第 1 章 引论
建议 1：理解 Pythonic 概念
Pythonic
Tim Peters 的 《The Zen of Python》相信学过 Python 的都耳熟能详，在交互式环境中输入import this可以查看，其实有意思的是这段 Python 之禅的源码：
d = {}
<span class="hljs-keyword">for</span> <span class="hljs-built_in">c</span> <span class="hljs-keyword">in</span> (<span class="hljs-number">65</span>, <span class="hljs-number">97</span>):
<span class="hljs-keyword">for</span> i <span class="hljs-keyword">in</span> range(<span class="hljs-number">26</span>):
d[chr(i+<span class="hljs-built_in">c</span>)] = chr((i+<span class="hljs-number">13</span>) % <span class="hljs-number">26</span> + <span class="hljs-built_in">c</span>)
<span class="hljs-built_in">print</span> <span class="hljs-string">""</span>.<span class="hljs-built_in">join</span>([d.<span class="hljs-keyword">get</span>(<span class="hljs-built_in">c</span>, <span class="hljs-built_in">c</span>) <span class="hljs-keyword">for</span> <span class="hljs-built_in">c</span> <span class="hljs-keyword">in</span> s])
哈哈哈，相信这是大佬在跟我们举反例吧。
书中还举了一个快排的例子：
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">quicksort</span><span class="hljs-params">(array)</span>:</span>
less = []
greater = []
<span class="hljs-keyword">if</span> len(array) <= <span class="hljs-number">1</span>:
<span class="hljs-keyword">return</span> array
pivot =array.pop()
<span class="hljs-keyword">for</span> x <span class="hljs-keyword">in</span> array:
<span class="hljs-keyword">if</span> x <= pivot:
less.append(x)
<span class="hljs-keyword">else</span>:
greater.append(x)
<span class="hljs-keyword">return</span> quicksort(less) + [pivot] + quicksort(greater)
代码风格
通过对语法、库和应用程序的理解来编写代码，充分体现 Python 自身的特色：
<span class="hljs-comment"># 变量交换</span>
a, b = b, a
<span class="hljs-comment"># 上下文管理</span>
with open(path, <span class="hljs-string">'r'</span>) <span class="hljs-keyword">as</span> f:
do_sth_with(f)
<span class="hljs-comment"># 不应当过分地追求奇技淫巧</span>
a = [<span class="hljs-number">1</span>, <span class="hljs-number">2</span>, <span class="hljs-number">3</span>, <span class="hljs-number">4</span>]
a[::<span class="hljs-number">-1</span>] <span class="hljs-comment"># 不推荐。好吧，自从学了切片我一直用的这个</span>
<span class="hljs-keyword">list</span>(reversed(a)) <span class="hljs-comment"># 推荐</span>
然后表扬了 Flask 框架，提到了 generator 之类的特性尤为 Pythonic，有个包和模块的约束：
- 包和模块的命名采用小写、单数形式，而且短小
- 包通常仅作为命名空间，如只含空的__init__.py文件
建议 2：编写 Pythonic 代码
命名的规范：
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">find_num</span><span class="hljs-params">(searchList, num)</span>:</span>
<span class="hljs-keyword">for</span> listValue <span class="hljs-keyword">in</span> searchList:
<span class="hljs-keyword">if</span> num == listValue:
<span class="hljs-keyword">return</span> <span class="hljs-keyword">True</span>
<span class="hljs-keyword">else</span>:
<span class="hljs-keyword">pass</span>
尝试去通读官方手册，掌握不断发展的新特性，这将使你编写代码的执行效率更高，推荐深入学习 Flask、gevent 和 requests。
建议 3：理解 Python 与 C 语言的不同之处
提到了三点：
- Python 使用代码缩进的方式来分割代码块，不要混用 Tab 键和空格
- Python 中单、双引号的使用
- 三元操作符：x if bool else y
建议 4：在代码中适当添加注释
这一点已经受教了，现在编写代码都会合理地加入块注释、行注释和文档注释，可以使用__doc__输出。
建议 5：通过适当添加空行使代码布局更为优雅、合理
建议 6：编写函数的 4 个原则
- 函数设计要尽量短小，嵌套层次不宜过深
- 函数申明应该做到合理、简单、易于使用
- 函数参数设计应该考虑向下兼容
- 一个函数只做一件事，尽量保证函数语句粒度的一致性
Python 中函数设计的好习惯还包括：不要在函数中定义可变对象作为默认值，使用异常替换返回错误，保证通过单元测试等。
<span class="hljs-comment"># 关于函数设计的向下兼容</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">readfile</span><span class="hljs-params">(filename)</span>:</span> <span class="hljs-comment"># 第一版本</span>
<span class="hljs-keyword">pass</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">readfile</span><span class="hljs-params">(filename, log)</span>:</span> <span class="hljs-comment"># 第二版本</span>
<span class="hljs-keyword">pass</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">readfile</span><span class="hljs-params">(filename, logger=logger.info)</span>:</span> <span class="hljs-comment"># 合理的设计</span>
<span class="hljs-keyword">pass</span>
最后还有个函数可读性良好的例子：
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">GetContent</span><span class="hljs-params">(ServerAdr, PagePath)</span>:</span>
http = httplib.HTTP(ServerAdr)
http.putrequest(<span class="hljs-string">'GET'</span>, PagePath)
http.putheader(<span class="hljs-string">'Accept'</span>, <span class="hljs-string">'text/html'</span>)
