self.arg = arg和self . dict ['arg'] = arg在Python中的区别

罗德里戈·普林西比

当我定义课程时，我总是去

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

a = A（'hello'）

打印a.arg

'你好'

但是我在https://github.com/pylons/webob/blob/master/src/webob/request.py的第133和134行中找到的内容让我想到了在A类中所做的与以下操作之间的区别是什么：

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

b = B（再见）

打印b.arg

'再见'

埃利·科维戈（Eli Korvigo）

有几个主要含义：

使用self.__dict__添加属性规避__setattr__，这可能与某种行为，你可能希望避免在一些地方被重载。

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attribute打印任何内容b。

在某些情况下不太灵活

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