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Python 中类的构造方法 __New__

somenzz 人气:0

1、概述

python 的类中,所有以双下划线__包起来的方法,叫魔术方法,魔术方法在类或对象的某些事件发出后可以自动执行,让类具有神奇的魔力,比如常见的构造方法__new__ 、初始化方法__init__ 、析构方法__del__ ,今天来聊一聊__new__的妙用,主要分享以下几点:

2、__new__ 和 __init__ 的区别

示例代码:

class A: 
    def __new__(cls, *args, **kwargs): 
        print("new", cls, args, kwargs) 
        return super().__new__(cls) 
 
    def __init__(self, *args, **kwargs): 
        print("init", self, args, kwargs) 
 
 
def how_object_construction_works(): 
    x = A(1, 2, 3, x=4) 
    print(x)     
    print("===================") 
    x = A.__new__(A, 1, 2, 3, x=4) 
    if isinstance(x, A): 
        type(x).__init__(x, 1, 2, 3, x=4) 
    print(x) 
 
if __name__ == "__main__": 
    how_object_construction_works() 


上述代码定义了一个类 A,在调用 A(1, 2, 3, x=4) 时先执行 new,再执行 init,等价于:

x = A.__new__(A, 1, 2, 3, x=4) 
if isinstance(x, A): 
    type(x).__init__(x, 1, 2, 3, x=4) 


代码的运行结果如下:

new <class '__main__.A'> (1, 2, 3) {'x': 4} 
init <__main__.A object at 0x7fccaec97610> (1, 2, 3) {'x': 4} 
<__main__.A object at 0x7fccaec97610> 
=================== 
new <class '__main__.A'> (1, 2, 3) {'x': 4} 
init <__main__.A object at 0x7fccaec97310> (1, 2, 3) {'x': 4} 
<__main__.A object at 0x7fccaec97310> 


new 的主要作用就是让程序员可以自定义类的创建行为,以下是其主要应用场景:

3、应用1:改变内置的不可变类型

我们知道,元组是不可变类型,但是我们继承 tuple ,然后可以在 new 中,对其元组的元素进行修改,因为 new 返回之前,元组还不是元组,这在 init 函数中是无法实现的。比如说,实现一个大写的元组,代码如下:

class UppercaseTuple(tuple): 
    def __new__(cls, iterable): 
        upper_iterable = (s.upper() for s in iterable) 
        return super().__new__(cls, upper_iterable) 
 
    # 以下代码会报错,初始化时是无法修改的 
    # def __init__(self, iterable): 
    #     print(f'init {iterable}') 
    #     for i, arg in enumerate(iterable): 
    #         self[i] = arg.upper() 
 
if __name__ == '__main__': 
    print("UPPERCASE TUPLE EXAMPLE") 
    print(UppercaseTuple(["hello", "world"])) 
 
# UPPERCASE TUPLE EXAMPLE 
# ('HELLO', 'WORLD') 

4、应用2:实现一个单例

class Singleton: 
    _instance = None 
 
    def __new__(cls, *args, **kwargs): 
        if cls._instance is None: 
            cls._instance = super().__new__(cls, *args, **kwargs) 
        return cls._instance 
 
 
if __name__ == "__main__": 
 
    print("SINGLETON EXAMPLE") 
    x = Singleton() 
    y = Singleton() 
    print(f"{x is y=}") 
# SINGLETON EXAMPLE 
# x is y=True 

5、应用3:客户端缓存

当客户端的创建成本比较高时,比如读取文件或者数据库,可以采用以下方法,同一个客户端属于同一个实例,节省创建对象的成本,这本质就是多例模式。

class Client: 
    _loaded = {} 
    _db_file = "file.db" 
 
    def __new__(cls, client_id): 
        if (client := cls._loaded.get(client_id)) is not None: 
            print(f"returning existing client {client_id} from cache") 
            return client 
        client = super().__new__(cls) 
        cls._loaded[client_id] = client 
        client._init_from_file(client_id, cls._db_file) 
        return client 
 
    def _init_from_file(self, client_id, file): 
        # lookup client in file and read properties 
        print(f"reading client {client_id} data from file, db, etc.") 
        name = ... 
        email = ... 
        self.name = name 
        self.email = email 
        self.id = client_id 
 
 
if __name__ == '__main__': 
    print("CLIENT CACHE EXAMPLE") 
    x = Client(0) 
    y = Client(0) 
    print(f"{x is y=}") 
    z = Client(1) 
# CLIENT CACHE EXAMPLE 
# reading client 0 data from file, db, etc. 
# returning existing client 0 from cache 
# x is y=True 
# reading client 1 data from file, db, etc. 

6、应用4:不同文件不同的解密方法

先在脚本所在目录创建三个文件:plaintext_hello.txt、rot13_hello.txt、otp_hello.txt,程序会根据不同的文件选择不同的解密算法

import codecs 
import itertools 
 
 
class EncryptedFile: 
    _registry = {}  # 'rot13' -> ROT13Text 
 
    def __init_subclass__(cls, prefix, **kwargs): 
        super().__init_subclass__(**kwargs) 
        cls._registry[prefix] = cls 
 
    def __new__(cls, path: str, key=None): 
        prefix, sep, suffix = path.partition(":///") 
        if sep: 
            file = suffix 
        else: 
            file = prefix 
            prefix = "file" 
        subclass = cls._registry[prefix] 
        obj = object.__new__(subclass) 
        obj.file = file 
        obj.key = key 
        return obj 
 
    def read(self) -> str: 
        raise NotImplementedError 
 
 
class Plaintext(EncryptedFile, prefix="file"): 
    def read(self): 
        with open(self.file, "r") as f: 
            return f.read() 
 
 
class ROT13Text(EncryptedFile, prefix="rot13"): 
    def read(self): 
        with open(self.file, "r") as f: 
            text = f.read() 
        return codecs.decode(text, "rot_13") 
 
 
class OneTimePadXorText(EncryptedFile, prefix="otp"): 
    def __init__(self, path, key): 
        if isinstance(self.key, str): 
            self.key = self.key.encode() 
 
    def xor_bytes_with_key(self, b: bytes) -> bytes: 
        return bytes(b1 ^ b2 for b1, b2 in zip(b, itertools.cycle(self.key))) 
 
    def read(self): 
        with open(self.file, "rb") as f: 
            btext = f.read() 
        text = self.xor_bytes_with_key(btext).decode() 
        return text 
 
 
if __name__ == "__main__": 
    print("ENCRYPTED FILE EXAMPLE") 
    print(EncryptedFile("plaintext_hello.txt").read()) 
    print(EncryptedFile("rot13:///rot13_hello.txt").read()) 
    print(EncryptedFile("otp:///otp_hello.txt", key="1234").read()) 
# ENCRYPTED FILE EXAMPLE 
# plaintext_hello.txt 
# ebg13_uryyb.gkg 
# ^FCkYW_X^GLE 

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