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

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_mem

In [4]: out._inner_mem

In [5]: out.inner_mem

In [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_mem

In [7]: out = ClassOut()

In [8]: out.out_mem

In [9]: ClassOut.out_mem

In [10]: ClassOut.out_mem1

In [11]: out._inner_mem

In [12]: out.out_mem = 'out_mem modified by object'

In [13]: ClassOut.out_mem

In [14]: out.out_mem

In [15]: out.new_mem = 'clas'

In [16]: out.new_mem

In [17]: o = ClassOut()

In [18]: o.out_mem

In [19]: ClassOut.out_mem

In [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'

8.1. 不能直接给对象设置属性?

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

详细见 Python-slots-doc