div class="line">print('%8.f' % (x))</div>
</div>
<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="#id23">字符串（Strings）</a></h1>
<p>由字符组成。</p>
<div class="line-block">
<div class="line">fruit = 'banana!'</div>
<div class="line">first_letter = fruit[0]</div>
<div class="line">second_letter = fruit[1]</div>
</div>
<p>索引（index）从0开始，所以1代表第二个字符。只用整数。</p>
<p>负整数代表从字符串末尾开始。如fruit[-1]代表fruit字符串最后一个字符。</p>
<div class="line-block">
<div class="line">i = 1</div>
<div class="line">fruit[i]</div>
<div class="line">fruit[i+1]</div>
</div>
<p>len()函数。返回字符串字符个数。len(fruit)。</p>
<div class="line-block">
<div class="line">L = len(fruit)</div>
<div class="line">fruit[L-1]，最后一个字符。与fruit[-1]等价。</div>
</div>
<p>遍历（traverse）字符串。</p>
<blockquote>
<div class="line-block">
<div class="line">fruit = 'banana'</div>
<div class="line">for c in fruit:</div>
<div class="line-block">
<div class="line">print(c)</div>
</div>
</div>
</blockquote>
<p>反向遍历。</p>
<blockquote>
<div class="line-block">
<div class="line">fruit = 'banana'</div>
<div class="line">for i in range(len(fruit)-1,-1,-1):</div>
<div class="line-block">
<div class="line">print(fruit[i])</div>
</div>
</div>
<div class="line-block">
<div class="line">fruit = 'banana'</div>
<div class="line">for c in fruit[::-1]: # [start,stop,step]</div>
<div class="line-block">
<div class="line">print(c)</div>
</div>
</div>
<div class="line-block">
<div class="line">fruit = 'banana'</div>
<div class="line">for c in ''.join(reversed(fruit)):</div>
<div class="line-block">
<div class="line">print(c)</div>
</div>
</div>
</blockquote>
<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="#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>
<div class="line">suffix = 'ack'</div>
<div class="line">for c in prefixes:</div>
<div class="line-block">
<div class="line">if c == 'O' or c == 'Q':</div>
<div class="line-block">
<div class="line">print(c + 'u' + suffix)</div>
</div>
<div class="line">else:</div>
<div class="line-block">
<div class="line">print(c + suffix)</div>
</div>
</div>
</div>
</div>
<div class="section" id="slice">
<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">
<div class="line">s = 'Monty Python'</div>
<div class="line">s[0:5]</div>
<div class="line">s[6:12]</div>
<div class="line">s[:5]</div>
<div class="line">s[6:]</div>
<div class="line">s[:]</div>
</div>
<p>n一般小于m。如果n大于等于m，那么就返回空字符串。</p>
<p>空字符串的长度是0。</p>
<p>字符串是immutable的。不能改变已有的字符串。</p>
<div class="line-block">
<div class="line">greeting = 'Hello, world!'</div>
<div class="line">greeting[0] = 'J'</div>
</div>
<div class="line-block">
<div class="line">greeting = 'Hello, world!'</div>
<div class="line">new_greeting = 'J' + greeting[1:]</div>
</div>
</div>
<div class="section" id="id6">
<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">
<div class="line">i = 0</div>
<div class="line">while i < len(word):</div>
<div class="line-block">
<div class="line">if word[i] == c:</div>
<div class="line-block">
<div class="line">return i</div>
</div>
<div class="line">i = i + 1</div>
</div>
<div class="line">return -1</div>
</div>
</div>
<div class="line-block">
<div class="line">print(find('banana', 'a'))</div>
</div>
<p>练习一：加第三个参数，设定从哪个字符开始搜起。</p>
<p>练习二：加第三个参数，设定从哪个方向开始搜起。</p>
<p>String对象有内置函数find。</p>
<p>数字符串中某个字符的个数。</p>
<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="#id27">String类（对象）方法</a></h1>
<div class="line-block">
<div class="line">upper()</div>
<div class="line">lower()</div>
</div>
<p>方法调用：invocation/call</p>
<div class="line-block">
<div class="line">word.find('na')</div>
<div class="line">word.find('na', 3)</div>
<div class="line">name.find('b', 1, 2)</div>
</div>
</div>
<div class="section" id="in">
<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="#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">
<div class="line">==</div>
<div class="line">>, >=</div>
<div class="line"><, <=</div>
</div>
<p>练习：写is_reverse函数，使得is_reverse('god', 'dog')返回True。</p>
<p>find_from函数的两种实现。如果能够找出错误，给1分奖励。</p>
<p>字符串是对象（object）。</p>
<p>对象的本质涵义 - data construct。</p>
<p>计算复杂度。</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="#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">
<div class="line">[ ]</div>
<div class="line">[10, 20, 30, 40]</div>
<div class="line">['crunchy frog', 'ram bladder', 'lark vomit']</div>
</div>
</blockquote>
<p>列表中的元素不需要是同一类型的: <tt class="docutils literal">['spam', 2.0, 5, [10, 20]]</tt></p>
<p>列表[10,20]在另外一个列表中，这叫嵌套列表。</p>
<p>['spam', 1, ['Brie', 'Roquefort', 'Pol le Veq'], [1, 2, 3]]，长度是多少？</p>
<p>列表是 Mutable类型。值可以在原地变。（注意与String的区别）。</p>
<p>IndexError</p>
<p>遍历</p>
<dl class="docutils">
<dt>for cheese in cheeses:</dt>
<dd>print(cheese)</dd>
<dt>for i in range(len(numbers)):</dt>
<dd>numbers[i] = numbers[i] * 2</dd>
<dt>for x in []:</dt>
<dd>print('This never happens.')</dd>
</dl>
