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        <center><font color="#4094C7" size="4">Python3 自学笔记第一章【变量和简单数据类型】</font></center>

<hr>
<h1 id="1-1变量的命名和使用"><a href="#1-1变量的命名和使用" class="headerlink" title=" - 1.1变量的命名和使用 "></a><font color="#FF0000"> - 1.1变量的命名和使用 </font></h1><blockquote>
<p>变量名只能包含字母、数字和下划线。变量名可以字母或者下划线打头，但不能以数字开头，例如，可以将变量命名为message_1，但不能将其命名为1_message</p>
</blockquote>
<blockquote>
<p>变量名不能包含空格，但可使用下划线来分割其中的单词，例如，变量名greeting_message可行，但变量名greeting message会引发错误</p>
</blockquote>
<blockquote>
<p>不要将Python关键字和函数名用作变量名，即不要使用Python保留用于特殊用途的单词，如print</p>
</blockquote>
<blockquote>
<p>变量名应既简短又具有描述性，例如，name比n好，student_name比s_n好，name_length比length_of_persons_name好</p>
</blockquote>
<blockquote>
<p>慎用小写字母l和大写字母O，因为它们可能被人看错成数字1和0</p>
</blockquote>
<h1 id="1-2字符串"><a href="#1-2字符串" class="headerlink" title=" - 1.2字符串 "></a><font color="#FF0000"> - 1.2字符串 </font></h1><p> 字符串就是一系列字符，在Python中，用引号括起来的都是字符串，其中的引号可以是单引号也可以双引号：</p>
<figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="string">"This is a string."</span></span><br><span class="line"><span class="string">'This is also a string.'</span></span><br></pre></td></tr></table></figure>
<p>这种灵活性让我们能够在字符串中包含引号和撇号：</p>
<figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line"><span class="string">'I told my friend,"Python is my favorite language!"'</span></span><br><span class="line"><span class="string">"The language 'Python' is named er Monty Python,not the snake."</span></span><br><span class="line"><span class="string">"One of Python's strengths is i diverse and supportive community."</span></span><br></pre></td></tr></table></figure>
<h1 id="1-2-1使用方法修改字符串的大小写"><a href="#1-2-1使用方法修改字符串的大小写" class="headerlink" title=" - 1.2.1使用方法修改字符串的大小写"></a><font color="#FF0000"> - 1.2.1使用方法修改字符串的大小写</font></h1><p>三种处理方法如下：<br><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line">title()     <span class="comment">#将字符串每个单词的首字母都改为大写</span></span><br><span class="line">upper()     <span class="comment">#将字符串的每个字母都改为大写</span></span><br><span class="line">lower()     <span class="comment">#将字符串的每个字母都改为小写</span></span><br></pre></td></tr></table></figure></p>
<p>例如：<br><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line">message = <span class="string">"I love you!"</span></span><br><span class="line">print(name.title())</span><br><span class="line">print(name.upper())</span><br><span class="line">print(name.lower())</span><br></pre></td></tr></table></figure></p>
<p>输出结果如下：<br><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line">I Love You!</span><br><span class="line">I LOVE YOU!</span><br><span class="line">i love you!</span><br></pre></td></tr></table></figure></p>
<h1 id="1-2-2合并（拼接）字符串"><a href="#1-2-2合并（拼接）字符串" class="headerlink" title=" - 1.2.2合并（拼接）字符串"></a><font color="#FF0000"> - 1.2.2合并（拼接）字符串</font></h1><p>Python使用加号（+）来合并字符串，举例说明：</p>
<figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line">first_name = <span class="string">"I"</span></span><br><span class="line">second_name = <span class="string">"love"</span></span><br><span class="line">third_name = <span class="string">"python"</span></span><br><span class="line">full_name = first_name + <span class="string">" "</span> + second_name + <span class="string">" "</span> + third_time</span><br><span class="line">print(full_name.title() + <span class="string">"!"</span>)</span><br></pre></td></tr></table></figure>
<p>输出结果如下：</p>
<figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">I Love Python!</span><br></pre></td></tr></table></figure>
<h1 id="1-2-3使用制表符或换行符来添加空白"><a href="#1-2-3使用制表符或换行符来添加空白" class="headerlink" title=" - 1.2.3使用制表符或换行符来添加空白"></a><font color="#FF0000"> - 1.2.3使用制表符或换行符来添加空白</font></h1><p>添加横向制表符：</p>
<figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">&gt;&gt;&gt;print(<span class="string">"\tPython"</span>)</span><br><span class="line">    Python</span><br></pre></td></tr></table></figure>
<p>添加换行符：</p>
<figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line">&gt;&gt;&gt;print(<span class="string">"C\nC++\nPython\nJavaScript"</span>)</span><br><span class="line">C</span><br><span class="line">C++</span><br><span class="line">Python</span><br><span class="line">JavaScript</span><br></pre></td></tr></table></figure>
<center><font color="#FF0000"> 附表：Python转义符 </font><br><br><img src="https://i.imgur.com/Do9ja5A.png" alt=""><br><br></center>

<h1 id="1-2-4删除空白"><a href="#1-2-4删除空白" class="headerlink" title=" - 1.2.4删除空白"></a><font color="#FF0000"> - 1.2.4删除空白</font></h1><p>在Python中可用 lstrip()、rstrip()、strip() 分别删除字符串开头、结尾、全部的空白，举例说明：</p>
<figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line">&gt;&gt;&gt;message = <span class="string">' python '</span></span><br><span class="line">&gt;&gt;&gt;message</span><br><span class="line"><span class="string">' python '</span></span><br><span class="line">&gt;&gt;&gt;message.lstrip()</span><br><span class="line"><span class="string">'python '</span></span><br><span class="line">&gt;&gt;&gt;message.rstrip()</span><br><span class="line"><span class="string">' python'</span></span><br><span class="line">&gt;&gt;&gt;message.strip()</span><br><span class="line"><span class="string">'python'</span></span><br></pre></td></tr></table></figure>
<p>如果要永久删除字符串中的空白，必须将删除操作的结果存回到变量中：</p>
<figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line">&gt;&gt;&gt;message = <span class="string">' python '</span></span><br><span class="line">&gt;&gt;&gt;message = message.strip()</span><br><span class="line">&gt;&gt;&gt;message</span><br><span class="line"><span class="string">'python'</span></span><br></pre></td></tr></table></figure>
<h1 id="1-3数字"><a href="#1-3数字" class="headerlink" title=" - 1.3数字"></a><font color="#FF0000"> - 1.3数字</font></h1><p>在编程中，经常使用数字来记录游戏得分、表示可视化数据、储存Web应用信息等。Python根据数字的用法以不同的方式处理它们</p>
<h1 id="1-3-1整数"><a href="#1-3-1整数" class="headerlink" title=" - 1.3.1整数"></a><font color="#FF0000"> - 1.3.1整数</font></h1><p>在Python中，可对整数执行加（+）减（-）乘（*）除（/）乘方（**）运算，同时也支持运算次序：</p>
<figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br></pre></td><td class="code"><pre><span class="line">&gt;&gt;&gt;<span class="number">3</span> + <span class="number">2</span></span><br><span class="line"><span class="number">5</span></span><br><span class="line">&gt;&gt;&gt;<span class="number">3</span> - <span class="number">2</span></span><br><span class="line"><span class="number">1</span></span><br><span class="line">&gt;&gt;&gt;<span class="number">3</span> * <span class="number">2</span></span><br><span class="line"><span class="number">6</span></span><br><span class="line">&gt;&gt;&gt;<span class="number">3</span> \ <span class="number">2</span></span><br><span class="line"><span class="number">1.5</span></span><br><span class="line">&gt;&gt;&gt;<span class="number">3</span> ** <span class="number">2</span></span><br><span class="line"><span class="number">9</span></span><br><span class="line">&gt;&gt;&gt;<span class="number">3</span> ** <span class="number">3</span></span><br><span class="line"><span class="number">27</span></span><br><span class="line">&gt;&gt;&gt;<span class="number">2</span> + <span class="number">3</span> * <span class="number">4</span></span><br><span class="line"><span class="number">14</span></span><br><span class="line">&gt;&gt;&gt;(<span class="number">2</span> + <span class="number">3</span>) * <span class="number">4</span></span><br><span class="line"><span class="number">20</span></span><br></pre></td></tr></table></figure>
