Python程序员最常犯的10个错误，你中招了吗？_python两个代码同时读一个文件,会出bug吗-优快云博客

本文列举了Python开发过程中常见的10个错误，包括滥用表达式作为函数参数的默认值、不理解闭包中的变量绑定等问题，并提供了相应的解决方案。

摘要： Python简介 Python是一种具有动态语义的、面向对象的解释型高级编程语言。因其内置了高级数据结构，并支持动态类型和动态绑定，使用Python进行快速应用程序开发十分便利。同时作为一门脚本语言，它兼容部分现有的组件和服务。

Python简介

Python是一种具有动态语义的、面向对象的解释型高级编程语言。因其内置了高级数据结构，并支持动态类型和动态绑定，使用Python进行快速应用程序开发十分便利。同时作为一门脚本语言，它兼容部分现有的组件和服务。Python还支持模块和各种库的扩展，有助于实现模块化编程和提高代码复用率。

关于本文

刚接触这门语言的新手可能会对Python简洁灵活的语法有些不适应，或是低估了Python强大的性能。鉴于此，本文列出了Python开发人员常犯的10个小错误，资深程序猿也难免会中招哦。

0582085fcad54ac457da10cc973b153357fa0658

本文供Python高级开发人员参考，Python小白可以参考下面这篇文章：

http://www.onlamp.com/pub/a/python/2004/02/05/learn_python.html

常见错误1：滥用表达式作为函数参数的默认值

Python允许开发者指定函数参数的默认值，这也是Python的一大特色，但当默认值可变时，可能会给开发者带来一些困扰。例如下面定义的函数：

<span style="color:#333333"><span style="color:#f8f8f2"><code class="language-c"><span style="color:#f8f8f2">>></span><span style="color:#f8f8f2">></span> def <span style="color:#e6db74"><span style="color:#a6e22e">foo</span></span><span style="color:#f8f8f2"><span style="color:#f8f8f2">(</span></span><span style="color:#f8f8f2">bar</span><span style="color:#f8f8f2"><span style="color:#f8f8f2">=</span></span><span style="color:#f8f8f2"><span style="color:#f8f8f2">[</span></span><span style="color:#f8f8f2"><span style="color:#f8f8f2">]</span></span><span style="color:#f8f8f2"><span style="color:#f8f8f2">)</span></span><span style="color:#f8f8f2">:</span> # bar is optional and defaults to <span style="color:#f8f8f2">[</span><span style="color:#f8f8f2">]</span> <span style="color:#66d9ef"><span style="color:#f92672">if</span></span> not specified

<span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span> bar<span style="color:#f8f8f2">.</span><span style="color:#e6db74"><span style="color:#a6e22e">append</span></span><span style="color:#f8f8f2"><span style="color:#f8f8f2">(</span></span><span style="color:#a6e22e"><span style="color:#f8f8f2"><span style="color:#e6db74">"baz"</span></span></span><span style="color:#f8f8f2"><span style="color:#f8f8f2">)</span></span> # but <span style="color:#f92672">this</span> line could be problematic<span style="color:#f8f8f2">,</span> as we'll see<span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span>

<span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span> <span style="color:#66d9ef"><span style="color:#f92672">return</span></span> bar</code></span></span>

看出bug了吗？那就是在每次调用函数前没有对可变参数进行赋值，而认为该参数就是默认值。比如上面的代码，有人可能期望在反复调用foo()时返回'baz'，以为每次调用foo()时，bar的值都为[]，即一个空列表。

但是，让我们来看看代码运行结果：

<span style="color:#333333"><span style="color:#f8f8f2"><code class="language-c"><span style="color:#f8f8f2">>></span><span style="color:#f8f8f2">></span> <span style="color:#e6db74">foo</span><span style="color:#f8f8f2">(</span><span style="color:#f8f8f2">)</span>

<span style="color:#f8f8f2">[</span><span style="color:#a6e22e"><span style="color:#e6db74">"baz"</span></span><span style="color:#f8f8f2">]</span>

<span style="color:#f8f8f2">>></span><span style="color:#f8f8f2">></span> <span style="color:#e6db74">foo</span><span style="color:#f8f8f2">(</span><span style="color:#f8f8f2">)</span>

<span style="color:#f8f8f2">[</span><span style="color:#a6e22e"><span style="color:#e6db74">"baz"</span></span><span style="color:#f8f8f2">,</span> <span style="color:#a6e22e"><span style="color:#e6db74">"baz"</span></span><span style="color:#f8f8f2">]</span>      

<span style="color:#f8f8f2">>></span><span style="color:#f8f8f2">></span> <span style="color:#e6db74">foo</span><span style="color:#f8f8f2">(</span><span style="color:#f8f8f2">)</span>

<span style="color:#f8f8f2">[</span><span style="color:#a6e22e"><span style="color:#e6db74">"baz"</span></span><span style="color:#f8f8f2">,</span> <span style="color:#a6e22e"><span style="color:#e6db74">"baz"</span></span><span style="color:#f8f8f2">,</span> <span style="color:#a6e22e"><span style="color:#e6db74">"baz"</span></span><span style="color:#f8f8f2">]</span></code></span></span>

嗯？为什么每次调用foo()后会不断把"baz"添加到已有的列表，而不是新建一个新列表呢？答案就是，函数参数的默认值仅在定义函数时执行一次。因此，仅在第一次定义foo()时，bar初始化为默认值（即空列表），此后，每次调用foo()函数时，参数bar都是第一次初始化时生成的列表。

常见的解决方案：

<span style="color:#333333"><span style="color:#f8f8f2"><code class="language-c"><span style="color:#f8f8f2">>></span><span style="color:#f8f8f2">></span> def <span style="color:#e6db74"><span style="color:#a6e22e">foo</span></span><span style="color:#f8f8f2"><span style="color:#f8f8f2">(</span></span><span style="color:#f8f8f2">bar</span><span style="color:#f8f8f2"><span style="color:#f8f8f2">=</span></span><span style="color:#f8f8f2">None</span><span style="color:#f8f8f2"><span style="color:#f8f8f2">)</span></span><span style="color:#f8f8f2">:</span>

<span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span> <span style="color:#66d9ef"><span style="color:#f92672">if</span></span> bar is None<span style="color:#f8f8f2">:</span> # or <span style="color:#66d9ef"><span style="color:#f92672">if</span></span> not bar<span style="color:#f8f8f2">:</span>

<span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span> bar <span style="color:#f8f8f2">=</span> <span style="color:#f8f8f2">[</span><span style="color:#f8f8f2">]</span>

<span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span> bar<span style="color:#f8f8f2">.</span><span style="color:#e6db74">append</span><span style="color:#f8f8f2">(</span><span style="color:#a6e22e"><span style="color:#e6db74">"baz"</span></span><span style="color:#f8f8f2">)</span>

<span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span> <span style="color:#66d9ef"><span style="color:#f92672">return</span></span> bar

<span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span>

<span style="color:#f8f8f2">>></span><span style="color:#f8f8f2">></span> <span style="color:#e6db74">foo</span><span style="color:#f8f8f2">(</span><span style="color:#f8f8f2">)</span>

<span style="color:#f8f8f2">[</span><span style="color:#a6e22e"><span style="color:#e6db74">"baz"</span></span><span style="color:#f8f8f2">]</span>

<span style="color:#f8f8f2">>></span><span style="color:#f8f8f2">></span> <span style="color:#e6db74">foo</span><span style="color:#f8f8f2">(</span><span style="color:#f8f8f2">)</span>

