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
是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()
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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 )
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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 >
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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')
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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
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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.
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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