http.putheader(<span class="hljs-string">'Accept'</span>, <span class="hljs-string">'text/plain'</span>)
http.endheaders()
httpcode, httpmsg, headers = http.getreply()
<span class="hljs-keyword">if</span> httpcode != <span class="hljs-number">200</span>:
<span class="hljs-keyword">raise</span> <span class="hljs-string">"Could not get document: Check URL and Path."</span>
doc = http.getfile()
data = doc.read() <span class="hljs-comment"># 此处是不是应该使用 with ？</span>
doc.close
<span class="hljs-keyword">return</span> data
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">ExtractData</span><span class="hljs-params">(inputstring, start_line, end_line)</span>:</span>
lstr = inputstring.splitlines() <span class="hljs-comment"># split</span>
j = <span class="hljs-number">0</span>
<span class="hljs-keyword">for</span> i <span class="hljs-keyword">in</span> lstr:
j += <span class="hljs-number">1</span>
<span class="hljs-keyword">if</span> i.strip() == start_line: slice_start = j
<span class="hljs-keyword">elif</span> i.strip() == end_line: slice_end = j
<span class="hljs-keyword">return</span> lstr[slice_start:slice_end]
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">SendEmail</span><span class="hljs-params">(sender, receiver, smtpserver, username, password, content)</span>:</span>
subject = <span class="hljs-string">"Contented get from the web"</span>
msg = MIMEText(content, <span class="hljs-string">'plain'</span>, <span class="hljs-string">'utf-8'</span>)
msg[<span class="hljs-string">'Subject'</span>] = Header(subject, <span class="hljs-string">'utf-8'</span>)
smtp = smtplib.SMTP()
smtp.connect(smtpserver)
smtp.login(username, password)
smtp.sendmail(sender, receiver, msg.as_string())
smtp.quit()
建议 7：将常量集中到一个文件
在 Python 中应当如何使用常量：
- 通过命名风格提醒使用者该变量代表常量，如常量名全部大写
- 通过自定义类实现常量功能：将存放常量的文件命名为constant.py，并在其中定义一系列常量
<span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">_const</span>:</span>
<span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">ConstError</span><span class="hljs-params">(TypeError)</span>:</span> <span class="hljs-keyword">pass</span>
<span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">ConstCaseError</span><span class="hljs-params">(ConstError)</span>:</span> <span class="hljs-keyword">pass</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__setattr__</span><span class="hljs-params">(self, name, value)</span>:</span>
<span class="hljs-keyword">if</span> self.__dict__.has_key(name):
<span class="hljs-keyword">raise</span> self.ConstError, <span class="hljs-string">"Can't change const.%s"</span> % name
<span class="hljs-keyword">if</span> <span class="hljs-keyword">not</span> name.isupper():
<span class="hljs-keyword">raise</span> self.ConstCaseError, \
<span class="hljs-string">'const name "%s" is not all uppercase'</span> % name
self.__dict__(name) = value
<span class="hljs-keyword">import</span> sys
sys.modules[__name__] = _const()
<span class="hljs-keyword">import</span> const
const.MY_CONSTANT = <span class="hljs-number">1</span>
const.MY_SECOND_CONSTANT = <span class="hljs-number">2</span>
const.MY_THIRD_CONSTANT = <span class="hljs-string">'a'</span>
const.MY_FORTH_CONSTANT = <span class="hljs-string">'b'</span>
其他模块中引用这些常量时，按照如下方式进行即可：
<span class="hljs-keyword">from</span> constant <span class="hljs-keyword">import</span> <span class="hljs-keyword">const</span> print(<span class="hljs-keyword">const</span>.MY_CONSTANT)
第 2 章 编程惯用法
建议 8：利用 assert 语句来发现问题
<span class="hljs-meta">>></span>> y = <span class="hljs-number">2</span> <span class="hljs-meta">>></span>> assert x == y, <span class="hljs-string">"not equals"</span> Traceback (most recent call last): File <span class="hljs-string">"<stdin>"</span>, line <span class="hljs-number">1</span>, <span class="hljs-keyword">in</span> <<span class="hljs-class"><span class="hljs-keyword">module</span>></span> <span class="hljs-symbol">AssertionError:</span> <span class="hljs-keyword">not</span> equals <span class="hljs-meta">>></span>> x = <span class="hljs-number">1</span> <span class="hljs-meta">>></span>> y = <span class="hljs-number">2</span> <span class="hljs-comment"># 以上代码相当于</span> <span class="hljs-meta">>></span>> if __debug_<span class="hljs-number">_</span> and not x == <span class="hljs-symbol">y:</span> ... raise AssertionError(<span class="hljs-string">"not equals"</span>) ... Traceback (most recent call last): File <span class="hljs-string">"<stdin>"</span>, line <span class="hljs-number">2</span>, <span class="hljs-keyword">in</span> <<span class="hljs-class"><span class="hljs-keyword">module</span>></span> <span class="hljs-symbol">AssertionError:</span> <span class="hljs-keyword">not</span> equals
