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