<span style="color:#f8f8f2">[</span><span style="color:#a6e22e"><span style="color:#e6db74">"baz"</span></span><span style="color:#f8f8f2">]</span>

<span style="color:#f8f8f2">>></span><span style="color:#f8f8f2">></span> <span style="color:#e6db74">foo</span><span style="color:#f8f8f2">(</span><span style="color:#f8f8f2">)</span>

<span style="color:#f8f8f2">[</span><span style="color:#a6e22e"><span style="color:#e6db74">"baz"</span></span><span style="color:#f8f8f2">]</span></code></span></span>

常见错误2：错误地使用类变量

代码示例：

<span style="color:#333333"><span style="color:#f8f8f2"><code class="language-c"><span style="color:#f8f8f2">>></span><span style="color:#f8f8f2">></span> <span style="color:#f92672">class</span> <span style="color:#e6db74"><span style="color:#a6e22e">A</span></span><span style="color:#f8f8f2"><span style="color:#f8f8f2">(</span></span><span style="color:#f8f8f2">object</span><span style="color:#f8f8f2"><span style="color:#f8f8f2">)</span></span><span style="color:#f8f8f2">:</span>

<span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span> x <span style="color:#f8f8f2">=</span> <span style="color:#ae81ff"><span style="color:#ae81ff">1</span></span>

<span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span>

<span style="color:#f8f8f2">>></span><span style="color:#f8f8f2">></span> <span style="color:#f92672">class</span> <span style="color:#e6db74"><span style="color:#a6e22e">B</span></span><span style="color:#f8f8f2"><span style="color:#f8f8f2">(</span></span><span style="color:#f8f8f2">A</span><span style="color:#f8f8f2"><span style="color:#f8f8f2">)</span></span><span style="color:#f8f8f2">:</span>

<span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span> pass

<span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span>

<span style="color:#f8f8f2">>></span><span style="color:#f8f8f2">></span> <span style="color:#f92672">class</span> <span style="color:#e6db74"><span style="color:#a6e22e">C</span></span><span style="color:#f8f8f2"><span style="color:#f8f8f2">(</span></span><span style="color:#f8f8f2">A</span><span style="color:#f8f8f2"><span style="color:#f8f8f2">)</span></span><span style="color:#f8f8f2">:</span>

<span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span> pass

<span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span>

<span style="color:#f8f8f2">>></span><span style="color:#f8f8f2">></span> print A<span style="color:#f8f8f2">.</span>x<span style="color:#f8f8f2">,</span> B<span style="color:#f8f8f2">.</span>x<span style="color:#f8f8f2">,</span> C<span style="color:#f8f8f2">.</span>x

<span style="color:#ae81ff">1</span> <span style="color:#ae81ff">1</span> <span style="color:#ae81ff">1</span></code></span></span>

运行结果没问题。

<span style="color:#333333"><span style="color:#f8f8f2"><code class="language-c"><span style="color:#f8f8f2">>></span><span style="color:#f8f8f2">></span> B<span style="color:#f8f8f2">.</span>x <span style="color:#f8f8f2">=</span> <span style="color:#ae81ff"><span style="color:#ae81ff">2</span></span>

<span style="color:#f8f8f2">>></span><span style="color:#f8f8f2">></span> print A<span style="color:#f8f8f2">.</span>x<span style="color:#f8f8f2">,</span> B<span style="color:#f8f8f2">.</span>x<span style="color:#f8f8f2">,</span> C<span style="color:#f8f8f2">.</span>x

<span style="color:#ae81ff"><span style="color:#ae81ff">1</span></span> <span style="color:#ae81ff"><span style="color:#ae81ff">2</span></span> <span style="color:#ae81ff"><span style="color:#ae81ff">1</span></span></code></span></span>

结果也正确。

<span style="color:#333333"><span style="color:#f8f8f2"><code class="language-c"><span style="color:#f8f8f2">>></span><span style="color:#f8f8f2">></span> A<span style="color:#f8f8f2">.</span>x <span style="color:#f8f8f2">=</span> <span style="color:#ae81ff"><span style="color:#ae81ff">3</span></span>

<span style="color:#f8f8f2">>></span><span style="color:#f8f8f2">></span> print A<span style="color:#f8f8f2">.</span>x<span style="color:#f8f8f2">,</span> B<span style="color:#f8f8f2">.</span>x<span style="color:#f8f8f2">,</span> C<span style="color:#f8f8f2">.</span>x

<span style="color:#ae81ff"><span style="color:#ae81ff">3</span></span> <span style="color:#ae81ff"><span style="color:#ae81ff">2</span></span> <span style="color:#ae81ff"><span style="color:#ae81ff">3</span></span></code></span></span>

什么鬼？我们只改变了A.x.，为什么C.x 也变了？

在Python中，类变量是以字典形式进行内部处理，遵循方法解析顺序（Method Resolution Order ，MRO）。因此，在上述代码中，因为在类C中没有找到属性x，它就会从父类中查找x的值（尽管Python支持多重继承，但上述代码只存在一个父类A）。换句话说，C没有独立于类A的属于自己的x。因此，C.x实际上指的是A.x。除非处理得当，否则就会导致Python出现错误。

如果想更深入了解Python的类特性，请戳：

https://www.toptal.com/python/python-class-attributes-an-overly-thorough-guide

常见错误3：错误指定异常代码块的参数

假设你有如下代码：

<span style="color:#333333"><span style="color:#f8f8f2"><code class="language-c"><span style="color:#f8f8f2">>></span><span style="color:#f8f8f2">></span> <span style="color:#f92672">try</span><span style="color:#f8f8f2">:</span>

<span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span> l <span style="color:#f8f8f2">=</span> <span style="color:#f8f8f2">[</span><span style="color:#a6e22e"><span style="color:#e6db74">"a"</span></span><span style="color:#f8f8f2">,</span> <span style="color:#a6e22e"><span style="color:#e6db74">"b"</span></span><span style="color:#f8f8f2">]</span>

<span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span> <span style="color:#66d9ef"><span style="color:#f92672">int</span></span><span style="color:#f8f8f2">(</span>l<span style="color:#f8f8f2">[</span><span style="color:#ae81ff"><span style="color:#ae81ff">2</span></span><span style="color:#f8f8f2">]</span><span style="color:#f8f8f2">)</span>

<span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span> except ValueError<span style="color:#f8f8f2">,</span> IndexError<span style="color:#f8f8f2">:</span> # To <span style="color:#f92672">catch</span> both exceptions<span style="color:#f8f8f2">,</span> right<span style="color:#f8f8f2">?</span>

<span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span> pass

<span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span>

Traceback <span style="color:#f8f8f2">(</span>most recent call last<span style="color:#f8f8f2">)</span><span style="color:#f8f8f2">:</span>

File <span style="color:#e6db74">"</span><span style="color:#f8f8f2"><span style="color:#e6db74"><</span></span><span style="color:#f92672"><span style="color:#e6db74">stdin</span></span><span style="color:#f8f8f2"><span style="color:#e6db74">></span></span><span style="color:#e6db74">"</span><span style="color:#f8f8f2">,</span> line <span style="color:#ae81ff"><span style="color:#ae81ff">3</span></span><span style="color:#f8f8f2">,</span> in <span style="color:#f8f8f2"><</span>module<span style="color:#f8f8f2">></span>

IndexError<span style="color:#f8f8f2">:</span> <span style="color:#e6db74">list</span> index out of range</code></span></span>

这里的问题是except语句不接受以这种方式指定的异常列表。在Python2.x中，except Exception语句中变量e可用来把异常信息绑定到第二个可选参数上，以便进一步查看异常的情况。因此，在上述代码中，except语句并没有捕捉到IndexError异常；而是将出现的异常绑定到了参数IndexError中。

