快捷導(dǎo)航

Python 冷門(mén)魔術(shù)方法小結(jié)

更新時(shí)間：2025年04月16日 10:13:34 作者：予早隨筆

本文主要介紹了Python 冷門(mén)魔術(shù)方法小結(jié),文中通過(guò)示例代碼介紹的非常詳細(xì),對(duì)大家的學(xué)習(xí)或者工作具有一定的參考學(xué)習(xí)價(jià)值,需要的朋友們下面隨著小編來(lái)一起學(xué)習(xí)學(xué)習(xí)吧

__init_subclass__

https://docs.python.org/3/reference/datamodel.html#object.__init_subclass__Python 3.6 新增。

父類派生子類后會(huì)調(diào)用該方法，方法中 cls 指向派生出的子類。

若__init_subclass__被定義為普通方法，將會(huì)被隱式轉(zhuǎn)換為類方法，故不必使用@classmethod裝飾器
metaclass 不會(huì)被傳給__init_subclass__
傳給一個(gè)新類的關(guān)鍵字參數(shù)會(huì)被傳給父類的__init_subclass__。為保證與父類__init_subclass__兼容，應(yīng)當(dāng)將關(guān)鍵字參數(shù)中僅子類__init_subclass__需要的參數(shù)去掉后再傳入父類__init_subclass__

class Base:
    def __init_subclass__(cls, /, name, **kwargs):
        super().__init_subclass__(**kwargs)
        print("Base __init_subclass__ called")
        cls.x = {}
        cls.name = name


class A(Base, name="Jack"):
    def __init__(self):
        super().__init__()
        print("A __init__ called")

    def __new__(cls, *args, **kwargs):
        super().__new__(cls)
        print("A __new__ called")


print(A.x)
print(A.name)
# Base __init_subclass__ called
# {}
# Jack

class A:
    def __init__(self):
        super().__init__()
        print("A __init__ called")

    def __new__(cls, *args, **kwargs):
        super().__new__(cls)
        print("A __new__ called")

    @classmethod
    def __init_subclass__(cls, **kwargs):
        super().__init_subclass__()
        print(kwargs)
        print("A __init_subclass__ called")


class B(A, bbb=12):
    def __init__(self):
        super().__init__()
        print("B __init__ called")

    def __new__(cls, *args, **kwargs):
        super().__new__(cls)
        print("B __new__ called")

    def __init_subclass__(cls, **kwargs):
        super().__init_subclass__()
        print("B __init_subclass__ called")


class C(B, ccc=12):
    def __init__(self):
        super().__init__()
        print("C __init__ called")

    def __new__(cls, *args, **kwargs):
        super().__new__(cls)
        print("C __new__ called")

    def __init_subclass__(cls, **kwargs):
        super().__init_subclass__()
        print("C __init_subclass__ called")

__class_getitem__

__class_getitem__方法的目的是允許標(biāo)準(zhǔn)庫(kù)泛型類的運(yùn)行時(shí)形參化以更方便地對(duì)這些類應(yīng)用類型提示。

若__class_getitem__被定義為普通方法，將會(huì)被隱式轉(zhuǎn)換為類方法，故不必使用@classmethod裝飾器
__class_getitem__方法應(yīng)當(dāng)返回一個(gè) GenericAlias 類型

https://docs.python.org/3/reference/datamodel.html#object.__class_getitem__

from typing import List


class A:

    def __class_getitem__(cls, item):
        print(item)
        return "abc"


print(A[0])

if __name__ == '__main__':
    int_arr_type = List[int]
    list1: int_arr_type = [1]
    list2: int_arr_type = []
    print(int_arr_type)

instancecheck與subclasscheck

__instancecheck__可以自定義實(shí)例檢查邏輯
__subclasscheck__可以自定義子類檢查邏輯

from typing import Any

class Meta(type):
    def __instancecheck__(self, instance: Any) -> bool:
        print("Instance Check")
        print(instance)
        return True

    def __subclasscheck__(self, subclass: type) -> bool:
        print("Subclass Check")
        print(subclass)
        if subclass is int:
            return True
        return False


class A(metaclass=Meta):
    pass


o = A()

print(isinstance(123, A))

print()

print(issubclass(int, A))

# Instance Check
# 123
# True
# 
# Subclass Check
# <class 'int'>
# True

prepare

https://peps.python.org/pep-3115

Python 3.0 新增。

__prepare__用于為類準(zhǔn)備命名空間。

__prepare__定義在元類中并且必須被顯式定義為類方法
__prepare__方法返回一個(gè)映射對(duì)象
Python 執(zhí)行類定義語(yǔ)句，會(huì)首先調(diào)用其元類__prepare__方法，獲得一個(gè)映射對(duì)象，該映射對(duì)象會(huì)被作為該類的命名空間，類的屬性和方法會(huì)被存儲(chǔ)在該映射對(duì)象中，隨后映射對(duì)象會(huì)成為該類的__dict__屬性。

import collections
from typing import Any, Mapping

# 創(chuàng)建一個(gè) OrderedDict 對(duì)象作為字典
global_dict = collections.OrderedDict()

global_dict["global_dict_name"] = "global_dict"


class MyMeta(type):

    @classmethod
    def __prepare__(metacls, __name: str, __bases: tuple[type, ...], **kwargs: Any) -> Mapping[str, object]:
        print(metacls, __name, __bases, kwargs)
        return global_dict


class MyClass(metaclass=MyMeta):
    a = 1
    b = 2
    c = 3


print(MyClass.__dict__)
print("global_dict_name" in MyClass.__dict__)

# <class '__main__.MyMeta'> MyClass () {}
# {'global_dict_name': 'global_dict', '__module__': '__main__', '__firstlineno__': 18, 'a': 1, 'b': 2, 'c': 3, '__static_attributes__': (), '__dict__': <attribute '__dict__' of 'MyClass' objects>, '__weakref__': <attribute '__weakref__' of 'MyClass' objects>, '__doc__': None}
# True

__mro_entries__

https://docs.python.org/3/reference/datamodel.html#object.__mro_entries__

若某個(gè)類的父類不是type的實(shí)例（此時(shí)父類就是一個(gè)普通對(duì)象），則定義該類時(shí)其基類會(huì)被替換為父類中定義的__mro_entries__方法的返回值，若父類中沒(méi)有定義__mro_entries__方法，此時(shí)類的定義會(huì)報(bào)錯(cuò)AttributeError: 'xxx' object has no attribute 'mro'，因?yàn)闊o(wú)法進(jìn)行MRO。

class A1:
    ...

class A2:
    def __mro_entries__(self, bases):
        return (dict,)

class B1(A1):
    print(type(A1), type(A1) is type)
    ...

class B2(A2()):
    print(type(A2()), type(A2()) is not type)
    ...

print(B1.mro())
print(B2.mro())
# <class 'type'> True
# <class '__main__.A2'> True
# [<class '__main__.B1'>, <class '__main__.A1'>, <class 'object'>]
# [<class '__main__.B2'>, <class 'dict'>, <class 'object'>]

到此這篇關(guān)于Python 冷門(mén)魔術(shù)方法小結(jié)的文章就介紹到這了,更多相關(guān)Python 冷門(mén)魔術(shù)方法內(nèi)容請(qǐng)搜索腳本之家以前的文章或繼續(xù)瀏覽下面的相關(guān)文章希望大家以后多多支持腳本之家！

您可能感興趣的文章: