Python – Bound , Unbound & static method

測試環境為 CentOS 8 x86_64 (虛擬機)

參考文章 – https://stackoverflow.com/questions/114214/class-method-differences-in-python-bound-unbound-and-static

下面來看一下什麼是 bound & unbound method 與他們的差異.

[root@localhost ~]# python3
Python 3.6.8 (default, Mar 25 2022, 11:15:52) 
[GCC 8.5.0 20210514 (Red Hat 8.5.0-10)] on linux
Type "help", "copyright", "credits" or "license" for more information.

Class 內定義了 bound (method_one) & unbound (method_two) method

class Test:
    def method_one(self):
        print ("Called method_one")
    def method_two():
        print ("Called method_two") 

在定義好的 class 其 method 皆為 function.

>>> type(Test.method_one)
<class 'function'>
>>> type(Test.method_two)
<class 'function'>

• Bound Method
  我們無法直接透過 class 來使用 method_one 這個 method.

  >>> Test.method_one()
  Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
  TypeError: method_one() missing 1 required positional argument: 'self'
  

  method_one 需傳入 self 參數,我們須先產生 instance 後才能使用它.

  >>> a_test = Test()
  

  透過 instance.method 或是 class.method(instance) 的方式來使用.

  >>> a_test.method_one()
  Called method_one
  >>> Test.method_one(a_test)
  Called method_one
  

  a_test.method_one 與 a_test.method_two 皆為 Bound method ,但 a_test.method_two 無法透過這個方式來使用.

  >>> a_test.method_one
  <bound method Test.method_one of <__main__.Test object at 0x7f1c77c15eb8>>
  >>> a_test.method_two
  <bound method Test.method_two of <__main__.Test object at 0x7f1c77c15eb8>>
  

• Unbound Method
  在定義好的 class 其 method 沒有傳入 self 參數的 method 稱為 Unbound method , 要怎麼使用 unbound (method_two) method ,直接透過 class.method 即可使用.

  >>> Test.method_two()
  Called method_two
  

  無法透過 instance 來使用.

  >>> a_test.method_two()
  Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
  TypeError: method_two() takes 0 positional arguments but 1 was given
  

• @staticmethod
  需使用 @staticmethod 才能讓 method_two 透過 instance 來使用.

  class Test:
      def method_one(self):
          print ("Called method_one")
      @staticmethod
      def method_two():
          print ("Called method_two") 
  

  >>> a_test = Test()
  >>> a_test.method_one()
  Called method_one
  >>> a_test.method_two()
  Called method_two
  

  a_test.method_two 型別為 function.

  >>> a_test.method_one
  <bound method Test.method_one of <__main__.Test object at 0x7f1c77b9f240>>
  >>> type(a_test.method_one)
  <class 'method'>
  

  >>> a_test.method_two
  <function Test.method_two at 0x7f1c77bfeb70>
  >>> type(a_test.method_two)
  <class 'function'>