Python In the class , All with double underline __ How to wrap it up , It's called magic method , Magic methods can be executed automatically after some events of a class or object are issued , Let classes have magical magic , For example, common construction methods __new__、 Initialization method __init__、 destructor __del__, Let's talk today __new__ The magic effect of , The main points are as follows ：
__new__ and __init__ The difference between
application 1： Change the built-in immutable type
application 2： Implement a single example
application 3： Client cache
application 4： Different files have different decryption methods
application 5：Metaclasses
__new__ and __init__ The difference between 1、 Call timing is different ：new Is the way to actually create an instance ,init For instance initialization ,new Precede init function .
2、 Different return values ,new Returns an instance of a class , and init Don't return any information .
3、new yes class Methods , and init It's the method of the object .
Sample code ：
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() The above code defines a class A, Calling A(1, 2, 3, x=4) We'll do it first new, Re execution init, Equivalent to ：
x = A.__new__(A, 1, 2, 3, x=4)
if isinstance(x, A):
type(x).__init__(x, 1, 2, 3, x=4)The code runs as follows ：
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 The main function of creating custom classes is to let programmers create custom behaviors , Here are Python Construction method of __new__ Common scenarios ：
We know , Tuples are immutable types , But we inherit tuple , Then you can go to new in , Modify the element of its tuple , because new Before returning , Tuples are not tuples , This is in init Functions cannot be implemented . for instance , Implement an uppercase tuple , The code is as follows ：
class UppercaseTuple(tuple):
def __new__(cls, iterable):
upper_iterable = (s.upper() for s in iterable)
return super().__new__(cls, upper_iterable)
# The following code will report an error , It cannot be modified during initialization
# 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')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=TrueWhen the creation cost of the client is relatively high , Such as reading files or databases , The following methods can be used , The same client belongs to the same instance , Save the cost of creating objects , This is essentially a multi instance pattern .
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.First create three files in the directory where the script is located ：plaintext_hello.txt、rot13_hello.txt、otp_hello.txt, The program will choose different decryption algorithms according to different files
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^GLEND
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