[python]如何创建一个不可变的类

所谓的不可变的类就是类的实例不可修改。

下面我们先看一下普通的类:

In [1]: class A(object):
   ...:     pass
   ...:

In [2]: a = A()

In [3]: a.abc = 1

In [4]: a.abc
Out[4]: 1

普通的类的实例可以在运行时添加新的属性。

那么如何定义一个不可变的类呢? 下面就来看一个不可变类的例子:

In [8]: class B(object):
  ....:     __slots__ = ['abc']
  ....:     def __init__(self, abc):
  ....:         super(B, self).__setattr__('abc', abc)
  ....:     def __setattr__(self, name, value):
  ....:         raise AttributeError("'%s' has no attribute %s" % (self.__class__, name))
  ....:

In [9]: b = B(123)

In [10]: b.abc
Out[10]: 123

In [11]: b.abc = 4
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-15-fecd1642a038> in <module>()
----> 1 b.abc = 4

<ipython-input-12-030c2c96393c> in __setattr__(self, name, value)
      4         super(B, self).__setattr__('abc', abc)
      5     def __setattr__(self, name, value):
----> 6         raise AttributeError("'%s' has no attribute %s" % (self.__class__, name))
      7

AttributeError: '<class '__main__.B'>' has no attribute abc

In [12]: b.__dict__
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-18-25106575ab93> in <module>()
----> 1 b.__dict__

AttributeError: 'B' object has no attribute '__dict__'

In [13]: b.x = 1
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-20-0205c051c209> in <module>()
----> 1 b.x = 1

<ipython-input-12-030c2c96393c> in __setattr__(self, name, value)
      4         super(B, self).__setattr__('abc', abc)
      5     def __setattr__(self, name, value):
----> 6         raise AttributeError("'%s' has no attribute %s" % (self.__class__, name))
      7

AttributeError: '<class '__main__.B'>' has no attribute x

这里有两个要点:一个是 __slots__ 另一个是 __setattr__

通过定义 __slots__ 替换掉了类的实例的 __dict__ 属性,阻止新增属性。 通过覆盖 __setattr__ 方法,阻止修改现有属性的值。

有任何意见欢迎在下方留言或者加我微信交流


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