python 魔法方法概述及示例

Love The Way You Lie 2022-10-19 00:54 120阅读 0赞

## 部分魔法方法的概念总结 ##

<table> 
 <thead> 
  <tr> 
   <th>语句</th> 
   <th>魔法方法</th> 
   <th>Python 概念</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>obj[key]<br> obj[i:j]<br>obj[i:j:k]</td> 
   <td>__getitem__(key)</td> 
   <td>可订阅对象</td> 
  </tr> 
  <tr> 
   <td>with obj:…</td> 
   <td>__enter__/__exit__</td> 
   <td>上下文管理器</td> 
  </tr> 
  <tr> 
   <td>for i in obj:…</td> 
   <td>__iter__/__next__<br> __len__/__getitem__</td> 
   <td>可迭代对象<br> 序列</td> 
  </tr> 
  <tr> 
   <td>obj.&lt;attribute&gt;</td> 
   <td>__getattr__</td> 
   <td>动态属性检索</td> 
  </tr> 
  <tr> 
   <td>obj(*args,**kwargs)</td> 
   <td>__call__(*args,**kwargs)</td> 
   <td>可调用对象</td> 
  </tr> 
 </tbody> 
</table>

## 示例 ##

##### 1. \_\_getitem\_\_ 可订阅对象 #####

my_numbers = (4,5,3,9)
    print (my_numbers[-1])  # 9
    print (my_numbers[-3])  # 5
    
    my_numbers = (1,1,2,3,5,8,13,21)
    print (my_numbers[2:5]) # (2, 3, 5) 使用切片获取多个元素 从第一个数字索引(包含)开始，直到第二个数字索引(不包含)的所有元素
    
    print(my_numbers[:3])    #(1, 1, 2) 将获取 数字索引3(不包含)之前的所有元素
    print(my_numbers[3:])    #(3, 5, 8, 13, 21) 将获取 数字索引3(包含)之后的所有元素
    print(my_numbers[::])    #(1, 1, 2, 3, 5, 8, 13, 21) 获取 整个元组
    print(my_numbers[1:7:2]) #(1, 3, 8) 此处 第三个参数代表步长 用于表示在区间内迭代时每次要跳过多少个元素
    print(my_numbers)        #(1, 1, 2, 3, 5, 8, 13, 21)

以上功能能够实现，要归功于\_\_getitem\_\_ 此魔法方法；当类似myobject\[key\]之类的方法被调用时会掉用这个魔法方法，它将键值(方括号内的值)作为参数传递。

class Items:
        def __init__(self,*values):
            self._values = list(values)
        
        def __len__(self):
            return len(self._values)
    
        def __getitem__(self, item):
            return self._values.__getitem__(item)
    
    my_number_items = Items(1,2,3,4,5,6)
    print(my_number_items[0])    # 会执行 \_\_getitem__ 方法
    print(my_number_items[:3])   # 会执行 \_\_getitem__ 方法

##### 2. \_\_enter\_\_/\_\_exit\_\_ 上下文管理器 #####

###### 使用： ######

'''with 语句将调用 __enter__ 方法， 无论这个方法返回什么，都将分配给as 后边的变量; 当执行完process_file(fd) 方法后 会调用__exit__ 方法， 即使上下文管理器中发生异常或错误 __exit__方法任然会被调用'''
    with open(filename) as fd:
    	process_file(fd)

###### 示例： ######

def stop_database():
        print ("stop")
    
    def start_database():
        print ("start")
    
    class DBHandler:
        def __enter__(self):
            stop_database()
            return self
    
        def __exit__(self, exc_type, exc_val, exc_tb):
            start_database()
    
    def db_backup():
        print("pg_dump database")
    
    def main():
        with DBHandler():
            db_backup()
    
    main()
    #stop
    #pg_dump database
    #start
    
    import contextlib
    
    @contextlib.contextmanager
    def db_handler():
        stop_database()
        yield
        start_database()
    
    with db_handler():
        db_backup
    
    # 注：contextlib.ContextDecorator Python3.6 及之后版本可用
    class dbhandler_decorator(contextlib.ContextDecorator):
        def __enter__(self):
            stop_database()
    
        def __exit__(self, ext_type, ex_value, ex_traceback):
            start_database()
    
    @dbhandler_decorator()
    def offline_backup():
        print("pg_dump database")
    
    offline_backup()
    #stop
    #pg_dump database
    #start

###### 3. \_\_iter\_\_/\_\_next\_\_ 创建可迭代对象 (注： Python3.6 及之后版本可用) ######

from datetime import timedelta, date
    
    class DataRangeIterable():
        """ An iterable that contains its own iterator object"""
        def __init__(self, start_date, end_date):
            self.start_date = start_date
            self.end_date = end_date
            self._present_day = start_date
    
        def __iter__(self):
            # current_day = self.start_date
            # while current_day < self.end_date:
            # yield current_day
            # current_day += timedelta(days=1)
            return self
    
        def __next__(self):
            if self._present_day > self.end_date:
                raise StopIteration
            today = self._present_day
            self._present_day += timedelta(days=1)
            return today
    
    for day in DataRangeIterable(date(2018,1,1),date(2018,1,5)):
    	print(day)
    
    # 2018-01-01
    # 2018-01-02
    # 2018-01-03
    # 2018-01-04

###### 4. \_\_len\_\_/\_\_getitem\_\_ 创建序列 ######

class DataRangeIterable():
        """ An iterable that contains its own iterator object"""
        def __init__(self, start_date, end_date):
            self.start_date = start_date
            self.end_date = end_date
            # self._present_day = start_date
            self._range = self._create_range()
    
        def _create_range(self):
            days = []
            current_day = self.start_date
            while current_day < self.end_date:
                days.append(current_day)
                current_day += timedelta(days=1)
            return days
    
        def __getitem__(self,day_no):
            return self._range[day_no]
        
        def __len__(self):
            return len(self._range)
    
    for day in DataRangeIterable(date(2018,1,1),date(2018,1,5)):
        print(day)
        
    # 2018-01-01
    # 2018-01-02
    # 2018-01-03
    # 2018-01-04

###### 5. \_\_getattr\_\_ 动态属性检索 ######

class DynamicAttributes:
        def __init__(self, attribute):
            self.attribute = attribute
    
        def __getattr__(self,attr):
            if attr.startswith("fallback_"):
                name = attr.replace("fallback_", "")
                return f"[fallback resolved] { name}"
            raise AttributeError(f"{ self.__class__.__name__} has no attribute { attr}")
    
    dyn = DynamicAttributes("value")
    print(dyn.attribute)     # value
    
    print(dyn.fallback_test)  # [fallback resolved] test
    
    print(dyn.fallback)    # 引发 raise时会抛出此提示 AttributeError: DynamicAttributes has no attribute fallback
    
    dyn.__dict__["fallback_new"] = "new value"
    print(dyn.fallback_new)  # new value
     
    print(getattr(dyn,"something", "default")) # default

###### 6. \_\_call\_\_(\*args,\*\*kwargs) 可调用对象 ######

我们尝试像执行常规函数一样执行对象时，会调用名为\_\_call\_\_的魔法方法。传递给对象的每个参数都将单独传递给\_\_call\_\_方法。

from collections import defaultdict
    class CallCount:
        def __init__(self):
            self._counts =  defaultdict(int)
    
        def __call__(self, argument):
            self._counts[argument] += 1
            return self._counts[argument]
    
    cc = CallCount()
    print(cc(1))          # 1
    print(cc(2))          # 1
    print(cc(1))          # 2
    print(cc(1))          # 3
    print(cc("something"))# 1