Python – __new__ & __init__

測試環境為 CentOS 8 x86_64 (虛擬機)

在 Class 裡 __new__ , __init__ 的差別.

• object.__new__(cls[, …]) – https://docs.python.org/3/reference/datamodel.html#object.__new__
  建構函式, Called to create a new instance of class cls. __new__()
• object.__init__(self[, …]) – https://docs.python.org/3/reference/datamodel.html#object.__init__
  初始化函式, Called after the instance has been created (by __new__())

__new__() 與 __init__() 都是在建立 Instance 會去執行的 , __new__() 是在產生 Instance 前執行, __init__() 是產生 Instance 之後.

• 參考範例 – https://www.gushiciku.cn/pl/pI4S/zh-tw

  [root@localhost ~]# vi new_init1.py
  class Test:
      def __new__(cls):
          print('__new__')
          return super().__new__(cls)
      def __init__(self):
          print('__init__')
  
  Test=Test()
  

  執行結果

  [root@localhost ~]# python3 new_init1.py
  __new__
  __init__
  

  說明:
  __new__() 的返回值為新的 Instance,通過 super().__new__(cls[, …]) 來使用.

          return super().__new__(cls)
  

• 參考範例 – https://codertw.com/%E7%A8%8B%E5%BC%8F%E8%AA%9E%E8%A8%80/374044/

  [root@localhost ~]# vi new_init2.py
  class PositiveInteger(int):
      def __new__(cls, value):
          return super(PositiveInteger, cls).__new__(cls, abs(value))
  i = PositiveInteger(-3)
  print (i)
  

  執行結果

  [root@localhost ~]# python3 new_init2.py
  3
  

  說明:
  在回傳新 Instance 時就透過 abs 函數把數值轉成正數.

          return super(PositiveInteger, cls).__new__(cls, abs(value))