Notes: Magic Method

Magic Methods

Constructor

__init__, Class initialization method. Local constructor method is different from the ordinary. It will automatically call the object is created.

Create a constructor

class FooBar():
    def __init__(self):
        self.somevar = 42
        
        
f = FooBar()
f.somevar   # 42

Destructor

__del__This method before the object is destroyed (garbage collected) call, but given not know the exact time of the call, do not use as much as possible __del__.

Substantial sequence and mapping protocols

**__len__(self)**

This method returns the number of items included in the collection of sequences, is the number of elements, for mapping a bond - the value of the number. If __len__returns zero (and not implemented __nonzero__), the object regarded as false (like empty tuples, strings, lists, dictionaries, like) in a boolean context.

**__getitem__(self, key)**

This method returns the value associated with the specified key and.

**__setitem__(self, key, value)**

This method is associated with the linkage to the stored value. So that you can use __getitem__to get.

**__delitem__(self, key)**

This method is in use on the part of the object __del__to be invoked when the statement, delete the value associated with the key.

Create an infinite sequence

def check_index(key):
    """
    指定的键是否是可接受的索引？

    键必须是非负整数，如果不是，将引发TypeError异常。
    如果是负数，将引发IndexError异常（因为这个序列的长度是无穷的）。
    """ 
    if not isinstance(key, int): raise TypeError
    if key < 0: raise IndexError

class AritmeticSequence():

    def __init__(self, start=0, step=0):
        """[summary]
        
        初始化这个算数序列
        
        Keyword Arguments:
            start {number} -- 序列中的第一个值 (default: {0})
            step {number} -- 两个相邻值的差 (default: {0})
            changed {dict} -- 一个字典，包含用户修改后的值
        """ 
        self.start = start        # 存储起始值
        self.step = step          # 存储步长值
        self.changed = {}         # 没有任何元素被修改

    def __getitem__(self, key):
        """
        
        从算数序列中获取一个元素
        """ 

        check_index(key)

        try: return self.changed[key]               # 修改过？
        except KeyError:                            # 如果没有修改过
            return self.start + key * self.step     # 就计算元素的值

    def __setitem__(self, key, value):
        """
        
        修改算数序列中的元素
    
        """ 
        check_index(key)

        self.changed[key] = value

 
s = AritmeticSequence()
for i in range(10):
    print(s[i], end=" ")
    
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