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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="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>
</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>
</ul>
</li>
<li><a class="reference internal" href="#id11" id="id33">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>
<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="#id13">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="#id14">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="#id15">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="#id16">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="#id17">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="#id18">数与格式化显示</a></h1>
<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>
</div>
<div class="section" id="strings">
<h1><a class="toc-backref" href="#id19">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="#id20">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="#id21">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="#id22">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="#id23">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="#id24">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>
<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="#id26">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="#id27">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="#id28">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="#id29">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>
</div>
<div class="section" id="id10">
<h2><a class="toc-backref" href="#id30">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>
<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="#id32">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>
</div>
<div class="line">return d</div>
</div>
</div>
<div class="line-block">
<div class="line">h = histogram('good')</div>
<div class="line">print(h)</div>
</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>
</div>
<div class="section" id="id11">
<h1><a class="toc-backref" href="#id33">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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