Python学习笔记 __slots__ 限制类属性

Python允许在定义class的时候，定义一个特殊的slots变量，来限制该class能添加的属性

>>> obj = object()
>>> obj.name = "whatever"
Traceback (most recent call last):
  File "", line 1, in <module>
AttributeError: 'object' object has no attribute 'name'

但是为什么这样就可以呢:

>>> class Object(object):pass
...
>>> Obj = Object()
>>> Obj.name = "whatever"
>>> Obj.name
'whatever'
>>>

答: 现在你给第二个代码块中的Object加上属性 slots 试试:

>>> class Object(object):
...     __slots__ = {}
...
>>> Obj = Object()
>>> Obj.name = "whatever"
Traceback (most recent call last):
  File "", line 1, in 
AttributeError: 'Object' object has no attribute 'name'

会发现抛出了同样的异常。 object 、 list 、 dict 等内置函数都如此。

拥有 slots 属性的类在实例化对象时不会自动分配 dict ，而 obj.attr 即 obj.dict[‘attr’], 所以会引起 AttributeError

对于拥有 slots 属性的类的实例 Obj 来说，只能对 Obj 设置 slots 中有的属性:

>>> class Object(object):
...     __slots__ = {"a","b"}
...
>>> Obj = Object()
>>> Obj.a = 1
>>> Obj.a
1
>>> Obj.c = 1
Traceback (most recent call last):
  File "", line 1, in 
AttributeError: 'Object' object has no attribute 'c'