运行是加入-O参数可以禁用断言。
建议 9：数据交换的时候不推荐使用中间变量
<span class="hljs-meta">>></span>> Timer(<span class="hljs-string">'temp = x; x = y; y = temp;'</span>, <span class="hljs-string">'x = 2; y = 3'</span>).timeit() <span class="hljs-number">0</span>.<span class="hljs-number">05</span>9251302998745814 <span class="hljs-meta">>></span>> Timer(<span class="hljs-string">'x, y = y, x'</span>, <span class="hljs-string">'x = 2; y = 3'</span>).timeit() <span class="hljs-number">0</span>.<span class="hljs-number">050073164</span>99904846
对于表达式x, y = y, x，在内存中执行的顺序如下：
- 先计算右边的表达式y, x，因此先在内存中创建元组(y, x)，其标识符和值分别为y, x及其对应的值，其中y和x是在初始化已经存在于内存中的对象
- 计算表达式左边的值并进行赋值，元组被依次分配给左边的标识符，通过解压缩，元组第一标识符y分配给左边第一个元素x，元组第二标识符x分配给左边第一个元素y，从而达到交换的目的
下面是通过字节码的分析：
<span class="hljs-meta">>>> </span><span class="hljs-keyword">import</span> dis
<span class="hljs-meta">>>> </span><span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">swap1</span><span class="hljs-params">()</span>:</span>
<span class="hljs-meta">... </span> x = <span class="hljs-number">2</span>
<span class="hljs-meta">... </span> y = <span class="hljs-number">3</span>
<span class="hljs-meta">... </span> x, y = y, x
<span class="hljs-meta">... </span>
<span class="hljs-meta">>>> </span><span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">swap2</span><span class="hljs-params">()</span>:</span>
<span class="hljs-meta">... </span> x = <span class="hljs-number">2</span>
<span class="hljs-meta">... </span> y = <span class="hljs-number">3</span>
<span class="hljs-meta">... </span> temp = x
<span class="hljs-meta">... </span> x = y
<span class="hljs-meta">... </span> y = temp
<span class="hljs-meta">... </span>
<span class="hljs-meta">>>> </span>dis.dis(swap1)
<span class="hljs-number">2</span> <span class="hljs-number">0</span> LOAD_CONST <span class="hljs-number">1</span> (<span class="hljs-number">2</span>)
<span class="hljs-number">3</span> STORE_FAST <span class="hljs-number">0</span> (x)
<span class="hljs-number">3</span> <span class="hljs-number">6</span> LOAD_CONST <span class="hljs-number">2</span> (<span class="hljs-number">3</span>)
<span class="hljs-number">9</span> STORE_FAST <span class="hljs-number">1</span> (y)
<span class="hljs-number">4</span> <span class="hljs-number">12</span> LOAD_FAST <span class="hljs-number">1</span> (y)
<span class="hljs-number">15</span> LOAD_FAST <span class="hljs-number">0</span> (x)
<span class="hljs-number">18</span> ROT_TWO <span class="hljs-comment"># 交换两个栈的最顶层元素</span>
<span class="hljs-number">19</span> STORE_FAST <span class="hljs-number">0</span> (x)
<span class="hljs-number">22</span> STORE_FAST <span class="hljs-number">1</span> (y)
<span class="hljs-number">25</span> LOAD_CONST <span class="hljs-number">0</span> (<span class="hljs-keyword">None</span>)
<span class="hljs-number">28</span> RETURN_VALUE
<span class="hljs-meta">>>> </span>dis.dis(swap2)
<span class="hljs-number">2</span> <span class="hljs-number">0</span> LOAD_CONST <span class="hljs-number">1</span> (<span class="hljs-number">2</span>)
<span class="hljs-number">3</span> STORE_FAST <span class="hljs-number">0</span> (x)
<span class="hljs-number">3</span> <span class="hljs-number">6</span> LOAD_CONST <span class="hljs-number">2</span> (<span class="hljs-number">3</span>)
<span class="hljs-number">9</span> STORE_FAST <span class="hljs-number">1</span> (y)
<span class="hljs-number">4</span> <span class="hljs-number">12</span> LOAD_FAST <span class="hljs-number">0</span> (x)
<span class="hljs-number">15</span> STORE_FAST <span class="hljs-number">2</span> (temp)
<span class="hljs-number">5</span> <span class="hljs-number">18</span> LOAD_FAST <span class="hljs-number">1</span> (y)
<span class="hljs-number">21</span> STORE_FAST <span class="hljs-number">0</span> (x)
<span class="hljs-number">6</span> <span class="hljs-number">24</span> LOAD_FAST <span class="hljs-number">2</span> (temp)
<span class="hljs-number">27</span> STORE_FAST <span class="hljs-number">1</span> (y)
