一篇文章让你彻底搞清楚Python中self的含义

骑猪看日落 2022-08-21 01:12 169阅读 0赞

转载地址：http://python.jobbole.com/81921/

刚开始学习Python的类写法的时候觉得很是麻烦，为什么定义时需要而调用时又不需要，为什么不能内部简化从而减少我们敲击键盘的次数？你看完这篇文章后就会明白所有的疑问。

### self代表类的实例，而非类。 ###

实例来说明

Python

class Test: def prt(self): print(self) print(self.\_\_class\_\_) t = Test() t.prt()

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   <td> 
    <div style=""> 
     <div> 
      <span>class</span> 
      <span> </span> 
      <span>Test</span> 
      <span>:</span> 
     </div> 
     <div> 
      <span>&nbsp;&nbsp;&nbsp;&nbsp;</span> 
      <span>def</span> 
      <span> </span> 
      <span>prt</span> 
      <span>(</span> 
      <span>self</span> 
      <span>)</span> 
      <span>:</span> 
     </div> 
     <div> 
      <span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</span> 
      <span>print</span> 
      <span>(</span> 
      <span>self</span> 
      <span>)</span> 
     </div> 
     <div> 
      <span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</span> 
      <span>print</span> 
      <span>(</span> 
      <span>self</span> 
      <span>.</span> 
      <span>__class__</span> 
      <span>)</span> 
     </div> 
     <div>
       &nbsp; 
     </div> 
     <div> 
      <span>t</span> 
      <span> </span> 
      <span>=</span> 
      <span> </span> 
      <span>Test</span> 
      <span>(</span> 
      <span>)</span> 
     </div> 
     <div> 
      <span>t</span> 
      <span>.</span> 
      <span>prt</span> 
      <span>(</span> 
      <span>)</span> 
     </div> 
    </div> </td> 
  </tr> 
 </tbody> 
</table>

执行结果如下

Python

<\_\_main\_\_.Test object at 0x000000000284E080> <class '\_\_main\_\_.Test'>

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    <div style=""> 
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      <span>&lt;</span> 
      <span>__main__</span> 
      <span>.</span> 
      <span>Test</span> 
      <span> </span> 
      <span>object</span> 
      <span> </span> 
      <span>at</span> 
      <span> </span> 
      <span>0x000000000284E080</span> 
      <span>&gt;</span> 
     </div> 
     <div> 
      <span>&lt;</span> 
      <span>class</span> 
      <span> </span> 
      <span>'__main__.Test'</span> 
      <span>&gt;</span> 
     </div> 
    </div> </td> 
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从上面的例子中可以很明显的看出，self代表的是类的实例。而self.class则指向类。

### self不必非写成self ###

有很多童鞋是先学习别的语言然后学习Python的，所以总觉得self怪怪的，想写成this，可以吗？

当然可以，还是把上面的代码改写一下。

Python

class Test: def prt(this): print(this) print(this.\_\_class\_\_) t = Test() t.prt()

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    <div style=""> 
     <div> 
      <span>class</span> 
      <span> </span> 
      <span>Test</span> 
      <span>:</span> 
     </div> 
     <div> 
      <span>&nbsp;&nbsp;&nbsp;&nbsp;</span> 
      <span>def</span> 
      <span> </span> 
      <span>prt</span> 
      <span>(</span> 
      <span>this</span> 
      <span>)</span> 
      <span>:</span> 
     </div> 
     <div> 
      <span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</span> 
      <span>print</span> 
      <span>(</span> 
      <span>this</span> 
      <span>)</span> 
     </div> 
     <div> 
      <span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</span> 
      <span>print</span> 
      <span>(</span> 
      <span>this</span> 
      <span>.</span> 
      <span>__class__</span> 
      <span>)</span> 
     </div> 
     <div>
       &nbsp; 
     </div> 
     <div> 
      <span>t</span> 
      <span> </span> 
      <span>=</span> 
      <span> </span> 
      <span>Test</span> 
      <span>(</span> 
      <span>)</span> 
     </div> 
     <div> 
      <span>t</span> 
      <span>.</span> 
      <span>prt</span> 
      <span>(</span> 
      <span>)</span> 
     </div> 
    </div> </td> 
  </tr> 
 </tbody> 
</table>

