Completely master the double-down method in Python-Python Tutorial-php.cn

<p>This article brings you relevant knowledge about <a href="//m.sbmmt.com/course/list/31.html" target="_blank">Python</a>. There are some special methods in Python whose method names begin and end with double underscores, so they are also called double underscore methods. Let’s take a look at it together, I hope it will be helpful to everyone. </p> <p><img src="https://img.php.cn/upload/article/000/000/067/62d4f1fd38dfa174.jpg" alt="Completely master the double-down method in Python" ></p> <p>[Related recommendations: <a href="//m.sbmmt.com/course/list/31.html" target="_blank">Python3 video tutorial</a>]</p> <h2>Preface</h2> <p>Everyone has something to remember when writing Python code There is no such doubt. </p> <p>Why does the <code> </code> number in mathematics become a splicing function in string operations, such as <code>'ab' 'cd'</code>The result is <code>abcd</code>; and the number <code>*</code> becomes a repeated function, such as <code>'ab' * 2</code>, the result is <code>abab</code>. </p> <p>Why some objects<code>print</code>can output data, but <code>print</code>custom class objects output a bunch of incomprehensible code<code><__main__. MyCls object at 0x105732250></code>. </p> <p>It’s not because the system has been specially customized, but because there is a special type of method in Python that is automatically called on certain occasions. For example, after the <code>__add__</code> method is defined in the string class <code>str</code>, when the code encounters string addition <code>'ab' 'cd'</code>, it will Automatically call the <code>__add__</code> method to complete string splicing. </p> <p>Because the method names of this type of special method start and end with double underscores, they are also called double-underscore methods. </p> <p>There are many double download methods in Python. Today we will explain it in detail. </p> <p style="text-align:center"><img alt="" src="https://img.php.cn/upload/article/000/000/067/dc4eae5d11b7bd06ec193c866274280b-0.jpg"></p> <p style="text-align:center">Double download method in Python</p> <h2>1. The init method</h2> <p><code>__init__</code> is a method that many people come into contact with The first<code>Double down method</code>. </p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:py;">class A: def __init__(self, a): self.a = a</pre><div class="contentsignin">Copy after login</div></div><p>When calling <code>A()</code> to instantiate an object, the <code>__init__</code> method will be automatically called to complete the initialization of the object. </p><h2>2. Double under method of operator</h2><p>Define operator-related <code>double under method</code> in the class, you can directly add, subtract, multiply, divide, and compare on the class object Wait for operations. </p><p>Here, define a ruler class <code>Rule</code>, which contains an attribute <code>r_len</code> representing the length of the ruler. </p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:py;">class Rule: def __init__(self, r_len): self.r_len = r_len</pre><div class="contentsignin">Copy after login</div></div><h3>2.1 Comparison operators</h3><p>If you want to compare different rulers according to their length, you need to define comparison operators in the <code>Rule</code> class. </p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:py;">class Rule: def __init__(self, r_len): self.r_len = r_len # < 运算符 def __lt__(self, other): return self.r_len < other.r_len # <= 运算符 def __le__(self, other): return self.r_len <= other.r_len # > 运算符 def __gt__(self, other): return self.r_len > other.r_len # >= 运算符 def __ge__(self, other): return self.r_len >= other.r_len</pre><div class="contentsignin">Copy after login</div></div><p>Four comparisons <code><</code>, <code><=</code>, <code>></code> and <code>>=</code> are defined here operator, so that the <code>Rule</code> objects can be compared using the following code. </p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:py;">rule1 = Rule(10) rule2 = Rule(5) print(rule1 > rule2) # True print(rule1 >= rule2) # True print(rule1 < rule2) # False print(rule1 <= rule2) # False</pre><div class="contentsignin">Copy after login</div></div><p>When <code>></code> is used to compare <code>rule1</code> and <code>rule2</code>, the <code>rule1</code> object will automatically call <code> __gt__</code> method, and pass the <code>rule2</code> object to the <code>other</code> parameter to complete the comparison. </p><p>The following is the double-under method of the comparison operator</p><p style="text-align:center"><img alt="" src="https://img.php.cn/upload/article/000/000/067/dc4eae5d11b7bd06ec193c866274280b-1.png"/></p><p style="max-width:90%">The double-under method of the comparison operator</p><h3>2.2 Arithmetic operators</h3><p>Can support addition, subtraction, multiplication and division of class objects. </p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:py;">def __add__(self, other): return Rule(self.r_len + other.r_len)</pre><div class="contentsignin">Copy after login</div></div><p>The <code>__add__</code> method is defined here, corresponding to the <code> </code> operator. It will add the lengths of the two rulers and generate a new ruler. </p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:py;">rule1 = Rule(10) rule2 = Rule(5) rule3 = rule1 + rule2</pre><div class="contentsignin">Copy after login</div></div><p>The following is the double-under method of arithmetic operators</p><p style="text-align:center"><img alt="" src="https://img.php.cn/upload/article/000/000/067/dc4eae5d11b7bd06ec193c866274280b-2.png"/></p><h3>2.3 Reverse arithmetic operator</h3><p>It supports other types of variables with <code>Rule</code>Class addition. Taking the <code>__radd__</code> method as an example </p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:py;">def __radd__(self, other): return self.r_len + other</pre><div class="contentsignin">Copy after login</div></div><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:py;">rule1 = Rule(10) rule2 = 10 + rule1</pre><div class="contentsignin">Copy after login</div></div><p>When the program executes <code>10 rule1</code>, it will try to call the <code>__add__</code> of the <code>int</code> class but The <code>int</code> class class does not define a method for adding the <code>Rule</code> class object, so the program will call the <code> of the object </code>rule1<code> on the right of </code> <code> __radd__</code> method, and pass <code>10</code> to the <code>other</code> parameter. </p><p>So this operator is also called the right addition operator. The operators it supports are the same as the arithmetic operators above. Just add <code>r</code> before the method name. </p><h3>2.4 Incremental assignment operator</h3><p>The incremental assignment operators are <code> =</code>, <code>-=</code>, <code>*=</code>, <code>/=</code>etc. </p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:py;">def __iadd__(self, other): self.r_len += other return self</pre><div class="contentsignin">Copy after login</div></div><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:py;">rule1 = Rule(10) rule1 += 5</pre><div class="contentsignin">Copy after login</div></div><p>Except for the <code>__pmod__</code> method, everything else is the same as the arithmetic operators, with i added before the aspect name. </p><h3>2.4 Bit Operators</h3><p>This part supports binary operations such as negation, shift, AND or NOT. Since the <code>Rule</code> class does not involve bit operations, let's change to another example. </p><p>The class that defines binary string <code>BinStr</code>, contains the <code>bin_str</code> attribute, which represents a binary string. </p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:py;">class BinStr: def __init__(self, bin_str): self.bin_str = bin_str</pre><div class="contentsignin">Copy after login</div></div><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:py;">x = BinStr('1010') #创建二进制字符串对象 print(x.bin_str) # 1010</pre><div class="contentsignin">Copy after login</div></div><p>Define a negation operator for <code>BinStr</code><code>~</code></p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:py;"># ~ 运算符 def __invert__(self): inverted_bin_str = ''.join(['1' if i == '0' else '0' for i in self.bin_str]) return BinStr(inverted_bin_str)</pre><div class="contentsignin">Copy after login</div></div><p><code>__invert__</code>方法中，遍历<code>bin_str</code>字符串，将每位取反，并返回一个新的<code>BinStr</code>类对象。</p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:py;">x = BinStr('1011') invert_x = ~x print(invert_x.bin_str) # 0100</pre><div class="contentsignin">Copy after login</div></div><p>下面是位运算符的双下方法</p><p style="text-align:center"><img alt="" src="https://img.php.cn/upload/article/000/000/067/4894e53be3e9850194264d258e8620e0-3.png"/></p><p>这部分也支持反向位运算符和增量赋值位运算符，规则跟算数运算符一样，这里就不再赘述。</p><h2>3.字符串表示</h2><p>这部分涉及两个双下方法<code>__repr__</code>和<code>__format__</code>，在某些特殊场景，如<code>print</code>，会自动调用，将对象转成字符串。</p><p>还是以<code>BinStr</code>为例，先写<code>__repr__</code>方法。</p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:py;">def __repr__(self): decimal = int('0b'+self.bin_str, 2) return f'二进制字符串：{self.bin_str}，对应的十进制数字：{decimal}'</pre><div class="contentsignin">Copy after login</div></div><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:py;">x = BinStr('1011') print(x) # 输出：二进制字符串：1011，对应的十进制数字：11</pre><div class="contentsignin">Copy after login</div></div><p>当程序执行<code>print(x)</code>时，会自动调用<code>__repr__</code>方法，获取对象<code>x</code>对应的字符串。</p><p>再写<code>__format__</code>方法，它也是将对象格式化为字符串。