<span class="hljs-number">30</span> LOAD_CONST <span class="hljs-number">0</span> (<span class="hljs-keyword">None</span>)
<span class="hljs-number">33</span> RETURN_VALUE
建议 10：充分利用 Lazy evaluation 的特性
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">fib</span><span class="hljs-params">()</span>:</span>
a, b = <span class="hljs-number">0</span>, <span class="hljs-number">1</span>
<span class="hljs-keyword">while</span> <span class="hljs-keyword">True</span>:
<span class="hljs-keyword">yield</span> a
a, b = b, a + b
哈哈哈，我猜到肯定是生成器实现菲波拉契序列的例子，不过对比我写的版本，唉。。。
建议 11：理解枚举替代实现的缺陷
利用 Python 的动态特征，可以实现枚举：
<span class="hljs-comment"># 方式一</span>
<span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">Seasons</span>:</span>
Spring, Summer, Autumn, Winter = range(<span class="hljs-number">4</span>)
<span class="hljs-comment"># 方式二</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">enum</span><span class="hljs-params">(*posarg, **keysarg)</span>:</span>
<span class="hljs-keyword">return</span> type(<span class="hljs-string">"Enum"</span>, (object,), dict(zip(posarg, range(len(posarg))), **keysarg))
Seasons = enum(<span class="hljs-string">"Spring"</span>, <span class="hljs-string">"Summer"</span>, <span class="hljs-string">"Autumn"</span>, Winter=<span class="hljs-number">1</span>)
Seasons.Spring
<span class="hljs-comment"># 方式三</span>
<span class="hljs-meta">>>> </span><span class="hljs-keyword">from</span> collections <span class="hljs-keyword">import</span> namedtuple
<span class="hljs-meta">>>> </span>Seasons = namedtuple(<span class="hljs-string">'Seasons'</span>, <span class="hljs-string">'Spring Summer Autumn Winter'</span>)._make(range(<span class="hljs-number">4</span>))
<span class="hljs-meta">>>> </span>Seasons.Spring
<span class="hljs-number">0</span>
<span class="hljs-comment"># 但通过以上方式实现枚举都有不合理的地方</span>
<span class="hljs-meta">>>> </span>Seasons._replace(Spring=<span class="hljs-number">2</span>) │
Seasons(Spring=<span class="hljs-number">2</span>, Summer=<span class="hljs-number">1</span>, Autumn=<span class="hljs-number">2</span>, Winter=<span class="hljs-number">3</span>)
<span class="hljs-comment"># Python3.4 中加入了枚举，仅在父类没有任何枚举成员的时候才允许继承</span>
建议 12：不推荐使用 type 来进行类型检查
作为动态语言，Python 解释器会在运行时自动进行类型检查并根据需要进行隐式类型转换，当变量类型不同而两者之间又不能进行隐式类型转换时便抛出TypeError异常。
<span class="hljs-meta">>>> </span><span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">add</span><span class="hljs-params">(a, b)</span>:</span> <span class="hljs-meta">... </span> <span class="hljs-keyword">return</span> a + b <span class="hljs-meta">... </span> <span class="hljs-meta">>>> </span>add(<span class="hljs-number">1</span>, <span class="hljs-number">2j</span>) (<span class="hljs-number">1</span>+<span class="hljs-number">2j</span>) <span class="hljs-meta">>>> </span>add(<span class="hljs-string">'a'</span>, <span class="hljs-string">'b'</span>) <span class="hljs-string">'ab'</span> <span class="hljs-meta">>>> </span>add(<span class="hljs-number">1</span>, <span class="hljs-number">2</span>) <span class="hljs-number">3</span> <span class="hljs-meta">>>> </span>add(<span class="hljs-number">1.0</span>, <span class="hljs-number">2.3</span>) <span class="hljs-number">3.3</span> <span class="hljs-meta">>>> </span>add([<span class="hljs-number">1</span>, <span class="hljs-number">2</span>], [<span class="hljs-number">3</span>, <span class="hljs-number">4</span>]) [<span class="hljs-number">1</span>, <span class="hljs-number">2</span>, <span class="hljs-number">3</span>, <span class="hljs-number">4</span>] <span class="hljs-meta">>>> </span>add(<span class="hljs-number">1</span>, <span class="hljs-string">'a'</span>) Traceback (most recent call last): File <span class="hljs-string">"<stdin>"</span>, line <span class="hljs-number">1</span>, <span class="hljs-keyword">in</span> <module> File <span class="hljs-string">"<stdin>"</span>, line <span class="hljs-number">2</span>, <span class="hljs-keyword">in</span> add TypeError: unsupported operand type(s) <span class="hljs-keyword">for</span> +: <span class="hljs-string">'int'</span> <span class="hljs-keyword">and</span> <span class="hljs-string">'str'</span>
所以实际应用中，我们常常需要进行类型检查，但是不推荐使用type()，因为基于内建类型扩展的用户自定义类型，type()并不能准确返回结果：