改成this后，运行结果完全一样。

当然，最好还是尊重约定俗成的习惯，使用self。

### self可以不写吗 ###

在Python的解释器内部，当我们调用t.prt()时，实际上Python解释成Test.prt(t)，也就是说把self替换成类的实例。

有兴趣的童鞋可以把上面的t.prt()一行改写一下，运行后的实际结果完全相同。

实际上已经部分说明了self在定义时不可以省略，如果非要试一下，那么请看下面：

Python

class Test: def prt(): print(self) t = Test() t.prt()

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    <div style=""> 
     <div> 
      <span>class</span> 
      <span> </span> 
      <span>Test</span> 
      <span>:</span> 
     </div> 
     <div> 
      <span>&nbsp;&nbsp;&nbsp;&nbsp;</span> 
      <span>def</span> 
      <span> </span> 
      <span>prt</span> 
      <span>(</span> 
      <span>)</span> 
      <span>:</span> 
     </div> 
     <div> 
      <span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</span> 
      <span>print</span> 
      <span>(</span> 
      <span>self</span> 
      <span>)</span> 
     </div> 
     <div>
       &nbsp; 
     </div> 
     <div> 
      <span>t</span> 
      <span> </span> 
      <span>=</span> 
      <span> </span> 
      <span>Test</span> 
      <span>(</span> 
      <span>)</span> 
     </div> 
     <div> 
      <span>t</span> 
      <span>.</span> 
      <span>prt</span> 
      <span>(</span> 
      <span>)</span> 
     </div> 
    </div> </td> 
  </tr> 
 </tbody> 
</table>

运行时提醒错误如下：prt在定义时没有参数，但是我们运行时强行传了一个参数。

由于上面解释过了t.prt()等同于Test.prt(t)，所以程序提醒我们多传了一个参数t。

Python

Traceback (most recent call last): File "h.py", line 6, in <module> t.prt() TypeError: prt() takes 0 positional arguments but 1 was given

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      <span>Traceback</span> 
      <span> </span> 
      <span>(</span> 
      <span>most </span> 
      <span>recent </span> 
      <span>call </span> 
      <span>last</span> 
      <span>)</span> 
      <span>:</span> 
     </div> 
     <div> 
      <span>&nbsp;&nbsp;</span> 
      <span>File</span> 
      <span> </span> 
      <span>"h.py"</span> 
      <span>,</span> 
      <span> </span> 
      <span>line</span> 
      <span> </span> 
      <span>6</span> 
      <span>,</span> 
      <span> </span> 
      <span>in</span> 
      <span> </span> 
      <span>&lt;</span> 
      <span>module</span> 
      <span>&gt;</span> 
     </div> 
     <div> 
      <span>&nbsp;&nbsp;&nbsp;&nbsp;</span> 
      <span>t</span> 
      <span>.</span> 
      <span>prt</span> 
      <span>(</span> 
      <span>)</span> 
     </div> 
     <div> 
      <span>TypeError</span> 
      <span>:</span> 
      <span> </span> 
      <span>prt</span> 
      <span>(</span> 
      <span>)</span> 
      <span> </span> 
      <span>takes</span> 
      <span> </span> 
      <span>0</span> 
      <span> </span> 
      <span>positional </span> 
      <span>arguments </span> 
      <span>but</span> 
      <span> </span> 
      <span>1</span> 
      <span> </span> 
      <span>was </span> 
      <span>given</span> 
     </div> 
    </div> </td> 
  </tr> 
 </tbody> 
</table>