想在一个except语句同时捕捉到多个异常的正确方式是，将第一个参数指定为元组，并将要捕捉的异常类型都写入该元组中。为了方便起见，可以使用as关键字，Python 2 和Python 3都支持这种语法格式：

<span style="color:#333333"><span style="color:#f8f8f2"><code class="language-c"><span style="color:#f8f8f2">>></span><span style="color:#f8f8f2">></span> <span style="color:#f92672">try</span><span style="color:#f8f8f2">:</span>

<span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span> l <span style="color:#f8f8f2">=</span> <span style="color:#f8f8f2">[</span><span style="color:#a6e22e"><span style="color:#e6db74">"a"</span></span><span style="color:#f8f8f2">,</span> <span style="color:#a6e22e"><span style="color:#e6db74">"b"</span></span><span style="color:#f8f8f2">]</span>

<span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span> <span style="color:#66d9ef"><span style="color:#f92672">int</span></span><span style="color:#f8f8f2">(</span>l<span style="color:#f8f8f2">[</span><span style="color:#ae81ff"><span style="color:#ae81ff">2</span></span><span style="color:#f8f8f2">]</span><span style="color:#f8f8f2">)</span>

<span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span> except <span style="color:#f8f8f2">(</span>ValueError<span style="color:#f8f8f2">,</span> IndexError<span style="color:#f8f8f2">)</span> as e<span style="color:#f8f8f2">:</span> 

<span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span> pass

<span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span>

<span style="color:#f8f8f2">>></span><span style="color:#f8f8f2">></span></code></span></span>

常见错误4：错误理解Python中变量的作用域

Python变量作用域遵循LEGB规则，LEGB是Local，Enclosing，Global，Builtin的缩写，分别代表本地作用域、封闭作用域、全局作用域和内置作用域，这个规则看起来一目了然。事实上，Python的这种工作方式较为独特，会导致一些编程错误，例如：

<span style="color:#333333"><span style="color:#f8f8f2"><code class="language-c"><span style="color:#f8f8f2">>></span><span style="color:#f8f8f2">></span> x <span style="color:#f8f8f2">=</span> <span style="color:#ae81ff"><span style="color:#ae81ff">10</span></span>

<span style="color:#f8f8f2">>></span><span style="color:#f8f8f2">></span> def <span style="color:#e6db74">foo</span><span style="color:#f8f8f2">(</span><span style="color:#f8f8f2">)</span><span style="color:#f8f8f2">:</span>

<span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span> x <span style="color:#f8f8f2">+</span><span style="color:#f8f8f2">=</span> <span style="color:#ae81ff"><span style="color:#ae81ff">1</span></span>

<span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span> print x

<span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span>

<span style="color:#f8f8f2">>></span><span style="color:#f8f8f2">></span> <span style="color:#e6db74">foo</span><span style="color:#f8f8f2">(</span><span style="color:#f8f8f2">)</span>

Traceback <span style="color:#f8f8f2">(</span>most recent call last<span style="color:#f8f8f2">)</span><span style="color:#f8f8f2">:</span>

File <span style="color:#e6db74">"</span><span style="color:#f8f8f2"><span style="color:#e6db74"><</span></span><span style="color:#f92672"><span style="color:#e6db74">stdin</span></span><span style="color:#f8f8f2"><span style="color:#e6db74">></span></span><span style="color:#e6db74">"</span><span style="color:#f8f8f2">,</span> line <span style="color:#ae81ff"><span style="color:#ae81ff">1</span></span><span style="color:#f8f8f2">,</span> in <span style="color:#f8f8f2"><</span>module<span style="color:#f8f8f2">></span>

File <span style="color:#e6db74">"</span><span style="color:#f8f8f2"><span style="color:#e6db74"><</span></span><span style="color:#f92672"><span style="color:#e6db74">stdin</span></span><span style="color:#f8f8f2"><span style="color:#e6db74">></span></span><span style="color:#e6db74">"</span><span style="color:#f8f8f2">,</span> line <span style="color:#ae81ff"><span style="color:#ae81ff">2</span></span><span style="color:#f8f8f2">,</span> in foo

UnboundLocalError<span style="color:#f8f8f2">:</span> local variable <span style="color:#a6e22e"><span style="color:#e6db74">'x'</span></span> referenced before assignment</code></span></span>

问题出在哪？

上面的错误是因为在作用域内对变量赋值时，Python自动将该变量视为该作用域的本地变量，并对外部定义的同名变量进行了屏蔽。因此，原本正确的代码，在某个函数内部添加了一个赋值语句后，却意外收到了UnboundLocalError的报错信息。

关于UnboundLocalError更多内容请戳：

https://docs.python.org/2/faq/programming.html#why-am-i-getting-an-unboundlocalerror-when-the-variable-has-a-value

在使用列表时，Python程序员更容易掉入此类陷阱，例如：

<span style="color:#333333"><span style="color:#f8f8f2"><code class="language-c"><span style="color:#f8f8f2">>></span><span style="color:#f8f8f2">></span> lst <span style="color:#f8f8f2">=</span> <span style="color:#f8f8f2">[</span><span style="color:#ae81ff"><span style="color:#ae81ff">1</span></span><span style="color:#f8f8f2">,</span> <span style="color:#ae81ff"><span style="color:#ae81ff">2</span></span><span style="color:#f8f8f2">,</span> <span style="color:#ae81ff"><span style="color:#ae81ff">3</span></span><span style="color:#f8f8f2">]</span>

<span style="color:#f8f8f2">>></span><span style="color:#f8f8f2">></span> def <span style="color:#e6db74">foo1</span><span style="color:#f8f8f2">(</span><span style="color:#f8f8f2">)</span><span style="color:#f8f8f2">:</span>

<span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span> lst<span style="color:#f8f8f2">.</span><span style="color:#e6db74">append</span><span style="color:#f8f8f2">(</span><span style="color:#ae81ff"><span style="color:#ae81ff">5</span></span><span style="color:#f8f8f2">)</span> # This works ok<span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span>

<span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span>

<span style="color:#f8f8f2">>></span><span style="color:#f8f8f2">></span> <span style="color:#e6db74">foo1</span><span style="color:#f8f8f2">(</span><span style="color:#f8f8f2">)</span>

<span style="color:#f8f8f2">>></span><span style="color:#f8f8f2">></span> lst

<span style="color:#f8f8f2">[</span><span style="color:#ae81ff"><span style="color:#ae81ff">1</span></span><span style="color:#f8f8f2">,</span> <span style="color:#ae81ff"><span style="color:#ae81ff">2</span></span><span style="color:#f8f8f2">,</span> <span style="color:#ae81ff"><span style="color:#ae81ff">3</span></span><span style="color:#f8f8f2">,</span> <span style="color:#ae81ff"><span style="color:#ae81ff">5</span></span><span style="color:#f8f8f2">]</span>

<span style="color:#f8f8f2">>></span><span style="color:#f8f8f2">></span> lst <span style="color:#f8f8f2">=</span> <span style="color:#f8f8f2">[</span><span style="color:#ae81ff"><span style="color:#ae81ff">1</span></span><span style="color:#f8f8f2">,</span> <span style="color:#ae81ff"><span style="color:#ae81ff">2</span></span><span style="color:#f8f8f2">,</span> <span style="color:#ae81ff"><span style="color:#ae81ff">3</span></span><span style="color:#f8f8f2">]</span>

