Python 中类的构造方法 __New__的妙用

时间:2021-07-10 05:20:52

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:

  • __new__ 和 __init__ 的区别
  • 应用1:改变内置的不可变类型
  • 应用2:实现一个单例
  • 应用3:客户端缓存
  • 应用4:不同文件不同的解密方法
  • 应用5:Metaclasses

2、__new__ 和 __init__ 的区别

  • 调用时机不同:new 是真正创建实例的方法,init 用于实例的初始化,new 先于 init 运行。
  • 返回值不同,new 返回一个类的实例,而 init 不返回任何信息。
  • new class 的方法,而 init 是对象的方法。

示例代码:

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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:

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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:

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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:

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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

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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。

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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

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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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原文链接:https://developer.51cto.com/art/202110/686454.htm