<span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">UserInt</span><span class="hljs-params">(int)</span>:</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__init__</span><span class="hljs-params">(self, val=<span class="hljs-number">0</span>)</span>:</span>
self._val = int(val)
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__add__</span><span class="hljs-params">(self, val)</span>:</span>
<span class="hljs-keyword">if</span> isinstance(val, UserInt):
<span class="hljs-keyword">return</span> UserInt(self._val + val._val)
<span class="hljs-keyword">return</span> self._val + val
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__iadd__</span><span class="hljs-params">(self, val)</span>:</span>
<span class="hljs-keyword">raise</span> NotImplementedError(<span class="hljs-string">"not support operation"</span>)
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__str__</span><span class="hljs-params">(self)</span>:</span>
<span class="hljs-keyword">return</span> str(self._val)
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__repr__</span><span class="hljs-params">(self)</span>:</span>
<span class="hljs-keyword">return</span> <span class="hljs-string">"Integer %s"</span> % self._val
<span class="hljs-meta">>>> </span>n = UserInt()
<span class="hljs-meta">>>> </span>n
Integer <span class="hljs-number">0</span>
<span class="hljs-meta">>>> </span>print(n)
<span class="hljs-number">0</span>
<span class="hljs-meta">>>> </span>m = UserInt(<span class="hljs-number">2</span>)
<span class="hljs-meta">>>> </span>print(m)
<span class="hljs-number">2</span>
<span class="hljs-meta">>>> </span>type(n) <span class="hljs-keyword">is</span> int
<span class="hljs-keyword">False</span> <span class="hljs-comment"># 显然不合理</span>
<span class="hljs-meta">>>> </span>isinstance(n, int)
<span class="hljs-keyword">True</span>
我们可以使用isinstance来检查：isinstance(object, classinfo)
建议 13：尽量转换为浮点类型后再做除法
<span class="hljs-comment"># 计算平均成绩绩点</span> <span class="hljs-meta">>></span>> gpa = ((<span class="hljs-number">4</span>*<span class="hljs-number">96</span>+<span class="hljs-number">3</span>*<span class="hljs-number">85</span>+<span class="hljs-number">5</span>*<span class="hljs-number">98</span>+<span class="hljs-number">2</span>*<span class="hljs-number">70</span>)*<span class="hljs-number">4</span>) / ((<span class="hljs-number">4</span>+<span class="hljs-number">3</span>+<span class="hljs-number">5</span>+<span class="hljs-number">2</span>)*<span class="hljs-number">100</span>) <span class="hljs-meta">>></span>> gpa <span class="hljs-number">3.625714285714286</span> <span class="hljs-comment"># 终于知道自己的绩点是咋算的了</span>
建议 14：警惕 eval() 的安全漏洞
eval(expression[, globals[, locals]])将字符串 str 当成有效的表达式来求值并返回计算结果，globas为字典形式，locals为任何映射对象，它们分别表示全局和局部命名空间，两者都省略表达式将在调用的环境中执行，为什么需要警惕eval()呢：
<span class="hljs-comment"># 合理正确地使用</span>
<span class="hljs-meta">>></span>> eval(<span class="hljs-string">"1+1==2"</span>)
True
<span class="hljs-meta">>></span>> eval(<span class="hljs-string">'"a"+"b"'</span>)
<span class="hljs-string">'ab'</span>
<span class="hljs-comment"># 坏心眼的geek</span>
<span class="hljs-meta">>></span>> eval(<span class="hljs-string">'__import__("os").system("dir")'</span>)
Desktop Documents Downloads examples.desktop Music Pictures Public __pycache_<span class="hljs-number">_</span> Templates Videos
<span class="hljs-number">0</span>
<span class="hljs-meta">>></span>> eval(<span class="hljs-string">'__import__("os").system("del * /Q")'</span>) <span class="hljs-comment"># 嘿嘿嘿</span>
如果确实需要使用eval，建议使用安全性更好的ast.literal_eval。
建议 15：使用 enumerate() 获取序列迭代的索引和值
<span class="hljs-meta">>>> </span>li = [<span class="hljs-string">'a'</span>, <span class="hljs-string">'b'</span>, <span class="hljs-string">'c'</span>, <span class="hljs-string">'d'</span>, <span class="hljs-string">'e'</span>]
<span class="hljs-meta">>>> </span><span class="hljs-keyword">for</span> i, e <span class="hljs-keyword">in</span> enumerate(li):
<span class="hljs-meta">... </span> print(<span class="hljs-string">'index: '</span>, i, <span class="hljs-string">'element: '</span>, e)
<span class="hljs-meta">... </span>
index: <span class="hljs-number">0</span> element: a
index: <span class="hljs-number">1</span> element: b
index: <span class="hljs-number">2</span> element: c
