Python – getattr() , setattr() , hasattr() 與 delattr()

測試環境為 CentOS 8 x86_64 (虛擬機)

Python 的內建函式 – https://docs.python.org/zh-tw/3.11/library/functions.html 下面來看一下針對 object 的 getattr() , setattr() , hasattr() 與 delattr()

• hasattr(object, name)
  The result is True if the string is the name of one of the object’s attributes, False if not.
• getattr(object, name[, default])
  Return the value of the named attribute of object.
• setattr(object, name, value)
  The function assigns the value to the attribute, provided the object allows it.
• delattr(object, name)
  The function deletes the named attribute, provided the object allows it.

參考範例 – https://clay-atlas.com/blog/2019/11/30/python-chinese-tutorial-function-setattr-getattr/

這邊定義了 test 這個 class 包含 a 與 b 兩個屬性並給予值.

[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 test(object):
...     a = 123
...     b = 'today'
...

• 範例 : hasattr

  >>> hasattr(test, 'a')
  True
  >>> hasattr(test, 'b')
  True
  >>> hasattr(test, 'c')
  False
  

  說明:
  透過 hasattr 函數來檢視屬性是否存在,有回傳 True , 無回傳 False.

• 範例 : getattr

  >>> getattr(test, 'a')
  123
  >>> getattr(test, 'b')
  'today'
  

  說明:
  透過 getattr 函數來讀取屬性的值.

  >>> getattr(test, 'c')
  Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
  AttributeError: type object 'test' has no attribute 'c'
  

  說明:
  如屬性不存在則回傳錯誤.

  透過下面方式可以先判斷物件的屬性是否存在,若不存在就新增該屬性.

  >>> getattr(test, "d", setattr(test, "d", "ddtest"))
  'ddtest'
  >>> getattr(test, 'd')
  'ddtest'
  

• 範例 : setattr

  >>> setattr(test, 'a', 777)
  >>> setattr(test, 'b', 'yesterday')
  >>> getattr(test, 'a')
  777
  >>> getattr(test, 'b')
  'yesterday'
  

  說明:
  透過 setattr 函數來改變屬性的值.

  >>> setattr(test, 'c', 'test')
  >>> getattr(test, 'c')
  'test'
  

  說明:
  如果屬性不存在會新增該屬性並設定該屬性的值.

• 範例 : delattr

  >>> hasattr(test, 'a')
  True
  >>> delattr(test, 'a')
  >>> hasattr(test, 'a')
  False
  
  

  說明:
  透過 delattr 函數來刪除屬性.