当然，如果我们的定义和调用时均不传类实例是可以的，这就是类方法。

Python

class Test: def prt(): print(\_\_class\_\_) Test.prt()

运行结果如下

Python

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      <span>&lt;</span> 
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      <span> </span> 
      <span>'__main__.Test'</span> 
      <span>&gt;</span> 
     </div> 
    </div> </td> 
  </tr> 
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</table>

**在继承时，传入的是哪个实例，就是那个传入的实例，而不是指定义了self的类的实例。**

先看代码

Python

class Parent: def pprt(self): print(self) class Child(Parent): def cprt(self): print(self) c = Child() c.cprt() c.pprt() p = Parent() p.pprt()

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      <span>def</span> 
      <span> </span> 
      <span>pprt</span> 
      <span>(</span> 
      <span>self</span> 
      <span>)</span> 
      <span>:</span> 
     </div> 
     <div> 
      <span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</span> 
      <span>print</span> 
      <span>(</span> 
      <span>self</span> 
      <span>)</span> 
     </div> 
     <div>
       &nbsp; 
     </div> 
     <div> 
      <span>class</span> 
      <span> </span> 
      <span>Child</span> 
      <span>(</span> 
      <span>Parent</span> 
      <span>)</span> 
      <span>:</span> 
     </div> 
     <div> 
      <span>&nbsp;&nbsp;&nbsp;&nbsp;</span> 
      <span>def</span> 
      <span> </span> 
      <span>cprt</span> 
      <span>(</span> 
      <span>self</span> 
      <span>)</span> 
      <span>:</span> 
     </div> 
     <div> 
      <span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</span> 
      <span>print</span> 
      <span>(</span> 
      <span>self</span> 
      <span>)</span> 
     </div> 
     <div> 
      <span>c</span> 
      <span> </span> 
      <span>=</span> 
      <span> </span> 
      <span>Child</span> 
      <span>(</span> 
      <span>)</span> 
     </div> 
     <div> 
      <span>c</span> 
      <span>.</span> 
      <span>cprt</span> 
      <span>(</span> 
      <span>)</span> 
     </div> 
     <div> 
      <span>c</span> 
      <span>.</span> 
      <span>pprt</span> 
      <span>(</span> 
      <span>)</span> 
     </div> 
     <div> 
      <span>p</span> 
      <span> </span> 
      <span>=</span> 
      <span> </span> 
      <span>Parent</span> 
      <span>(</span> 
      <span>)</span> 
     </div> 
     <div> 
      <span>p</span> 
      <span>.</span> 
      <span>pprt</span> 
      <span>(</span> 
      <span>)</span> 
     </div> 
    </div> </td> 
  </tr> 
 </tbody> 
</table>

运行结果如下

Python

<\_\_main\_\_.Child object at 0x0000000002A47080> <\_\_main\_\_.Child object at 0x0000000002A47080> <\_\_main\_\_.Parent object at 0x0000000002A47240>

解释：

运行c.cprt()时应该没有理解问题，指的是Child类的实例。

但是在运行c.pprt()时，等同于Child.pprt(c)，所以self指的依然是Child类的实例，由于self中没有定义pprt()方法，所以沿着继承树往上找，发现在父类Parent中定义了pprt()方法，所以就会成功调用。

### 在描述符类中，self指的是描述符类的实例 ###

不太容易理解，先看实例：

Python

class Desc: def \_\_get\_\_(self, ins, cls): print('self in Desc: %s ' % self ) print(self, ins, cls) class Test: x = Desc() def prt(self): print('self in Test: %s' % self) t = Test() t.prt() t.x

