Python 定制类

╰半橙微兮° 2021-12-11 03:01 284阅读 0赞

1  
\_\_str\_\_() 和 \_\_repr\_\_()

修改打印一个实例时的默认值。

修改前：

>>> class Student(object):
    ...     def __init__(self,name):
    ...         self.name = name
    ... 
    >>> s=Student('syztoo')
    >>> print(s)
    <__main__.Student object at 0x7fde19208128>
    >>> s
    <__main__.Student object at 0x7fde192082b0>

定制 \_\_str\_\_() 方法：

>>> class Student(object):
    ...     def __init__(self,name):
    ...         self.name = name
    ...     def __str__(self):
    ...         return 'Student object (name: %s)' % self.name 
    ... 
    >>> s=Student('syztoo')
    >>> print(s)
    Student object (name: syztoo)
    >>> s
    <__main__.Student object at 0x7fde192082b0>

定制 \_\_repr\_\_() 方法：

>>> class Student(object):
    ...     def __init__(self,name):
    ...         self.name = name
    ...     def __str__(self):
    ...         return 'Student object (name: %s)' % self.name
    ...     __repr__ = __str__
    ... 
    >>> s=Student('syztoo')
    >>> print(s)
    Student object (name: syztoo)
    >>> s
    Student object (name: syztoo)

--------------------

\_\_iter\_\_()

如果一个类想被用于 `for ... in`循环，类似 list 或 tuple 那样，就必须实现一个`__iter__()` 方法，该方法返回一个迭代对象，然后，Python 的 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，直到遇到`StopIteration`错误时退出循环。

>>> class Fib():
    ...      def __init__(self):
    ...          self.a,self.b = 0,1
    ...      def __iter__(self):
    ...          return self
    ...      def __next__(self):
    ...          self.a,self.b = self.b,self.a+self.b
    ...          if self.a > 1000:
    ...              raise StopIteration()
    ...          return self.a
    ... 
    >>> for n in Fib():
    ...      print(n)
    ... 
    1
    1
    2
    3
    5
    8
    13
    21
    34
    55
    89
    144
    233
    377
    610
    987

--------------------

\_\_getitem\_\_(self，key)

使用 \_\_iter\_\_() 方法定制类后，虽然类可以作用于 for 循环，但并不能像列表那样通过索引取出元素，比如：

>>> Fib()[2]
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    TypeError: 'Fib' object does not support indexing

提示类对象不支持索引。

可以使用`__getitem__(self,key)` 方法为类定制索引，该方法需要一个固定参数key（及索引，从0开始），该方法返回key对应的值。

>>> class Fib(object):
    ...     def __getitem__(self,key):
    ...         a,b = 1,1
    ...         for i in range(key):
    ...             a,b = b,a+b
    ...         return a
    ... 
    >>> f = Fib()
    >>> f[0]
    1
    >>> f[100]
    573147844013817084101

定制切片和步长功能：

#!/usr/bin/env python3
    # -*- coding:utf-8 -*-
    
    class Fib(object):
        def __getitem__(self, n):
            if isinstance(n, int): # n是索引
                a, b = 1, 1
                for x in range(n):
                    a, b = b, a + b
                return a
            if isinstance(n, slice): # n是切片
                start = n.start
                stop = n.stop
                step = n.step
                if start is None:
                    start = 0
                a, b = 1, 1
                L = []
                for x in range(stop):
                    if step:
                        if x >= start and (x - start) % step == 0:
                            L.append(a)
                    else:
                        if x >= start:
                            L.append(a)
                    a, b = b, a + b
                return L
    
    f = Fib()
    print(f[100])
    print(f[10:20])
    print(f[:10])
    print(f[10:20:2])

--------------------

\_\_getattr\_\_()

class Student(object):
    
        def __init__(self):
            self.name = 'Michael'
    
        def __getattr__(self, attr):
            if attr=='score':
                return 99

当我们调用类不存在的属性时，Python解释器会尝试从 \_\_getattr\_\_() 方法中获取该属性，如果 \_\_getattr\_\_() 方法中，有定义该属性，那么我们有机会返回该属性的值：

>>> s = Student()
    >>> s.name
    'Michael'
    >>> s.score
    99

class Chain(object):
    
        def __init__(self, path=''):
            self._path = path
    
        def __getattr__(self, path):
            return Chain('%s/%s' % (self._path, path))
    
        def __str__(self):
            return self._path
    
        __repr__ = __str__
    
    >>> Chain().status.user.timeline.list
    '/status/user/timeline/list'

--------------------

\_\_call\_\_()

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。

class Student(object):
        def __init__(self, name):
            self.name = name
    
        def __call__(self):
            print('My name is %s.' % self.name)

调用方式如下：

>>> s = Student('Michael')
    >>> s() # self参数不要传入
    My name is Michael.

`__call__()`还可以定义参数：

class Chain(object):
        def __init__(self, path=''):
           self.__path = path
    
       def __getattr__(self, path):
           return Chain('%s/%s' % (self.__path, path))
    
       def __call__(self, path):
           return Chain('%s/%s' % (self.__path, path))
    
       def __str__(self):
           return self.__path
    
       __repr__ = __str__

>>> print(Chain().users('michael').repos)
    /users/michael/repos