python - self.arg = arg 和 self.__dict__ ['arg' ] = arg 在 Python 中的区别

Question

当我定义类时，我总是去

Class A(object):
    def __init__(self, arg):
        self.arg = arg
    def print_arg(self):
        print(self.arg)

a = A('hello')

print a.arg

'hello'

但是我在第 133 行和第 134 行发现的 的 https://github.com/Pylons/webob/blob/master/src/webob/request.py让我思考我在 A 类中所做的与以下内容之间的区别是什么:

Class B(object):
    def __init__(self, arg):
        self.__dict__['arg'] = arg
    def print_arg(self):
            print(self.arg)

b = B('goodbye')

print b.arg

'goodbye'

Answer 1

有几个主要的影响:

1. 使用 self.__dict__ 添加属性规避 __setattr__，这可能会因某些您可能希望在某些地方避免的行为而过载。

   In [15]: class Test(object):
       ...:     def __init__(self, a, b):
       ...:         self.a = a
       ...:         self.__dict__['b'] = b
       ...:     def __setattr__(self, name, value):
       ...:         print('Setting attribute "{}" to {}'.format(name, value))
       ...:         super(Test, self).__setattr__(name, value)
       ...:               
   
   In [16]: t = Test(1, 2)
   Setting attribute "a" to 1
   

   您可以看到没有为属性 b 打印任何内容。

2. 在某些情况下不够灵活

   In [9]: class WithSlots(object):
      ...:     __slots__ = ('a',)
      ...:     def __init__(self, a):
      ...:         self.__dict__['a'] = a
      ...:         
   
   In [10]: instance = WithSlots(1)
   ---------------------------------------------------------------------------
   AttributeError                            Traceback (most recent call last)
   <ipython-input-10-c717fcc835a7> in <module>()
   ----> 1 instance = WithSlots(1)
   
   <ipython-input-9-2d23b670e0fc> in __init__(self, a)
         2     __slots__ = ('a',)
         3     def __init__(self, a):
   ----> 4         self.__dict__['a'] = a
         5 
   
   AttributeError: 'WithSlots' object has no attribute '__dict__'
   
   In [11]: class WithSlots(object):
       ...:     __slots__ = ('a',)
       ...:     def __init__(self, a):
       ...:         self.a = a
       ...:         
       ...:         
   
   In [12]: instance = WithSlots(1) # -> works fine
   

3. 您不能在类定义之外执行此操作。