Python学习（七）—— 面向对象高级编程

╰+哭是因爲堅強的太久メ 2022-03-17 15:38 380阅读 0赞

### 一、使用\_\_slots\_\_ ###

众所周知，Python的动态绑定允许我们在程序运行的过程中动态给class或实例加上功能。但是如果我们想要限制实例的属性，则可以在定义class的时候，定义一个特殊的\_\_slots\_\_变量，来限制该class实例能添加的属性：

>>> class Student(object):
 ... __slots__ = ('name', 'age') #用tuple定义允许绑定的属性名称
 ...
 >>> s = Student() #创建新的实例
 >>> s.name = 'Michael'
 >>> s.age = 25
 >>> s.score = 99
 Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 AttributeError: 'Student' object has no attribute 'score'

需要注意的是，\_\_slots\_\_定义的属性仅对当前类实例起作用，对继承的子类是不起作用的。除非在子类中也定义\_\_slots\_\_，这样子类实例允许定义的属性就是自身的\_\_slots\_\_加上父类的\_\_slots\_\_。

### 二、使用@property ###

在class中定义一个属性时，直接将属性暴露出去的方法会使得属性可被任意修改，这显然是不可取的。一般情况下，我们可以通过定义get和set的方法来加以限制：

class Student(object):
 def get_score(self):
 return self._score
 def set_score(self, value):
 if not isinstance(value, int):
 raise ValueError('scoer must be an integet!')
 if value < 0 or value > 100:
 raise ValueError('score must between 0 ~ 100!')
 self._score = value

而Python提供了另一种简洁的方法：@property装饰器负责把一个方法变成属性调用

class Student(object):
 @property #相当于get
 def score(self):
 return self._score
 @score.setter #相当于set
 def score(self, value):
 if not isinstance(vlaue, int):
 raise ValueError('score must be an integer!')
 if value < 0 or value > 100:
 raise ValueError('score must between 0 ~ 100!')
 self._score = value
 #score方法可以当作属性一样调用

### 三、多重继承 ###

在设计类的继承关系时，通常主线都是单一继承下来的。但是，如果需要“混入”额外的功能，通过多重继承就可以实现。这种设计通常称之为Mixln。

Python自带的很多库也使用了Mixln。举个例子，Python自带了TCPServer和UDPServer这两类网络服务，而要同时服务多个用户就必须使用多进程或多线程模型，这两种模型由ForkingMixIn和ThreadingMixIn提供。通过组合，我们就可以创造出合适的服务来。

比如，编写一个多进程模式的TCP服务，定义如下：

class MyTCPServer(TCPServer, ForkingMixIn):
        pass

编写一个多线程模的UDP服务，定义如下：

class MyUDPServer(UDPServer, ThreadingMixIn):
        pass

这样一来，我们不需要复杂而庞大的继承链，只要选择组合不同的类的功能，就可以快速构造出所需的子类。

### 四、定制类 ###

看到类似\_\_slots\_\_这种形如\_\_xxx\_\_的变量或者函数名就要注意，这些在Python中是有特殊用途的。Python的class中有许多这样有特殊用途的函数，可以帮助我们定制类。