<span style="color:#f8f8f2">>></span><span style="color:#f8f8f2">></span> def <span style="color:#e6db74">foo2</span><span style="color:#f8f8f2">(</span><span style="color:#f8f8f2">)</span><span style="color:#f8f8f2">:</span>

<span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span> lst <span style="color:#f8f8f2">+</span><span style="color:#f8f8f2">=</span> <span style="color:#f8f8f2">[</span><span style="color:#ae81ff"><span style="color:#ae81ff">5</span></span><span style="color:#f8f8f2">]</span> # <span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span> but <span style="color:#f92672">this</span> bombs<span style="color:#f8f8f2">!</span>

<span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span>

<span style="color:#f8f8f2">>></span><span style="color:#f8f8f2">></span> <span style="color:#e6db74">foo2</span><span style="color:#f8f8f2">(</span><span style="color:#f8f8f2">)</span>

Traceback <span style="color:#f8f8f2">(</span>most recent call last<span style="color:#f8f8f2">)</span><span style="color:#f8f8f2">:</span>

File <span style="color:#e6db74">"</span><span style="color:#f8f8f2"><span style="color:#e6db74"><</span></span><span style="color:#f92672"><span style="color:#e6db74">stdin</span></span><span style="color:#f8f8f2"><span style="color:#e6db74">></span></span><span style="color:#e6db74">"</span><span style="color:#f8f8f2">,</span> line <span style="color:#ae81ff"><span style="color:#ae81ff">1</span></span><span style="color:#f8f8f2">,</span> in <span style="color:#f8f8f2"><</span>module<span style="color:#f8f8f2">></span>

File <span style="color:#e6db74">"</span><span style="color:#f8f8f2"><span style="color:#e6db74"><</span></span><span style="color:#f92672"><span style="color:#e6db74">stdin</span></span><span style="color:#f8f8f2"><span style="color:#e6db74">></span></span><span style="color:#e6db74">"</span><span style="color:#f8f8f2">,</span> line <span style="color:#ae81ff"><span style="color:#ae81ff">2</span></span><span style="color:#f8f8f2">,</span> in foo

UnboundLocalError<span style="color:#f8f8f2">:</span> local variable <span style="color:#a6e22e"><span style="color:#e6db74">'lst'</span></span> referenced before assignment</code></span></span>

奇怪，为什么foo1正常运行，而foo2崩溃了呢？

原因和上一个案例中出现的问题相似，但这里的错误更加细微。函数foo1没有对变量lst进行赋值操作，而函数foo2有赋值操作。

首先， lst += [5]是lst = lst + [5]的缩写形式，在函数foo2中试图对变量lst进行赋值操作（Python将变量lst默认为本地作用域的变量）。但是，lst += [5]语句是对lst变量自身进行的赋值操作（此时变量lst的作用域是函数foo2），但是在函数foo2中还未声明该变量，所以就报错啦！

常见错误5:在遍历列表时修改列表

下面代码中的错误很明显：

<span style="color:#333333"><span style="color:#f8f8f2"><code class="language-c"><span style="color:#f8f8f2">>></span><span style="color:#f8f8f2">></span> odd <span style="color:#f8f8f2">=</span> lambda x <span style="color:#f8f8f2">:</span> <span style="color:#e6db74"><span style="color:#f92672">bool</span></span><span style="color:#f8f8f2">(</span>x <span style="color:#f8f8f2">%</span> <span style="color:#ae81ff"><span style="color:#ae81ff">2</span></span><span style="color:#f8f8f2">)</span>

<span style="color:#f8f8f2">>></span><span style="color:#f8f8f2">></span> numbers <span style="color:#f8f8f2">=</span> <span style="color:#f8f8f2">[</span>n <span style="color:#66d9ef"><span style="color:#f92672">for</span></span> n in <span style="color:#e6db74">range</span><span style="color:#f8f8f2">(</span><span style="color:#ae81ff"><span style="color:#ae81ff">10</span></span><span style="color:#f8f8f2">)</span><span style="color:#f8f8f2">]</span>

<span style="color:#f8f8f2">>></span><span style="color:#f8f8f2">></span> <span style="color:#66d9ef"><span style="color:#f92672">for</span></span> i in <span style="color:#e6db74">range</span><span style="color:#f8f8f2">(</span><span style="color:#e6db74">len</span><span style="color:#f8f8f2">(</span>numbers<span style="color:#f8f8f2">)</span><span style="color:#f8f8f2">)</span><span style="color:#f8f8f2">:</span>

<span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span> <span style="color:#66d9ef"><span style="color:#f92672">if</span></span> <span style="color:#e6db74">odd</span><span style="color:#f8f8f2">(</span>numbers<span style="color:#f8f8f2">[</span>i<span style="color:#f8f8f2">]</span><span style="color:#f8f8f2">)</span><span style="color:#f8f8f2">:</span>

<span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span> del numbers<span style="color:#f8f8f2">[</span>i<span style="color:#f8f8f2">]</span> # BAD<span style="color:#f8f8f2">:</span> Deleting item from a <span style="color:#e6db74">list</span> <span style="color:#66d9ef"><span style="color:#f92672">while</span></span> iterating over it

<span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span>

Traceback <span style="color:#f8f8f2">(</span>most recent call last<span style="color:#f8f8f2">)</span><span style="color:#f8f8f2">:</span>

File <span style="color:#e6db74">"</span><span style="color:#f8f8f2"><span style="color:#e6db74"><</span></span><span style="color:#f92672"><span style="color:#e6db74">stdin</span></span><span style="color:#f8f8f2"><span style="color:#e6db74">></span></span><span style="color:#e6db74">"</span><span style="color:#f8f8f2">,</span> line <span style="color:#ae81ff"><span style="color:#ae81ff">2</span></span><span style="color:#f8f8f2">,</span> in <span style="color:#f8f8f2"><</span>module<span style="color:#f8f8f2">></span>

IndexError<span style="color:#f8f8f2">:</span> <span style="color:#e6db74">list</span> index out of range</code></span></span>

有经验的程序员都知道，在Python中遍历列表或数组时不应该删除该列表（数组）中的元素。虽然上面代码的错误很明显，但是在编写复杂代码时，资深程序员也难免会犯此类错误。

幸好Python集成了大量经典的编程范式，如果运用得当，可以大大简化代码并提高编程效率。简单的代码会降低出现上述bug的几率。列表解析式（list comprehensions）就是利器之一，它将完美避开上述bug，解决方案如下：

<span style="color:#333333"><span style="color:#f8f8f2"><code class="language-c"><span style="color:#f8f8f2">>></span><span style="color:#f8f8f2">></span> odd <span style="color:#f8f8f2">=</span> lambda x <span style="color:#f8f8f2">:</span> <span style="color:#e6db74"><span style="color:#f92672">bool</span></span><span style="color:#f8f8f2">(</span>x <span style="color:#f8f8f2">%</span> <span style="color:#ae81ff"><span style="color:#ae81ff">2</span></span><span style="color:#f8f8f2">)</span>

<span style="color:#f8f8f2">>></span><span style="color:#f8f8f2">></span> numbers <span style="color:#f8f8f2">=</span> <span style="color:#f8f8f2">[</span>n <span style="color:#66d9ef"><span style="color:#f92672">for</span></span> n in <span style="color:#e6db74">range</span><span style="color:#f8f8f2">(</span><span style="color:#ae81ff"><span style="color:#ae81ff">10</span></span><span style="color:#f8f8f2">)</span><span style="color:#f8f8f2">]</span>

