Python Class and Instance Instantiation Behavior

new and init Function

__new__
- Official document
- 為Class function
  - 用了特殊方法，所以實現時不用特別寫上@classmethod裝飾器
- 主要用途: 構建Instance
- return:
  - 自身Class對應的Instance，並作為__init__的第一個參數，及為self
  - 其他Class對應的Instance，會呼叫對應的__init__
__init__
- Official document
- 為Object function
- 主要用途: 初始化Object變量
- return: 不允許返回任何值

class Some:
    def __new__(cls, *agrs, **kwargs):
        print('>> __new__')
        instance = object.__new__(cls)
        return instance
    def __init__(self, name):
        print('>> __init__')
        self.name = name
if __name__ == '__main__':
    car = Some('car')
    print('name: {}'.format(car.name))
    print('type of Some: {}'.format(type(Some)))
    print('type of car: {}'.format(type(car)))
""" output 
>> __new__
>> __init__
name: Car
type of Some: <class 'type'>
type of car: <class '__main__.Some'>
""

Metaclass

在Python中，Class也是一個Object，代表Class與其他Object一樣，都有建立與初始化的過程與相關函數
- Class建立的時機是當所在的module被import的時候
Metaclass即是定義Class object建立與初始化過程的角色
Python預設使用type作為Metaclass的實作
- 若在自身Object中宣告的Metaclass則使用指定的Metaclass，否則尋找父類是否有指定並使用之；若一路往上都沒有找到宣告Metaclass的語句，則只用type作為Metaclass的實作
- 可以透過繼承type來實作Metaclass，達到改變Class的建立與初始化過程
- 在初始化install時，會呼叫Metaclass的__call__
  - 若在自己實作的Metaclass中沒有實作__call__，則呼叫type的__call__
  - type中的__call__預設會呼叫Class的__new__ 與 __init__
type本身也是一個type Metaclass的instance，換句話說，type是本身的instance
Metaclass可以是任何具有__call__()的物件，Python會呼叫所指定物件的__call__()方法，並將物件本身、類別名稱、父類別資訊與特性作為參數傳入

class SomeMeta(type):
    def __new__(cls, cls_name, bases, attrs):
        print('>> SomeMeta.__new__')
        return type.__new__(cls, cls_name, bases, attrs)
    def __init__(self, cls_name, bases, attrs):
        print('>> SomeMeta __init__')
    def __call__(self, *args, **kwargs):
        print('>> SomeMeta.__call__')
        return super(SomeMeta, self).__call__(*args, **kwargs)
class Some(metaclass=SomeMeta):
    def __new__(cls, *agrs, **kwargs):
        print('>> __new__')
        instance = object.__new__(cls)
        return instance
    def __init__(self, name):
        print('>> __init__')
        self.name = name
if __name__ == '__main__':
    print('\n=== __main__  ===\n')
    print('class of SomeMeta: {}'.format(SomeMeta.__class__))
    car = Some("Car")
    print("name: {}".format(car.name))
    print('type of Some: {}'.format(type(Some)))
    print('type of car: {}'.format(type(car)))
""" output
>> SomeMeta.__new__
>> SomeMeta __init__
=== __main__  ===
>> SomeMeta.__call__
>> __new__
>> __init__
name: Car
type of SomeMeta: <class 'type'>
type of Some: <class '__main__.SomeMeta'>
type of car: <class '__main__.Some'>
"""

Python Class and Instance Instantiation Behavior

__new__ and __init__ Function

Metaclass

new and init Function