index: <span class="hljs-number">3</span> element: d
index: <span class="hljs-number">4</span> element: e
<span class="hljs-comment"># enumerate(squence, start=0) 内部实现</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">enumerate</span><span class="hljs-params">(squence, start=<span class="hljs-number">0</span>)</span>:</span>
n = start
<span class="hljs-keyword">for</span> elem <span class="hljs-keyword">in</span> sequence:
<span class="hljs-keyword">yield</span> n, elem <span class="hljs-comment"># 666</span>
n += <span class="hljs-number">1</span>
<span class="hljs-comment"># 明白了原理我们自己也来实现一个反序的</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">reversed_enumerate</span><span class="hljs-params">(squence)</span>:</span>
n = <span class="hljs-number">-1</span>
<span class="hljs-keyword">for</span> elem <span class="hljs-keyword">in</span> reversed(sequence):
<span class="hljs-keyword">yield</span> len(sequence) + n, elem
n -= <span class="hljs-number">1</span>
建议 16：分清 == 与 is 的适用场景
操作符意义isobject identity==equal
is的作用是用来检查对象的标示符是否一致，也就是比较两个对象在内存中是否拥有同一块内存空间，相当于id(x) == id(y)，它并不适用于判断两个字符串是否相等。==才是用来判断两个对象的值是否相等，实际是调用了内部的__eq__，所以a==b相当于a.__eq__(b)，也就是说==是可以被重载的，而is不能被重载。
<span class="hljs-meta">>></span>> s1 = <span class="hljs-string">'hello world'</span> <span class="hljs-meta">>></span>> s2 = <span class="hljs-string">'hello world'</span> <span class="hljs-meta">>></span>> s1 == s2 True <span class="hljs-meta">>></span>> s1 is s2 False <span class="hljs-meta">>></span>> s1.__eq_<span class="hljs-number">_</span>(s2) True <span class="hljs-meta">>></span>> a = <span class="hljs-string">'Hi'</span> <span class="hljs-meta">>></span>> b = <span class="hljs-string">'Hi'</span> <span class="hljs-meta">>></span>> a == b True <span class="hljs-meta">>></span>> a is b True
咦~怎么上例中的a, b又是“同一对象”了？这跟 Python 的 string interning 机制有关，为了提高系统性能，对于较小的字符串会保留其值的一个副本，当创建新的字符串时直接指向该副本，所以a和b的 id 值是一样的，同样对于小整数[-5, 257)也是如此：
<span class="hljs-meta">>></span>> id(a) <span class="hljs-number">140709793837832</span> <span class="hljs-meta">>></span>> id(b) <span class="hljs-number">140709793837832</span> <span class="hljs-meta">>></span>> x = -<span class="hljs-number">5</span> <span class="hljs-meta">>></span>> y = -<span class="hljs-number">5</span> <span class="hljs-meta">>></span>> x is y True <span class="hljs-meta">>></span>> id(x) == id(y) True
建议 17：考虑兼容性，尽可能使用 Unicode
我之前也总结过编码的问题。由于最早的编码是 ASCII 码，只能表示 128 个字符，显然这对其它语言编码并不适用，Unicode就是为了不同的文字分配一套统一的编码。
建议 18：构建合理的包层次来管理 module
本质上每一个 Python 文件都是一个模块，使用模块可以增强代码的可维护性和可重用性，在较大的项目中，我们需要合理地组织项目层次来管理模块，这就是包(Package)的作用。
一句话说包：一个包含__init__.py 文件的目录。包中的模块可以通过.进行访问，即包名.模块名。那么这个__init__.py文件有什么用呢？最明显的作用就是它区分了包和普通目录，在该文件中申明模块级别的 import 语句从而变成了包级别可见，另外在该文件中定义__all__变量，可以控制需要导入的子包或模块。
这里给出一个较为合理的包组织方式，是FlaskWeb 开发：基于Python的Web应用开发实战一书中推荐而来的：
<span class="hljs-params">|-flasky
|</span>-app/ <span class="hljs-comment"># Flask 程序</span>
<span class="hljs-params">|-templates/ # 存放模板
|</span>-static/ <span class="hljs-comment"># 静态文件资源</span>
<span class="hljs-params">|-main/
|</span>-__init_<span class="hljs-number">_</span>.py
<span class="hljs-params">|-errors.py # 蓝本中的错误处理程序
|</span>-forms.py <span class="hljs-comment"># 表单对象</span>
<span class="hljs-params">|-views.py # 蓝本中定义的程序路由
|</span>-__init_<span class="hljs-number">_</span>.py
<span class="hljs-params">|-email.py # 电子邮件支持
|</span>-models.py <span class="hljs-comment"># 数据库模型</span>
<span class="hljs-params">|-migrations/ # 数据库迁移脚本
|</span>-tests/ <span class="hljs-comment"># 单元测试</span>
<span class="hljs-params">|-__init__.py
|</span>-test*.py
<span class="hljs-params">|-venv/ # 虚拟环境
|</span>-requirements/
<span class="hljs-params">|-dev.txt # 开发过程中的依赖包
|</span>-prod.txt <span class="hljs-comment"># 生产过程中的依赖包</span>
<span class="hljs-params">|-config.py # 储存程序配置
|</span>-manage.py <span class="hljs-comment"># 启动程序以及其他的程序任务</span>
第 3 章：基础语法
建议 19：有节制地使用 from…import 语句
Python 提供三种方式来引入外部模块：import语句、from…import语句以及__import__函数，其中__import__函数显式地将模块的名称作为字符串传递并赋值给命名空间的变量。
使用import需要注意以下几点：
- 优先使用import a的形式
- 有节制地使用from a import A
- 尽量避免使用from a import *
为什么呢？我们来看看 Python 的 import 机制，Python 在初始化运行环境的时候会预先加载一批内建模块到内存中，同时将相关信息存放在sys.modules中，我们可以通过sys.modules.items()查看预加载的模块信息，当加载一个模块时，解释器实际上完成了如下动作：
- 在sys.modules中搜索该模块是否存在，如果存在就导入到当前局部命名空间，如果不存在就为其创建一个字典对象，插入到sys.modules中