<span style="color:#f8f8f2">>></span><span style="color:#f8f8f2">></span> numbers<span style="color:#f8f8f2">[</span><span style="color:#f8f8f2">:</span><span style="color:#f8f8f2">]</span> <span style="color:#f8f8f2">=</span> <span style="color:#f8f8f2">[</span>n <span style="color:#66d9ef"><span style="color:#f92672">for</span></span> n in numbers <span style="color:#66d9ef"><span style="color:#f92672">if</span></span> not <span style="color:#e6db74">odd</span><span style="color:#f8f8f2">(</span>n<span style="color:#f8f8f2">)</span><span style="color:#f8f8f2">]</span> # ahh<span style="color:#f8f8f2">,</span> the beauty of it all

<span style="color:#f8f8f2">>></span><span style="color:#f8f8f2">></span> numbers

<span style="color:#f8f8f2">[</span><span style="color:#ae81ff"><span style="color:#ae81ff">0</span></span><span style="color:#f8f8f2">,</span> <span style="color:#ae81ff"><span style="color:#ae81ff">2</span></span><span style="color:#f8f8f2">,</span> <span style="color:#ae81ff"><span style="color:#ae81ff">4</span></span><span style="color:#f8f8f2">,</span> <span style="color:#ae81ff"><span style="color:#ae81ff">6</span></span><span style="color:#f8f8f2">,</span> <span style="color:#ae81ff"><span style="color:#ae81ff">8</span></span><span style="color:#f8f8f2">]</span></code></span></span>

更多有关列表解析式的详细内容，请戳：https://docs.python.org/2/tutorial/datastructures.html#tut-listcomps

常见错误6：不理解Python闭包中的变量绑定

代码示例：

<span style="color:#333333"><span style="color:#f8f8f2"><code class="language-c"><span style="color:#f8f8f2">>></span><span style="color:#f8f8f2">></span> def <span style="color:#e6db74"><span style="color:#a6e22e">create_multipliers</span></span><span style="color:#f8f8f2"><span style="color:#f8f8f2">(</span></span><span style="color:#f8f8f2"><span style="color:#f8f8f2">)</span></span><span style="color:#f8f8f2">:</span>

<span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span> <span style="color:#66d9ef"><span style="color:#f92672">return</span></span> <span style="color:#f8f8f2">[</span>lambda x <span style="color:#f8f8f2">:</span> i <span style="color:#f8f8f2">*</span> x <span style="color:#66d9ef"><span style="color:#f92672">for</span></span> i in <span style="color:#e6db74"><span style="color:#a6e22e">range</span></span><span style="color:#f8f8f2"><span style="color:#f8f8f2">(</span></span><span style="color:#ae81ff"><span style="color:#f8f8f2"><span style="color:#ae81ff">5</span></span></span><span style="color:#f8f8f2"><span style="color:#f8f8f2">)</span></span><span style="color:#f8f8f2">]</span>

<span style="color:#f8f8f2">>></span><span style="color:#f8f8f2">></span> <span style="color:#66d9ef"><span style="color:#f92672">for</span></span> multiplier in <span style="color:#e6db74"><span style="color:#a6e22e">create_multipliers</span></span><span style="color:#f8f8f2"><span style="color:#f8f8f2">(</span></span><span style="color:#f8f8f2"><span style="color:#f8f8f2">)</span></span><span style="color:#f8f8f2">:</span>

<span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span> print <span style="color:#e6db74"><span style="color:#a6e22e">multiplier</span></span><span style="color:#f8f8f2"><span style="color:#f8f8f2">(</span></span><span style="color:#ae81ff"><span style="color:#f8f8f2"><span style="color:#ae81ff">2</span></span></span><span style="color:#f8f8f2"><span style="color:#f8f8f2">)</span></span>

<span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span>
</code></span></span>

你以为运行结果会是：

<span style="color:#333333"><span style="color:#f8f8f2"><code class="language-c"><span style="color:#ae81ff"><span style="color:#ae81ff">0</span></span>

<span style="color:#ae81ff"><span style="color:#ae81ff">2</span></span>

<span style="color:#ae81ff"><span style="color:#ae81ff">4</span></span>

<span style="color:#ae81ff"><span style="color:#ae81ff">6</span></span>

<span style="color:#ae81ff"><span style="color:#ae81ff">8</span></span></code></span></span>

但实际输出结果是：8

<span style="color:#333333"><span style="color:#f8f8f2"><code class="language-c"><span style="color:#ae81ff"><span style="color:#ae81ff">8</span></span>

<span style="color:#ae81ff"><span style="color:#ae81ff">8</span></span>

<span style="color:#ae81ff"><span style="color:#ae81ff">8</span></span>

<span style="color:#ae81ff"><span style="color:#ae81ff">8</span></span></code></span></span>

惊不惊喜！

这种情况是由于Python延迟绑定（late binding）机制造成的，也就是说只有在内部函数被调用时才会搜索闭包中变量的值。所以在上述代码中，每次调用create_multipliers()函数中的return函数时，会在附近作用域中查询变量i的值。（此时，return中循环已结束，所以i值为4）。

常见解决方案：

<span style="color:#333333"><span style="color:#f8f8f2"><code class="language-c"><span style="color:#f8f8f2">>></span><span style="color:#f8f8f2">></span> def <span style="color:#e6db74"><span style="color:#a6e22e">create_multipliers</span></span><span style="color:#f8f8f2"><span style="color:#f8f8f2">(</span></span><span style="color:#f8f8f2"><span style="color:#f8f8f2">)</span></span><span style="color:#f8f8f2">:</span>

<span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span> <span style="color:#66d9ef"><span style="color:#f92672">return</span></span> <span style="color:#f8f8f2">[</span>lambda x<span style="color:#f8f8f2">,</span> i<span style="color:#f8f8f2">=</span>i <span style="color:#f8f8f2">:</span> i <span style="color:#f8f8f2">*</span> x <span style="color:#66d9ef"><span style="color:#f92672">for</span></span> i in <span style="color:#e6db74"><span style="color:#a6e22e">range</span></span><span style="color:#f8f8f2"><span style="color:#f8f8f2">(</span></span><span style="color:#ae81ff"><span style="color:#f8f8f2"><span style="color:#ae81ff">5</span></span></span><span style="color:#f8f8f2"><span style="color:#f8f8f2">)</span></span><span style="color:#f8f8f2">]</span>

<span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span>

<span style="color:#f8f8f2">>></span><span style="color:#f8f8f2">></span> <span style="color:#66d9ef"><span style="color:#f92672">for</span></span> multiplier in <span style="color:#e6db74"><span style="color:#a6e22e">create_multipliers</span></span><span style="color:#f8f8f2"><span style="color:#f8f8f2">(</span></span><span style="color:#f8f8f2"><span style="color:#f8f8f2">)</span></span><span style="color:#f8f8f2">:</span>

<span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span> print <span style="color:#e6db74"><span style="color:#a6e22e">multiplier</span></span><span style="color:#f8f8f2"><span style="color:#f8f8f2">(</span></span><span style="color:#ae81ff"><span style="color:#f8f8f2"><span style="color:#ae81ff">2</span></span></span><span style="color:#f8f8f2"><span style="color:#f8f8f2">)</span></span>

<span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span>

<span style="color:#ae81ff">0</span>

<span style="color:#ae81ff">2</span>

<span style="color:#ae81ff">4</span>

<span style="color:#ae81ff">6</span>

<span style="color:#ae81ff">8</span>
</code></span></span>

没错！我们利用了匿名函数lambda的默认参数来生成结果序列。有人觉得这种用法很简洁，有人会说它很巧妙，还有人会觉得晦涩难懂。如果你是Python开发人员，那么深刻理解上述语法对你而言非常重要。