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      <span>class</span> 
      <span> </span> 
      <span>Desc</span> 
      <span>:</span> 
     </div> 
     <div> 
      <span>&nbsp;&nbsp;&nbsp;&nbsp;</span> 
      <span>def</span> 
      <span> </span> 
      <span>__get__</span> 
      <span>(</span> 
      <span>self</span> 
      <span>,</span> 
      <span> </span> 
      <span>ins</span> 
      <span>,</span> 
      <span> </span> 
      <span>cls</span> 
      <span>)</span> 
      <span>:</span> 
     </div> 
     <div> 
      <span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</span> 
      <span>print</span> 
      <span>(</span> 
      <span>'self in Desc: %s '</span> 
      <span> </span> 
      <span>%</span> 
      <span> </span> 
      <span>self</span> 
      <span> </span> 
      <span>)</span> 
     </div> 
     <div> 
      <span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</span> 
      <span>print</span> 
      <span>(</span> 
      <span>self</span> 
      <span>,</span> 
      <span> </span> 
      <span>ins</span> 
      <span>,</span> 
      <span> </span> 
      <span>cls</span> 
      <span>)</span> 
     </div> 
     <div> 
      <span>class</span> 
      <span> </span> 
      <span>Test</span> 
      <span>:</span> 
     </div> 
     <div> 
      <span>&nbsp;&nbsp;&nbsp;&nbsp;</span> 
      <span>x</span> 
      <span> </span> 
      <span>=</span> 
      <span> </span> 
      <span>Desc</span> 
      <span>(</span> 
      <span>)</span> 
     </div> 
     <div> 
      <span>&nbsp;&nbsp;&nbsp;&nbsp;</span> 
      <span>def</span> 
      <span> </span> 
      <span>prt</span> 
      <span>(</span> 
      <span>self</span> 
      <span>)</span> 
      <span>:</span> 
     </div> 
     <div> 
      <span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</span> 
      <span>print</span> 
      <span>(</span> 
      <span>'self in Test: %s'</span> 
      <span> </span> 
      <span>%</span> 
      <span> </span> 
      <span>self</span> 
      <span>)</span> 
     </div> 
     <div> 
      <span>t</span> 
      <span> </span> 
      <span>=</span> 
      <span> </span> 
      <span>Test</span> 
      <span>(</span> 
      <span>)</span> 
     </div> 
     <div> 
      <span>t</span> 
      <span>.</span> 
      <span>prt</span> 
      <span>(</span> 
      <span>)</span> 
     </div> 
     <div> 
      <span>t</span> 
      <span>.</span> 
      <span>x</span> 
     </div> 
    </div> </td> 
  </tr> 
 </tbody> 
</table>

运行结果如下：

Python

self in Test: <\_\_main\_\_.Test object at 0x0000000002A570B8> self in Desc: <\_\_main\_\_.Desc object at 0x000000000283E208> <\_\_main\_\_.Desc object at 0x000000000283E208> <\_\_main\_\_.Test object at 0x0000000002A570B8> <class '\_\_main\_\_.Test'>

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      <span>self</span> 
      <span> </span> 
      <span>in</span> 
      <span> </span> 
      <span>Test</span> 
      <span>:</span> 
      <span> </span> 
      <span>&lt;</span> 
      <span>__main__</span> 
      <span>.</span> 
      <span>Test</span> 
      <span> </span> 
      <span>object</span> 
      <span> </span> 
      <span>at</span> 
      <span> </span> 
      <span>0x0000000002A570B8</span> 
      <span>&gt;</span> 
     </div> 
     <div> 
      <span>self</span> 
      <span> </span> 
      <span>in</span> 
      <span> </span> 
      <span>Desc</span> 
      <span>:</span> 
      <span> </span> 
      <span>&lt;</span> 
      <span>__main__</span> 
      <span>.</span> 
      <span>Desc </span> 
      <span>object</span> 
      <span> </span> 
      <span>at</span> 
      <span> </span> 
      <span>0x000000000283E208</span> 
      <span>&gt;</span> 
     </div> 
     <div> 
      <span>&lt;</span> 
      <span>__main__</span> 
      <span>.</span> 
      <span>Desc </span> 
      <span>object</span> 
      <span> </span> 
      <span>at</span> 
      <span> </span> 
      <span>0x000000000283E208</span> 
      <span>&gt;</span> 
      <span> </span> 
      <span>&lt;</span> 
      <span>__main__</span> 
      <span>.</span> 
      <span>Test</span> 
      <span> </span> 
      <span>object</span> 
      <span> </span> 
      <span>at</span> 
      <span> </span> 
      <span>0x0000000002A570B8</span> 
      <span>&gt;</span> 
      <span> </span> 
      <span>&lt;</span> 
      <span>class</span> 
      <span> </span> 
      <span>'__main__.Test'</span> 
      <span>&gt;</span> 
     </div> 
    </div> </td> 
  </tr> 
 </tbody> 
</table>