<table> 
 <thead> 
 <tr> 
 <th style="width:153px;">函数或变量（特殊）</th> 
 <th style="width:257px;">说明</th> 
 <th style="width:439px;">用法</th> 
 </tr> 
 </thead> 
 <tbody> 
 <tr> 
 <td style="width:153px;">__str__()</td> 
 <td style="width:257px;">返回用户看到的字符串</td> 
 <td style="width:439px;"> &gt;&gt;&gt; class Student(object): ...&nbsp; &nbsp; &nbsp; &nbsp; &nbsp;def __init__(self, name): ...&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;self.name = name ...&nbsp; &nbsp; &nbsp; &nbsp; &nbsp;def __str__(self): ...&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;return 'Student object (name: %s)' % self.name ... &gt;&gt;&gt; print(Student('Michael')) Student object (name: Michael) </td> 
 </tr> 
 <tr> 
 <td style="width:153px;">__repr__()</td> 
 <td style="width:257px;">返回程序开发者看到的字符串</td> 
 <td style="width:439px;"> &gt;&gt;&gt; class Student(object): ...&nbsp; &nbsp; &nbsp; &nbsp; &nbsp;def __init__(self, name): ...&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;self.name = name ...&nbsp; &nbsp; &nbsp; &nbsp; &nbsp;def __str__(self): ...&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;return 'Student object (name: %s)' % self.name ...&nbsp; &nbsp; &nbsp; &nbsp; &nbsp;__repr__ = __str__ ... &gt;&gt;&gt; s = Student('Michael') &gt;&gt;&gt; s Student object (name: Michael) </td> 
 </tr> 
 <tr> 
 <td style="width:153px;">__iter__()</td> 
 <td style="width:257px;">如果一个类想被用于for...in循环，类似list或tuple那样，就必须实现__iter__()方法。该方法返回一个迭代对象，for循环就会不断调用该迭代对象的__next__()方法拿到循环的下一个值，知道遇到StopIteration错误时退出循环。</td> 
 <td style="width:439px;"> 斐波那契数列： class Fib(object): &nbsp; &nbsp; def __init__(self): &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; self.a, self.b = 0, 1 #初始化两个计数器a,b &nbsp; &nbsp; def __iter__(self): &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; return self&nbsp; &nbsp; &nbsp; &nbsp; &nbsp;&nbsp; &nbsp;#实例本身就是迭代对象，故返回自己 &nbsp; &nbsp; def __next__(self): &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; self.a, self.b = self.b, self.a + self.b&nbsp; &nbsp; &nbsp;#计算下一个值 &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; if self.a &gt; 100000: #退出循环条件 &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; raise StopIteration() &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; return self.a&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; #返回下一个值 </td> 
 </tr> 
 <tr> 
 <td style="width:153px;">__getitem__</td> 
 <td style="width:257px;"> (1)让实例获得索引功能 (2)让实例获得切片功能 </td> 
 <td style="width:439px;"> class Fib(object): &nbsp; &nbsp; def __getitem__(self, n): &nbsp; &nbsp; &nbsp; &nbsp; if isinstance(n, int):&nbsp; #n是索引 &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; a, b = 1, 1 &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; for x in range(n): &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;a, b = b, a + b &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; return a &nbsp; &nbsp; &nbsp; &nbsp; if isinstance(n, slice): #n是切片 &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; start = n.start &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; stop = n.stop &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; if start is None: &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; start = 0 &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; a, b = 1, 1 &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; L = [] &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; for x in range(stop): &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; if x &gt;= start: &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; L.append(a) &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; a, b = b, a + b &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; return L </td> 
 </tr> 
 <tr> 
 <td style="width:153px;">__getattr__</td> 
 <td style="width:257px;">动态返回一个属性</td> 
 <td style="width:439px;"> class Student(objcet): &nbsp; &nbsp; def __init__(self, attr): &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; if attr = 'score': &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; return 99 &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; if attr='age': &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; return lambda: 25 &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; raise AttributeError('\'Student\' object has no attribute \'%s\'' % attr) </td> 
 </tr> 
 <tr> 
 <td style="width:153px;">__call__</td> 
 <td style="width:257px;">任何类，只需要定义一个__call__()方法，就可以直接对实例进行调用</td> 
 <td style="width:439px;"> 定义： class Student(object): &nbsp; &nbsp; &nbsp; def __init__(self, name): &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; self.name = name &nbsp; &nbsp; &nbsp; def __call__(self): &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; print('My name is %s.' % self.name) 调用方法如下： &gt;&gt;&gt; s = Student('Michael') &gt;&gt;&gt; s()&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;#self参数不要传入 My name is Michael. </td> 
 </tr> 
 </tbody> 
</table>

以上只介绍了最常用的几个定制方法，还有很多可定制的方法，请参考[Python官方文档][Python]。

### 五、使用枚举类 ###

在Python中要定义枚举类型，可以定义一个class类型，每个常量都是class的一个唯一实例。Python提供了Enum类来实现这个功能：

from enum import Enum
    Month = Enum('Month', ('Jan','Feb','Mar','Apr','May','Jun','Jul','Aug','Sep','Oct','Nov','Dec'))

这样我们就获得了Month类型的枚举类，可以直接使用Month.Jan来引用一个常量，或者枚举它的所有成员：

for name, member in Month.__members__.items():
        print(name, '=>', member, ',', member.value)   #value属性则是自动赋给成员的int常量，默认从1开始计数

如果需要更精确地控制枚举类型，可以从Enum派生出自定义类：

from enum import Enum, unique
    @unique        #@unique装饰器可以帮助我们检查保证没有重复值
    class Weekday(Enum):
        Sun = 0 #Sun的value被设定为0
        Mon = 1
        Tue = 2
        Wed = 3
        Thu = 4
        Fri = 5
        Sat = 6

访问这些枚举类型可以有若干种方法：

>>> day1 = Weekday.Mon
    >>> print(day1)
    Weekday.Mon
    >>> print(Weekday.Tue)
    Weekday.Tue
    >>> print(Weekday['Tue'])
    Weekday.Tue
    >>> print(Weekday.Tue.value)
    2
    >>> print(day1 == Weekday.Mon)
    True
    >>> print(day1 == Weekday.Tue)
    False
    >>> print(Weekday(1))
    Weekday.Mon
    >>> print(day1 == Weekday(1))
    True
    >>> Weekday(7)
    Traceback (most recent call last):
      ...
    ValueError: 7 is not a valid Weekday
    >>> for name, member in Weekday.__members__.items():
    ...     print(name, '=>', member)
    ...
    Sun => Weekday.Sun
    Mon => Weekday.Mon
    Tue => Weekday.Tue
    Wed => Weekday.Wed
    Thu => Weekday.Thu
    Fri => Weekday.Fri
    Sat => Weekday.Sat

可见，既可以用成员名称引用枚举常量，又可以直接根据value的值获得枚举常量。

[Python]: https://docs.python.org/3/reference/datamodel.html#special-method-names