<!-- 讨论软件工程认证数据输入问题。 -->
<p><tt class="docutils literal">+</tt> 操作符用来连接， <tt class="docutils literal">*</tt> 操作符用来重复。</p>
<p>列表的方法</p>
<blockquote>
<p>append</p>
<p>extend</p>
<p>sort</p>
<p>t = ['d', 'c', 'e', 'b', 'a']</p>
<p>t.sort() # 问t.sort()返回什么值？</p>
<p>t</p>
</blockquote>
<p>sum - reduce方法，把几个值变成一个值</p>
<p>map方法，把几个值变成另外几个值</p>
<dl class="docutils">
<dt>def f(x):</dt>
<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>
<blockquote>
<div class="line-block">
<div class="line">def f(x):</div>
<div class="line-block">
<div class="line">if x % 2 == 0:</div>
<div class="line-block">
<div class="line">return True</div>
</div>
<div class="line">return False</div>
</div>
</div>
<div class="line-block">
<div class="line">list(filter(f, [1,2,3,4]))</div>
</div>
</blockquote>
<p>pop</p>
<blockquote>
<div class="line-block">
<div class="line">t = ['a', 'b', 'c']</div>
<div class="line">x = t.pop(1) # pop可不带参数，不带参数返回哪个值？</div>
</div>
</blockquote>
<p>del</p>
<blockquote>
<div class="line-block">
<div class="line">t = ['a', 'b', 'c']</div>
<div class="line">del t[1]</div>
</div>
<div class="line-block">
<div class="line">t = ['a', 'b', 'c', 'd', 'e', 'f']</div>
<div class="line">del t[1:5]</div>
</div>
</blockquote>
<p>remove</p>
<blockquote>
<div class="line-block">
<div class="line">t = ['a', 'b', 'c']</div>
<div class="line">t.remove('b')</div>
</div>
</blockquote>
<p>split</p>
<blockquote>
<div class="line-block">
<div class="line">list_of_characters = list('spam')</div>
<div class="line">list_of_words = 'spam should be filtered'.split()</div>
<div class="line">list_of_words = 'spam-should-be-filtered'.split('-')</div>
</div>
</blockquote>
<p>join方法</p>
<blockquote>
<div class="line-block">
<div class="line">','.join(['1','2','3'])</div>
</div>
<div class="line-block">
<div class="line">a = 'banana'</div>
<div class="line">b = 'banana'</div>
<div class="line">a is b # a与b是不是指向同一个值</div>
<div class="line">a == b</div>
</div>
<div class="line-block">
<div class="line">a = [1, 2, 3]</div>
<div class="line">b = [1, 2, 3]</div>
<div class="line">a is b # not identical, a and b are not the same object</div>
<div class="line">a == b # equivalent though they have the same values</div>
</div>
</blockquote>
<p>别名（Aliasing）</p>
<p>a = [1, 2, 3]
b = a
b is a</p>
<p>把变量名与对象联系起来叫做reference。
a与b是指向[1,2,3]的两个references。
因为[1,2,3]是mutable的，所以使用a对[1,2,3]做改变同样影响到b对应的值。
error-prone（易错）</p>
</div>
<div class="section" id="id9">
<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>
<div class="line-block">
<div class="line">del t[0]</div>
</div>
</div>
<div class="line-block">
<div class="line">letters = ['a', 'b', 'c']</div>
<div class="line">delete_head(letters) # letters and t points to the same list object.</div>
<div class="line">letters</div>
</div>
</blockquote>
</div>
<div class="section" id="append">
<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>
<div class="line">t2 = t1.append(3)</div>
<div class="line">t1</div>
<div class="line">[1, 2, 3]</div>
<div class="line">t2</div>
</div>
<div class="line-block">
<div class="line">t3 = t1 + [4]</div>
<div class="line">t3</div>
<div class="line">[1, 2, 3, 4]</div>
<div class="line">t1</div>
<div class="line">[1, 2, 3]</div>
</div>
</blockquote>
<p>区别如下两个函数:</p>
<blockquote>
<dl class="docutils">
<dt>def bad_delete_head(t):</dt>
<dd>t = t[1:] # WRONG!</dd>
<dt>def tail(t):</dt>
<dd>return t[1:]</dd>
</dl>
</blockquote>
<div class="section" id="tdd-test-driven-development">
<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>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"><</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="#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="#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>
<div class="line-block">
<div class="line">d = {} or d = dict()</div>
</div>
<div class="line-block">
<div class="line">d = {'hot':'热', 'cool':'凉', 'cold':'冷'}</div>
<div class="line">d['warm'] = '温'</div>
<div class="line">d['warm']</div>
<div class="line">d['freezing'] # KeyError</div>
<div class="line">len(d)</div>
</div>
<div class="line-block">
<div class="line">'warm' in d</div>
<div class="line">'温' in d.values()</div>
</div>
</blockquote>
<p>key</p>
<p>value</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="#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>
<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>
<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="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>
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<div class="section" id="id14">
<h2><a class="toc-backref" href="#id41">函数执行顺序</a></h2>
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</div>
<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>
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