- 加载前确认是否需要对模块对应的文件进行编译，如果需要则先进行编译
- 执行动态加载，在当前命名空间中执行编译后的字节码，并将其中所有的对象放入模块对应的字典中
<span class="hljs-meta">>></span>> dir() [<span class="hljs-string">'__builtins__'</span>, <span class="hljs-string">'__doc__'</span>, <span class="hljs-string">'__loader__'</span>, <span class="hljs-string">'__name__'</span>, <span class="hljs-string">'__package__'</span>, <span class="hljs-string">'__spec__'</span>] <span class="hljs-meta">>></span>> import test testing <span class="hljs-class"><span class="hljs-keyword">module</span> <span class="hljs-title">import</span></span> <span class="hljs-meta">>></span>> dir() [<span class="hljs-string">'__builtins__'</span>, <span class="hljs-string">'__doc__'</span>, <span class="hljs-string">'__loader__'</span>, <span class="hljs-string">'__name__'</span>, <span class="hljs-string">'__package__'</span>, <span class="hljs-string">'__spec__'</span>, <span class="hljs-string">'test'</span>] <span class="hljs-meta">>></span>> import sys <span class="hljs-meta">>></span>> ‘test’ in sys.modules.keys() True <span class="hljs-meta">>></span>> id(test) <span class="hljs-number">140367239464744</span> <span class="hljs-meta">>></span>> id(sys.modules[<span class="hljs-string">'test'</span>]) <span class="hljs-number">140367239464744</span> <span class="hljs-meta">>></span>> dir(test) [<span class="hljs-string">'__builtins__'</span>, <span class="hljs-string">'__cached__'</span>, <span class="hljs-string">'__doc__'</span>, <span class="hljs-string">'__file__'</span>, <span class="hljs-string">'__loader__'</span>, <span class="hljs-string">'__name__'</span>, <span class="hljs-string">'__package__'</span>, <span class="hljs-string">'__spec__'</span>, <span class="hljs-string">'a'</span>, <span class="hljs-string">'b'</span>] <span class="hljs-meta">>></span>> sys.modules[<span class="hljs-string">'test'</span>].__dict_<span class="hljs-number">_</span>.keys() dict_keys([<span class="hljs-string">'__file__'</span>, <span class="hljs-string">'__builtins__'</span>, <span class="hljs-string">'__doc__'</span>, <span class="hljs-string">'__loader__'</span>, <span class="hljs-string">'__package__'</span>, <span class="hljs-string">'__spec__'</span>, <span class="hljs-string">'__name__'</span>, <span class="hljs-string">'b'</span>, <span class="hljs-string">'a'</span>, <span class="hljs-string">'__cached__'</span>])
从上可以看出，对于用户自定义的模块，import 机制会创建一个新的 module 将其加入当前的局部命名空间中，同时在 sys.modules 也加入该模块的信息，但本质上是在引用同一个对象，通过test.py所在的目录会多一个字节码文件。
建议 20：优先使用 absolute import 来导入模块
建议 21： i+=1 不等于 ++i
首先++i或–i在 Python 语法上是合法，但并不是我们通常理解的自增或自减操作：
<span class="hljs-meta">>></span>> ++<span class="hljs-number">1</span> <span class="hljs-comment"># +(+1)</span> <span class="hljs-number">1</span> <span class="hljs-meta">>></span>> --<span class="hljs-number">1</span> <span class="hljs-comment"># -(-1)</span> <span class="hljs-number">1</span> <span class="hljs-meta">>></span>> +++<span class="hljs-number">2</span> <span class="hljs-number">2</span> <span class="hljs-meta">>></span>> ---<span class="hljs-number">2</span> -<span class="hljs-number">2</span>
原来+或-只表示正负数符号。
建议 22：使用 with 自动关闭资源
对于打开的资源我们记得关闭它，如文件、数据库连接等，Python 提供了一种简单优雅的解决方案：with。
先来看with实现的原理吧。
with的实现得益于一个称为上下文管理器(context manager)的东西，它定义程序运行时需要建立的上下文，处理程序的进入和退出，实现了上下文管理协议，即对象中定义了__enter__()和__exit__()，任何实现了上下文协议的对象都可以称为一个上下文管理器：
- __enter__()：返回运行时上下文相关的对象
- __exit__(exception_type, exception_value, traceback)：退出运行时的上下文，处理异常、清理现场等
包含with语句的代码块执行过程如下：
<span class="hljs-keyword">with</span> 表达式 [<span class="hljs-keyword">as</span> 目标]:
代码块
<span class="hljs-comment"># 例</span>
<span class="hljs-meta">>>> </span><span class="hljs-keyword">with</span> open(<span class="hljs-string">'test.txt'</span>, <span class="hljs-string">'w'</span>) <span class="hljs-keyword">as</span> f:
<span class="hljs-meta">... </span> f.write(<span class="hljs-string">'test'</span>)
<span class="hljs-meta">... </span>
<span class="hljs-number">4</span>
<span class="hljs-meta">>>> </span>f.__enter__
<built-<span class="hljs-keyword">in</span> method __enter__ of _io.TextIOWrapper object at <span class="hljs-number">0x7f1b967aaa68</span>>
<span class="hljs-meta">>>> </span>f.__exit__
<built-<span class="hljs-keyword">in</span> method __exit__ of _io.TextIOWrapper object at <span class="hljs-number">0x7f1b967aaa68</span>>
