python类内init外声明的属性与init内声明的对象属性的访问和操作区别(面试题) 

 

 1.在ipython中输入以下代码，其输出会是什么？

In [1]: class ClassOut:   ...:     out_mem = 'out_mem'   ...:     print out_mem   ...:     def __init__(self):   ...:         inner_mem = 'inner_mem'   ...:         self.inner_mem = 'self.inner_mem'   ...:         self._inner_mem = 'self._inner_mem'   ...:         self.out_mem = 'self.out_mem'    ...:     In [2]: out = ClassOut()In [3]: out.out_memIn [4]: out._inner_memIn [5]: out.inner_memIn [6]: class ClassOut:   ...:     out_mem = 'out_mem'   ...:     print out_mem   ...:     def __init__(self):   ...:         inner_mem = 'inner_mem'   ...:         self.inner_mem = 'self.inner_mem'   ...:         self._inner_mem = 'self._inner_mem'   ...:         self.out_mem1 = 'self.out_mem1'   ...:         out_memIn [7]: out = ClassOut()In [8]: out.out_memIn [9]: ClassOut.out_memIn [10]: ClassOut.out_mem1In [11]: out._inner_memIn [12]: out.out_mem = 'out_mem modified by object'In [13]: ClassOut.out_memIn [14]: out.out_memIn [15]: out.new_mem = 'clas'In [16]: out.new_memIn [17]: o = ClassOut()In [18]: o.out_memIn [19]: ClassOut.out_memIn [20]: ClassOut.out_mem = 'out_mem modified by Class'In [21]: o.out_mem

 

 

 考察：

1. python解释器处理解释class

2. 类的初始化定义方法

3. 类的__init__方法 与 class object成员定义， class instance成员的定义

4. 类定义成员时的命名约定

5. class object  与 class instance的区别

6. class object 与 class instance在处理缺失成员访问时查询域优先级的区别

7. class 定义 __private_mem 不被外部发现的元婴？ （被重命名为_[class_name]__private_mem）!!!详见如下代码!

 

In [48]: class ClassOut:

    ...: out_mem = 'out_mem'

    ...: print out_mem

    ...: def __init__(self):

    ...: inner_mem = 'inner_mem'

    ...: self.inner_mem = 'self.inner_mem'

    ...: self._inner_mem = 'self._inner_mem'

    ...: self.out_mem1 = 'self.out_mem1'

    ...: self.__private_mem = 'self.__private_mem'

    ...:

out_mem

 

In [49]: no = ClassOut()

 

In [50]: no.__private_mem

---------------------------------------------------------------------------

AttributeError Traceback (most recent call last)

<ipython-input-50-36dd351a1b65> in <module>()

----> 1no.__private_mem

 

AttributeError: ClassOut instance has no attribute '__private_mem'

 

In [51]: no._ClassOut__private_mem

Out[51]: 'self.__private_mem'

 

 

 

 

 

>>> obj = object()>>> obj.name = "whatever"Traceback (most recent call last):  File "
      
       ", line 1, in 
       
        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.a1>>> Obj.c = 1Traceback (most recent call last):  File "
      
       ", line 1, in 
       
        AttributeError: 'Object' object has no attribute 'c'
       
      

详细见