常见错误7：模块之间出现循环依赖

假设你有两个文件，分别是a.py和b.py，两者相互导入，如下所示：

a.py模块中的代码：

<span style="color:#333333"><span style="color:#f8f8f2"><code class="language-c">import b

def <span style="color:#e6db74"><span style="color:#a6e22e">f</span></span><span style="color:#f8f8f2"><span style="color:#f8f8f2">(</span></span><span style="color:#f8f8f2"><span style="color:#f8f8f2">)</span></span><span style="color:#f8f8f2">:</span>

<span style="color:#66d9ef"><span style="color:#f92672">return</span></span> b<span style="color:#f8f8f2">.</span>x

print <span style="color:#e6db74"><span style="color:#a6e22e">f</span></span><span style="color:#f8f8f2"><span style="color:#f8f8f2">(</span></span><span style="color:#f8f8f2"><span style="color:#f8f8f2">)</span></span></code></span></span>

b.py模块中的代码：

<span style="color:#333333"><span style="color:#f8f8f2"><code class="language-c">import a

x <span style="color:#f8f8f2">=</span> <span style="color:#ae81ff"><span style="color:#ae81ff">1</span></span>

def <span style="color:#e6db74">g</span><span style="color:#f8f8f2">(</span><span style="color:#f8f8f2">)</span><span style="color:#f8f8f2">:</span>

print a<span style="color:#f8f8f2">.</span><span style="color:#e6db74">f</span><span style="color:#f8f8f2">(</span><span style="color:#f8f8f2">)</span></code></span></span>

首先，我们尝试导入a.py：

<span style="color:#333333"><span style="color:#f8f8f2"><code class="language-c"><span style="color:#f8f8f2">>></span><span style="color:#f8f8f2">></span> import a

<span style="color:#ae81ff"><span style="color:#ae81ff">1</span></span></code></span></span>

运行结果正确！这似乎有点出人意料，因为我们在这里进行循环导入，应该会报错呀！

答案是，在Python中如果仅存在一个循环导入，程序不会报错。如果一个模块已经被导入，Python会自动识别而不会再次导入。但是如果每个模块试图访问其他模块不同位置的函数或变量时，那么Error又双叒叕出现了。

回到上面的示例中，当导入a.py模块时，程序可以正常导入b.py模块，因为此时b.py模块未访问a.py中定义任何的变量或函数。b.py模块仅引用了a.py模中的a.f()函数。调用的a.f()函数隶属于g()函数，而a.py或b.py模块中并没有调用g()函数。所以程序没有报错。

但是，如果我们在未导入a.py模块之前先导入b.py模块，结果会怎样？

<span style="color:#333333"><span style="color:#f8f8f2"><code class="language-c"><span style="color:#f8f8f2">>></span><span style="color:#f8f8f2">></span> import b

<span style="color:#a6e22e">Traceback</span> <span style="color:#f8f8f2"><span style="color:#f8f8f2">(</span></span><span style="color:#f8f8f2">most recent call last</span><span style="color:#f8f8f2"><span style="color:#f8f8f2">)</span></span><span style="color:#f8f8f2">:</span>

File "<span style="color:#f8f8f2"><</span><span style="color:#f92672"><span style="color:#e6db74">stdin</span></span><span style="color:#f8f8f2">></span>"<span style="color:#f8f8f2">,</span> line <span style="color:#ae81ff">1</span><span style="color:#f8f8f2">,</span> in <span style="color:#f8f8f2"><</span>module<span style="color:#f8f8f2">></span>

File <span style="color:#a6e22e">"b.py"</span><span style="color:#f8f8f2">,</span> line <span style="color:#ae81ff">1</span><span style="color:#f8f8f2">,</span> in <span style="color:#f8f8f2"><</span>module<span style="color:#f8f8f2">></span>

import a

File <span style="color:#a6e22e">"a.py"</span><span style="color:#f8f8f2">,</span> line <span style="color:#ae81ff">6</span><span style="color:#f8f8f2">,</span> in <span style="color:#f8f8f2"><</span>module<span style="color:#f8f8f2">></span>

print <span style="color:#e6db74"><span style="color:#a6e22e">f</span></span><span style="color:#f8f8f2"><span style="color:#f8f8f2">(</span></span><span style="color:#f8f8f2"><span style="color:#f8f8f2">)</span></span>

File <span style="color:#a6e22e">"a.py"</span><span style="color:#f8f8f2">,</span> line <span style="color:#ae81ff">4</span><span style="color:#f8f8f2">,</span> in f

<span style="color:#66d9ef"><span style="color:#f92672">return</span></span> b<span style="color:#f8f8f2">.</span>x

AttributeError<span style="color:#f8f8f2">:</span> <span style="color:#a6e22e">'module'</span> object has no attribute <span style="color:#a6e22e">'x'</span></code></span></span>

报错了！问题在于，在导入b.py的过程中，它试图导入a.py模块，而a.py模块会调用f()函数，f()函数又试图访问b.x变量。但此时，还未对变量b.x进行定义，所以出现了AttributeError异常。

稍微修改下b.py，即在g()函数内部导入a.py就可以解决上述问题。

修改后的b.py：

<span style="color:#333333"><span style="color:#f8f8f2"><code class="language-c">x <span style="color:#f8f8f2">=</span> <span style="color:#ae81ff"><span style="color:#ae81ff">1</span></span>

def <span style="color:#e6db74">g</span><span style="color:#f8f8f2">(</span><span style="color:#f8f8f2">)</span><span style="color:#f8f8f2">:</span>


import a # This will be evaluated only when <span style="color:#e6db74">g</span><span style="color:#f8f8f2">(</span><span style="color:#f8f8f2">)</span> is called

print a<span style="color:#f8f8f2">.</span><span style="color:#e6db74">f</span><span style="color:#f8f8f2">(</span><span style="color:#f8f8f2">)</span></code></span></span>

现在我们再导入b.py模块，就不会报错啦！

<span style="color:#333333"><span style="color:#f8f8f2"><code class="language-c"><span style="color:#f8f8f2">>></span><span style="color:#f8f8f2">></span> import b

<span style="color:#f8f8f2">>></span><span style="color:#f8f8f2">></span> b<span style="color:#f8f8f2">.</span><span style="color:#e6db74">g</span><span style="color:#f8f8f2">(</span><span style="color:#f8f8f2">)</span>

<span style="color:#ae81ff"><span style="color:#ae81ff">1</span></span> # Printed a first time since module <span style="color:#a6e22e"><span style="color:#e6db74">'a'</span></span> calls <span style="color:#a6e22e"><span style="color:#e6db74">'print f()'</span></span> at the end

<span style="color:#ae81ff"><span style="color:#ae81ff">1</span></span> # Printed a second time<span style="color:#f8f8f2">,</span> <span style="color:#f92672">this</span> one is our call to <span style="color:#a6e22e"><span style="color:#e6db74">'g'</span></span></code></span></span>

常见错误8：文件命名与Python标准库模块的名称冲突

Python的优势之一就是其集成了丰富的标准库。正因为如此，稍不留神就会在为自己的文件命名时与Python自带标准库模块重名。例如，如果你的代码中有一个名为email.py的模块，恰好就和Python标准库中email.py模块重名了。）

上述问题比较复杂。举个例子，在导入模块A的时候，假如该模块A试图导入Python标准库中的模块B，但你已经定义了一个同名模块B，模块A会错误导入你自定义的模块B，而不是Python标准库中的模块B。这种错误很糟糕，因为程序员很难察觉到是因为命名冲突而导致的。