- 计算表达式的值，返回一个上下文管理器对象
- 加载上下文管理器对象的__exit__()以备后用
- 调用上下文管理器对象的__enter__()
- 将__enter__()的返回值赋给目标对象
- 执行代码块，正常结束调用__exit__()，其返回值直接忽略，如果发生异常，会调用__exit__()并将异常类型、值及 traceback 作为参数传递给__exit__()，__exit__()返回值为 false 异常将会重新抛出，返回值为 true 异常将被挂起，程序继续执行
于此，我们可以自定义一个上下文管理器：
<span class="hljs-meta">>>> </span><span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">MyContextManager</span><span class="hljs-params">(object)</span>:</span> <span class="hljs-meta">... </span> <span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__enter__</span><span class="hljs-params">(self)</span>:</span> <span class="hljs-meta">... </span> print(<span class="hljs-string">'entering...'</span>) <span class="hljs-meta">... </span> <span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__exit__</span><span class="hljs-params">(self, exception_type, exception_value, traceback)</span>:</span> <span class="hljs-meta">... </span> print(<span class="hljs-string">'leaving...'</span>) <span class="hljs-meta">... </span> <span class="hljs-keyword">if</span> exception_type <span class="hljs-keyword">is</span> <span class="hljs-keyword">None</span>: <span class="hljs-meta">... </span> print(<span class="hljs-string">'no exceptions!'</span>) <span class="hljs-meta">... </span> <span class="hljs-keyword">return</span> <span class="hljs-keyword">False</span> <span class="hljs-meta">... </span> <span class="hljs-keyword">elif</span> exception_type <span class="hljs-keyword">is</span> ValueError: <span class="hljs-meta">... </span> print(<span class="hljs-string">'value error!'</span>) <span class="hljs-meta">... </span> <span class="hljs-keyword">return</span> <span class="hljs-keyword">True</span> <span class="hljs-meta">... </span> <span class="hljs-keyword">else</span>: <span class="hljs-meta">... </span> print(<span class="hljs-string">'other error'</span>) <span class="hljs-meta">... </span> <span class="hljs-keyword">return</span> <span class="hljs-keyword">True</span> <span class="hljs-meta">... </span> <span class="hljs-meta">>>> </span><span class="hljs-keyword">with</span> MyContextManager(): <span class="hljs-meta">... </span> print(<span class="hljs-string">'Testing...'</span>) <span class="hljs-meta">... </span> entering... Testing... leaving... no exceptions! <span class="hljs-meta">>>> </span><span class="hljs-keyword">with</span> MyContextManager(): <span class="hljs-meta">... </span> print(<span class="hljs-string">'Testing...'</span>) <span class="hljs-meta">... </span> <span class="hljs-keyword">raise</span>(ValueError) <span class="hljs-meta">... </span> entering... Testing... leaving... value error!
Python 还提供contextlib模块，通过 Generator 实现，其中的 contextmanager 作为装饰器来提供一种针对函数级别上的上下文管理器，可以直接作用于函数/对象而不必关心__enter__()和__exit__()的实现。
推荐文章
建议 23：使用 else 子句简化循环（异常处理）
Python 的 else 子句提供了隐含的对循环是否由 break 语句引发循环结束的判断，有点绕哈，来看例子：
<span class="hljs-meta">>>> </span><span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">print_prime</span><span class="hljs-params">(n)</span>:</span>
<span class="hljs-meta">... </span> <span class="hljs-keyword">for</span> i <span class="hljs-keyword">in</span> range(<span class="hljs-number">2</span>, n):
<span class="hljs-meta">... </span> <span class="hljs-keyword">for</span> j <span class="hljs-keyword">in</span> range(<span class="hljs-number">2</span>, i):
<span class="hljs-meta">... </span> <span class="hljs-keyword">if</span> i % j == <span class="hljs-number">0</span>:
<span class="hljs-meta">... </span> <span class="hljs-keyword">break</span>
<span class="hljs-meta">... </span> <span class="hljs-keyword">else</span>:
<span class="hljs-meta">... </span> print(<span class="hljs-string">'{} is a prime number'</span>.format(i))
<span class="hljs-meta">... </span>
<span class="hljs-meta">>>> </span>print_prime(<span class="hljs-number">7</span>)
<span class="hljs-number">2</span> <span class="hljs-keyword">is</span> a prime number
<span class="hljs-number">3</span> <span class="hljs-keyword">is</span> a prime number
<span class="hljs-number">5</span> <span class="hljs-keyword">is</span> a prime number
可以看出，else 子句在循环正常结束和循环条件不成立时被执行，由 break 语句中断时不执行，同样，我们可以利用这颗语法糖作用在 while 和 try…except 中。
转载自http://www.codeceo.com/article/91-suggestions-about-python-part-one.html