</p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:py;">def __format__(self, format_spec): return format_spec % self.bin_str</pre><div class="contentsignin">Copy after login</div></div><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:py;">print('{0:二进制字符串：%s}'.format(x)) # 输出：二进制字符串：1011</pre><div class="contentsignin">Copy after login</div></div><p>当<code>.format</code>方法的前面字符串里包含<code>0:</code>时，就会自动调用<code>__format__</code>方法，并将字符串传给<code>format_spec</code>参数。</p><h2>4.数值转换</h2><p>调用<code>int(obj)</code>、<code>float(obj)</code>等方法，可以将对象转成相对应数据类型的数据。</p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:py;">def __int__(self): return int('0b'+self.bin_str, 2)</pre><div class="contentsignin">Copy after login</div></div><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:py;">x = BinStr('1011') print(int(x))</pre><div class="contentsignin">Copy after login</div></div><p>当调用<code>int(x)</code>时，会自动调用<code>__int__</code>方法，将二进制字符串转成十进制数字。</p><p>数值转换除了上面的两个外，还有<code>__abs__</code>、<code>__bool__</code>、<code>__complex__</code>、<code>__hash__</code>、<code>__index__</code>和<code>__str__</code>。</p><p><code>__str__</code>和<code>__repr__</code>一样，在<code>print</code>时都会被自动调用，但<code>__str__</code>优先级更高。</p><h2>5.集合相关的双下方法</h2><p>这部分可以像集合那样，定义对象长度、获取某个位置元素、切片等方法。</p><p>以<code>__len__</code>和<code>__getitem__</code>为例</p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:py;">def __len__(self): return len(self.bin_str) def __getitem__(self, item): return self.bin_str[item]</pre><div class="contentsignin">Copy after login</div></div><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:py;">x = BinStr('1011') print(len(x)) # 4 print(x[0]) # 1 print(x[0:3]) # 101</pre><div class="contentsignin">Copy after login</div></div><p><code>len(x)</code>会自动调用<code>__len__</code>返回对象的长度。</p><p>通过<code>[]</code>方式获取对象的元素时，会自动调用<code>__getitem__</code>方法，并将切片对象传给<code>item</code>参数，即可以获取单个元素，还可以获取切片。</p><p>集合相关的双下方法还包括<code>__setitem__</code>、<code>__delitem__</code>和<code>__contains__</code>。</p><h2>6.迭代相关的双下方法</h2><p>可以在对象上使用<code>for-in</code>遍历。</p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:py;">def __iter__(self): self.cur_i = -1 return self def __next__(self): self.cur_i += 1 if self.cur_i >= len(self.bin_str): raise StopIteration() # 退出迭代 return self.bin_str[self.cur_i]</pre><div class="contentsignin">Copy after login</div></div><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:py;">x = BinStr('1011') for i in x: print(i)</pre><div class="contentsignin">Copy after login</div></div><p>当在<code>x</code>上使用<code>for-in</code>循环时，会先调用<code>__iter__</code>方法将游标<code>cur_i</code>置为初始值<code>-1</code>，然后不断调用<code>__next__</code>方法遍历<code>self.bin_str</code>中的每一位。</p><p>这部分还有一个<code>__reversed__</code>方法用来反转对象。</p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:py;">def __reversed__(self): return BinStr(''.join(list(reversed(self.bin_str))))</pre><div class="contentsignin">Copy after login</div></div><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:py;">x = BinStr('1011') reversed_x = reversed(x) print(reversed_x) # 输出：二进制字符串：1101，对应的十进制数字：13</pre><div class="contentsignin">Copy after login</div></div><h2>7.类相关的双下方法</h2><p>做 web 开发的朋友，用类相关的双下方法会更多一些。</p><h3>7.1 实例的创建和销毁</h3><p>实例的创建是<code>__new__</code>和<code>__init__</code>方法，实例的销毁是<code>__del__</code>方法。</p><p><code>__new__</code>的调用早于<code>__init__</code>，它的作用是创建对象的实例（内存开辟一段空间），而后才将该实例传给<code>__init__</code>方法，完成实例的初始化。</p><p>由于<code>__new__</code>是类静态方法，因此它可以控制对象的创建，从而实现<strong>单例模式</strong>。</p><p><code>__del__</code>方法在实例销毁时，被自动调用，可以用来做一些清理工作和资源释放的工作。</p><h3>7.2 属性管理</h3><p>类属性的访问和设置。包括<code>__getattr__</code>、<code>__getattribute__</code>、<code>__setattr__</code>和<code>__delattr__</code>方法。</p><p><code>__getattr__</code>和<code>__getattribute__</code>的区别是，当访问类属性时，无论属性存不存在都会调用<code>__getattribute__</code>方法，只有当属性不存在时才会调用<code>__getattr__</code>方法。</p><h3>7.3 属性描述符</h3><p>控制属性的访问，一般用于把属性的取值控制在合理范围内。包括<code>__get__</code>、<code>__set__</code>和<code>__delete__</code>方法。</p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:py;">class XValidation: def __get__(self, instance, owner): return self.x def __set__(self, instance, value): if 0 <= value <= 100: self.x = value else: raise Exception('x不能小于0，不能大于100') def __delete__(self, instance): print('删除属性') class MyCls: x = XValidation() def __init__(self, n): self.x = n obj = MyCls(10) obj.x = 101 print(obj.x) # 抛异常：Exception: x不能小于0，不能大于100</pre><div class="contentsignin">Copy after login</div></div><p>上述例子，通过类属性描述符，可以将属性x的取值控制在<code>[0, 100]</code>之前，防止不合法的取值。</p> <h2>8.总结</h2> <p>虽然上面介绍的不是所有的双下方法，但也算是绝大多数了。</p> <p>虽然双下方法里可以编写任意代码，但大家尽量编写与方法要求一样的代码。如，在<code>__add__</code>方法实现的不是对象相加而是相减，虽然也能运行，但这样会造成很大困惑，不利于代码维护。</p> <p>【相关推荐：<a href="//m.sbmmt.com/course/list/31.html" target="_blank">Python3视频教程</a> 】</p>

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