<h1 id="1-3-2浮点数"><a href="#1-3-2浮点数" class="headerlink" title=" - 1.3.2浮点数"></a><font color="#FF0000"> - 1.3.2浮点数</font></h1><p>Python将带小数点的数字都称为浮点数：</p>
<figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line">&gt;&gt;&gt;<span class="number">0.1</span> + <span class="number">0.1</span></span><br><span class="line"><span class="number">0.2</span></span><br><span class="line">&gt;&gt;&gt;<span class="number">2</span> * <span class="number">0.2</span></span><br><span class="line"><span class="number">0.4</span></span><br></pre></td></tr></table></figure>
<p>需要注意的是，结果包含的小数位可能是不确定的，就现在而言，暂时忽略多余的小数位即可：</p>
<figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line">&gt;&gt;&gt;<span class="number">0.2</span> + <span class="number">0.1</span></span><br><span class="line"><span class="number">0.30000000000000004</span></span><br><span class="line">&gt;&gt;&gt;<span class="number">3</span> * <span class="number">0.1</span></span><br><span class="line"><span class="number">0.30000000000000004</span></span><br></pre></td></tr></table></figure>
<h1 id="1-3-3使用函数-str-避免错误"><a href="#1-3-3使用函数-str-避免错误" class="headerlink" title=" - 1.3.3使用函数 str() 避免错误"></a><font color="#FF0000"> - 1.3.3使用函数 str() 避免错误</font></h1><p>错误例子：</p>
<figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line">age = <span class="number">23</span></span><br><span class="line">message = <span class="string">"Happy "</span> + age + <span class="string">"rd Birthday!"</span></span><br><span class="line">print(message)</span><br></pre></td></tr></table></figure>
<p>运行时会报错：</p>
<figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line">Traceback (most recent call last):</span><br><span class="line">  File <span class="string">"birthday.py"</span>, line <span class="number">2</span>, <span class="keyword">in</span> &lt;module&gt;</span><br><span class="line">    message = <span class="string">"Happy "</span> + age + <span class="string">"rd Birthday!"</span></span><br><span class="line">TypeError: must be str, <span class="keyword">not</span> int</span><br></pre></td></tr></table></figure>
<p>这是一个类型错误，意味着Python无法识别我们使用的信息。在这个例子中，Python发现我们使用了一个值为整数（int）的变量，但它不知道该如何解读这个值，这个变量表示的可能是数值23，也可能是字符2和3。像上面这样的字符串中使用整数时，需要显式地指出我们希望Python将这个整数用作字符串。为此，可调用函数 str()，它让Python将非字符串值表示为字符串：</p>
<figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line">age = <span class="number">23</span></span><br><span class="line">message = <span class="string">"Happy "</span> + str(age) + <span class="string">"rd Birthday!"</span></span><br><span class="line">print(message)</span><br></pre></td></tr></table></figure>
<p>输出结果如下：</p>
<figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">Happy <span class="number">23</span>rd Birthday!</span><br></pre></td></tr></table></figure>
<h1 id="1-4注释"><a href="#1-4注释" class="headerlink" title=" - 1.4注释"></a><font color="#FF0000"> - 1.4注释</font></h1><p>注释让我们能够使用自然语言在程序中添加说明，Python中注释有三种方法：</p>
<figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line">print(<span class="string">"Hello Python!"</span>)</span><br><span class="line"></span><br><span class="line"><span class="comment">#这是单行注释</span></span><br><span class="line"></span><br><span class="line"><span class="string">'''这是多行注释</span></span><br><span class="line"><span class="string">这是多行注释'''</span></span><br><span class="line"></span><br><span class="line"><span class="string">"""这也是多行注释</span></span><br><span class="line"><span class="string">这也是多行注释"""</span></span><br></pre></td></tr></table></figure>

      
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