大部分童鞋开始有疑问了，为什么在Desc类中定义的self不是应该是调用它的实例t吗？怎么变成了Desc类的实例了呢？

注意：此处需要睁大眼睛看清楚了，这里调用的是t.x，也就是说是Test类的实例t的属性x，由于实例t中并没有定义属性x，所以找到了类属性x，而该属性是描述符属性，为Desc类的实例而已，所以此处并没有顶用Test的任何方法。

那么我们如果直接通过类来调用属性x也可以得到相同的结果。

下面是把t.x改为Test.x运行的结果。

Python

self in Test: <\_\_main\_\_.Test object at 0x00000000022570B8> self in Desc: <\_\_main\_\_.Desc object at 0x000000000223E208> <\_\_main\_\_.Desc object at 0x000000000223E208> None <class '\_\_main\_\_.Test'>

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   <td> 
    <div style=""> 
     <div> 
      <span>self</span> 
      <span> </span> 
      <span>in</span> 
      <span> </span> 
      <span>Test</span> 
      <span>:</span> 
      <span> </span> 
      <span>&lt;</span> 
      <span>__main__</span> 
      <span>.</span> 
      <span>Test</span> 
      <span> </span> 
      <span>object</span> 
      <span> </span> 
      <span>at</span> 
      <span> </span> 
      <span>0x00000000022570B8</span> 
      <span>&gt;</span> 
     </div> 
     <div> 
      <span>self</span> 
      <span> </span> 
      <span>in</span> 
      <span> </span> 
      <span>Desc</span> 
      <span>:</span> 
      <span> </span> 
      <span>&lt;</span> 
      <span>__main__</span> 
      <span>.</span> 
      <span>Desc </span> 
      <span>object</span> 
      <span> </span> 
      <span>at</span> 
      <span> </span> 
      <span>0x000000000223E208</span> 
      <span>&gt;</span> 
     </div> 
     <div> 
      <span>&lt;</span> 
      <span>__main__</span> 
      <span>.</span> 
      <span>Desc </span> 
      <span>object</span> 
      <span> </span> 
      <span>at</span> 
      <span> </span> 
      <span>0x000000000223E208</span> 
      <span>&gt;</span> 
      <span> </span> 
      <span>None</span> 
      <span> </span> 
      <span>&lt;</span> 
      <span>class</span> 
      <span> </span> 
      <span>'__main__.Test'</span> 
      <span>&gt;</span> 
     </div> 
    </div> </td> 
  </tr> 
 </tbody> 
</table>

题外话：由于在很多时候描述符类中仍然需要知道调用该描述符的实例是谁，所以在描述符类中存在第二个参数ins，用来表示调用它的类实例，所以t.x时可以看到第三行中的运行结果中第二项为<main.Test object at 0x0000000002A570B8>。而采用Test.x进行调用时，由于没有实例，所以返回None。

### **总结** ###

*  self在定义时需要定义，但是在调用时会自动传入。
 *  self的名字并不是规定死的，但是最好还是按照约定是用self
 *  self总是指调用时的类的实例。