因此，Python程序员要注意避免与Python标准库模块的命名冲突。毕竟，修改自己模块的名称比修改标准库的名称要容易的多！当然你也可以写一份Python改善建议书（Python Enhancement Proposal，PEP）提议修改标准库的名称。

常见错误9：不熟悉Python2和Python3之间的差异

先来看看foo.py文件中的代码：

<span style="color:#333333"><span style="color:#f8f8f2"><code class="language-c">import sys

def <span style="color:#e6db74"><span style="color:#a6e22e">bar</span></span><span style="color:#f8f8f2"><span style="color:#f8f8f2">(</span></span><span style="color:#f8f8f2">i</span><span style="color:#f8f8f2"><span style="color:#f8f8f2">)</span></span><span style="color:#f8f8f2">:</span>

<span style="color:#66d9ef"><span style="color:#f92672">if</span></span> i <span style="color:#f8f8f2">==</span> <span style="color:#ae81ff"><span style="color:#ae81ff">1</span></span><span style="color:#f8f8f2">:</span>

raise <span style="color:#e6db74">KeyError</span><span style="color:#f8f8f2">(</span><span style="color:#ae81ff"><span style="color:#ae81ff">1</span></span><span style="color:#f8f8f2">)</span>

<span style="color:#66d9ef"><span style="color:#f92672">if</span></span> i <span style="color:#f8f8f2">==</span> <span style="color:#ae81ff"><span style="color:#ae81ff">2</span></span><span style="color:#f8f8f2">:</span>

raise <span style="color:#e6db74">ValueError</span><span style="color:#f8f8f2">(</span><span style="color:#ae81ff"><span style="color:#ae81ff">2</span></span><span style="color:#f8f8f2">)</span>

def <span style="color:#e6db74">bad</span><span style="color:#f8f8f2">(</span><span style="color:#f8f8f2">)</span><span style="color:#f8f8f2">:</span>

e <span style="color:#f8f8f2">=</span> None

<span style="color:#f92672">try</span><span style="color:#f8f8f2">:</span>

<span style="color:#e6db74">bar</span><span style="color:#f8f8f2">(</span><span style="color:#66d9ef"><span style="color:#f92672">int</span></span><span style="color:#f8f8f2">(</span>sys<span style="color:#f8f8f2">.</span>argv<span style="color:#f8f8f2">[</span><span style="color:#ae81ff"><span style="color:#ae81ff">1</span></span><span style="color:#f8f8f2">]</span><span style="color:#f8f8f2">)</span><span style="color:#f8f8f2">)</span>

except KeyError as e<span style="color:#f8f8f2">:</span>

<span style="color:#e6db74">print</span><span style="color:#f8f8f2">(</span><span style="color:#a6e22e"><span style="color:#e6db74">'key error'</span></span><span style="color:#f8f8f2">)</span>

except ValueError as e<span style="color:#f8f8f2">:</span>

<span style="color:#e6db74">print</span><span style="color:#f8f8f2">(</span><span style="color:#a6e22e"><span style="color:#e6db74">'value error'</span></span><span style="color:#f8f8f2">)</span>

<span style="color:#e6db74">print</span><span style="color:#f8f8f2">(</span>e<span style="color:#f8f8f2">)</span>

<span style="color:#e6db74">bad</span><span style="color:#f8f8f2">(</span><span style="color:#f8f8f2">)</span></code></span></span>

在Python 2中，上述代码运行正常

<span style="color:#333333"><span style="color:#f8f8f2"><code class="language-c">$ python foo<span style="color:#f8f8f2">.</span>py <span style="color:#ae81ff"><span style="color:#ae81ff">1</span></span>

key error

<span style="color:#ae81ff"><span style="color:#ae81ff">1</span></span>

$ python foo<span style="color:#f8f8f2">.</span>py <span style="color:#ae81ff"><span style="color:#ae81ff">2</span></span>

value error

<span style="color:#ae81ff"><span style="color:#ae81ff">2</span></span></code></span></span>

但是在Python 3中运行时：

<span style="color:#333333"><span style="color:#f8f8f2"><code class="language-c">$ python3 foo<span style="color:#f8f8f2">.</span>py <span style="color:#ae81ff"><span style="color:#ae81ff">1</span></span>

key error

<span style="color:#a6e22e">Traceback</span> <span style="color:#f8f8f2"><span style="color:#f8f8f2">(</span></span><span style="color:#f8f8f2">most recent call last</span><span style="color:#f8f8f2"><span style="color:#f8f8f2">)</span></span><span style="color:#f8f8f2">:</span>

File <span style="color:#a6e22e">"foo.py"</span><span style="color:#f8f8f2">,</span> line <span style="color:#ae81ff">19</span><span style="color:#f8f8f2">,</span> in <span style="color:#f8f8f2"><</span>module<span style="color:#f8f8f2">></span>

<span style="color:#e6db74"><span style="color:#a6e22e">bad</span></span><span style="color:#f8f8f2"><span style="color:#f8f8f2">(</span></span><span style="color:#f8f8f2"><span style="color:#f8f8f2">)</span></span>

File <span style="color:#a6e22e">"foo.py"</span><span style="color:#f8f8f2">,</span> line <span style="color:#ae81ff">17</span><span style="color:#f8f8f2">,</span> in bad

<span style="color:#e6db74"><span style="color:#a6e22e">print</span></span><span style="color:#f8f8f2"><span style="color:#f8f8f2">(</span></span><span style="color:#f8f8f2">e</span><span style="color:#f8f8f2"><span style="color:#f8f8f2">)</span></span>

UnboundLocalError<span style="color:#f8f8f2">:</span> local variable <span style="color:#a6e22e">'e'</span> referenced before assignment</code></span></span>

什么情况？原来，在Python 3中，在except代码块作用域外无法访问异常对象。（原因是，Python 3会将内存堆栈中的循环引用进行保留，直到垃圾回收器运行后在内存中对其进行清理。）

更多内容请戳：

https://docs.python.org/3/reference/compound_stmts.html#except

解决方法之一是，在except代码块的作用域之外，加一句异常对象的引用就可以正常访问异常对象了。下面是处理后的代码，在Python2和Python3中的运行结果一致：

<span style="color:#333333"><span style="color:#f8f8f2"><code class="language-c">import sys

def <span style="color:#e6db74"><span style="color:#a6e22e">bar</span></span><span style="color:#f8f8f2"><span style="color:#f8f8f2">(</span></span><span style="color:#f8f8f2">i</span><span style="color:#f8f8f2"><span style="color:#f8f8f2">)</span></span><span style="color:#f8f8f2">:</span>

<span style="color:#66d9ef"><span style="color:#f92672">if</span></span> i <span style="color:#f8f8f2">==</span> <span style="color:#ae81ff"><span style="color:#ae81ff">1</span></span><span style="color:#f8f8f2">:</span>

raise <span style="color:#e6db74">KeyError</span><span style="color:#f8f8f2">(</span><span style="color:#ae81ff"><span style="color:#ae81ff">1</span></span><span style="color:#f8f8f2">)</span>

<span style="color:#66d9ef"><span style="color:#f92672">if</span></span> i <span style="color:#f8f8f2">==</span> <span style="color:#ae81ff"><span style="color:#ae81ff">2</span></span><span style="color:#f8f8f2">:</span>

raise <span style="color:#e6db74">ValueError</span><span style="color:#f8f8f2">(</span><span style="color:#ae81ff"><span style="color:#ae81ff">2</span></span><span style="color:#f8f8f2">)</span>

def <span style="color:#e6db74">good</span><span style="color:#f8f8f2">(</span><span style="color:#f8f8f2">)</span><span style="color:#f8f8f2">:</span>

exception <span style="color:#f8f8f2">=</span> None

<span style="color:#f92672">try</span><span style="color:#f8f8f2">:</span>

<span style="color:#e6db74">bar</span><span style="color:#f8f8f2">(</span><span style="color:#66d9ef"><span style="color:#f92672">int</span></span><span style="color:#f8f8f2">(</span>sys<span style="color:#f8f8f2">.</span>argv<span style="color:#f8f8f2">[</span><span style="color:#ae81ff"><span style="color:#ae81ff">1</span></span><span style="color:#f8f8f2">]</span><span style="color:#f8f8f2">)</span><span style="color:#f8f8f2">)</span>

except KeyError as e<span style="color:#f8f8f2">:</span>

exception <span style="color:#f8f8f2">=</span> e

<span style="color:#e6db74">print</span><span style="color:#f8f8f2">(</span><span style="color:#a6e22e"><span style="color:#e6db74">'key error'</span></span><span style="color:#f8f8f2">)</span>

except ValueError as e<span style="color:#f8f8f2">:</span>

exception <span style="color:#f8f8f2">=</span> e

<span style="color:#e6db74">print</span><span style="color:#f8f8f2">(</span><span style="color:#a6e22e"><span style="color:#e6db74">'value error'</span></span><span style="color:#f8f8f2">)</span>

<span style="color:#e6db74">print</span><span style="color:#f8f8f2">(</span>exception<span style="color:#f8f8f2">)</span>

<span style="color:#e6db74">good</span><span style="color:#f8f8f2">(</span><span style="color:#f8f8f2">)</span></code></span></span>

再次在Python3中运行代码：

<span style="color:#333333"><span style="color:#f8f8f2"><code class="language-c">$ python3 foo<span style="color:#f8f8f2">.</span>py <span style="color:#ae81ff"><span style="color:#ae81ff">1</span></span>

key error

<span style="color:#ae81ff"><span style="color:#ae81ff">1</span></span>

$ python3 foo<span style="color:#f8f8f2">.</span>py <span style="color:#ae81ff"><span style="color:#ae81ff">2</span></span>

value error

<span style="color:#ae81ff"><span style="color:#ae81ff">2</span></span></code></span></span>

问题解决了！

更多有关Python2和Python3之间的区别，请戳：

https://www.toptal.com/python#hiring-guide

常见错误10：误用_del_方法

假设名为mod.py的文件中有如下代码：

<span style="color:#333333"><span style="color:#f8f8f2"><code class="language-c">import foo

<span style="color:#f92672">class</span> <span style="color:#e6db74"><span style="color:#a6e22e">Bar</span></span><span style="color:#f8f8f2"><span style="color:#f8f8f2">(</span></span><span style="color:#f8f8f2">object</span><span style="color:#f8f8f2"><span style="color:#f8f8f2">)</span></span><span style="color:#f8f8f2">:</span>

<span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span>

def <span style="color:#e6db74">__<span style="color:#a6e22e">del__</span></span><span style="color:#f8f8f2"><span style="color:#f8f8f2">(</span></span><span style="color:#f8f8f2">self</span><span style="color:#f8f8f2"><span style="color:#f8f8f2">)</span></span><span style="color:#f8f8f2">:</span>

foo<span style="color:#f8f8f2">.</span><span style="color:#e6db74"><span style="color:#a6e22e">cleanup</span></span><span style="color:#f8f8f2"><span style="color:#f8f8f2">(</span></span><span style="color:#f8f8f2">self</span><span style="color:#f8f8f2"><span style="color:#f8f8f2">.</span></span><span style="color:#f8f8f2">myhandle</span><span style="color:#f8f8f2"><span style="color:#f8f8f2">)</span></span></code></span></span>

然后，你想在another_mod.py文件中进行如下操作：

<span style="color:#333333"><span style="color:#f8f8f2"><code class="language-c">import mod

mybar <span style="color:#f8f8f2">=</span> mod<span style="color:#f8f8f2">.</span><span style="color:#e6db74">Bar</span><span style="color:#f8f8f2">(</span><span style="color:#f8f8f2">)</span></code></span></span>

如果你试图运行another_mod.py，将会出现AttributeError异常。

为什么呢？因为当Python解释器关闭时，该模块的全局变量的值都会被置为None。因此，在上述示例中，在调用__del__函数时，foo的值已经为None。

关于Python解释器的更多内容，请戳：

https://mail.python.org/pipermail/python-bugs-list/2009-January/069209.html

调用atexit.register()函数可以解决上述的Python高阶编程问题。在调用atexit.register()函数后，当你的代码运行结束后（即正常退出程序的情况下），注册处理程序会在解释器关闭之前运行。

应用上述方法，修改后的mod.py文件如下：

<span style="color:#333333"><span style="color:#f8f8f2"><code class="language-c">import foo

import atexit

def <span style="color:#e6db74"><span style="color:#a6e22e">cleanup</span></span><span style="color:#f8f8f2"><span style="color:#f8f8f2">(</span></span><span style="color:#f8f8f2">handle</span><span style="color:#f8f8f2"><span style="color:#f8f8f2">)</span></span><span style="color:#f8f8f2">:</span>

foo<span style="color:#f8f8f2">.</span><span style="color:#e6db74"><span style="color:#a6e22e">cleanup</span></span><span style="color:#f8f8f2"><span style="color:#f8f8f2">(</span></span><span style="color:#f8f8f2">handle</span><span style="color:#f8f8f2"><span style="color:#f8f8f2">)</span></span>

<span style="color:#f92672">class</span> <span style="color:#e6db74"><span style="color:#a6e22e">Bar</span></span><span style="color:#f8f8f2"><span style="color:#f8f8f2">(</span></span><span style="color:#f8f8f2">object</span><span style="color:#f8f8f2"><span style="color:#f8f8f2">)</span></span><span style="color:#f8f8f2">:</span>

def <span style="color:#e6db74">__<span style="color:#a6e22e">init__</span></span><span style="color:#f8f8f2"><span style="color:#f8f8f2">(</span></span><span style="color:#f8f8f2">self</span><span style="color:#f8f8f2"><span style="color:#f8f8f2">)</span></span><span style="color:#f8f8f2">:</span>

<span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span><span style="color:#f8f8f2">.</span>

atexit<span style="color:#f8f8f2">.</span><span style="color:#66d9ef"><span style="color:#a6e22e">register</span></span><span style="color:#f8f8f2"><span style="color:#f8f8f2">(</span></span><span style="color:#f8f8f2">cleanup</span><span style="color:#f8f8f2"><span style="color:#f8f8f2">,</span></span><span style="color:#f8f8f2"> self</span><span style="color:#f8f8f2"><span style="color:#f8f8f2">.</span></span><span style="color:#f8f8f2">myhandle</span><span style="color:#f8f8f2"><span style="color:#f8f8f2">)</span></span></code></span></span>

当程序正常终止时，这种方法可以很方便的调用程序的清理功能。上述示例中，foo.cleanup函数会决定如何处理self.myhandle所绑定的对象，但是调用atexit.register（）函数就可以由你决定何时执行清理功能。

总结

Python是一种强大且灵活的编程语言，提供了很多编程机制和范式，它可以极大地提高我们的工作效率。但不论使用何种软件工具或编程语言，开发人员都应该彻底理解Python的语法规则和编程规范，否则将会陷入“一知半解，害已误人”的状态。

不断学习Python的语法规则，尤其文中提到这些问题，有助于降低代码的出错概率，也会提升Python编程的效率。

原文发布时间为：2018-04-5

本文作者：文摘菌

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