在Python中定义类变量的正确方法

Question

可能重复:
类__init __()函数内部和外部的变量

我注意到在Python中,人们以两种不同的方式初始化他们的类属性.

第一种方式是这样的:

class MyClass:
  __element1 = 123
  __element2 = "this is Africa"

  def __init__(self):
    #pass or something else

另一种风格如下:

class MyClass:
  def __init__(self):
    self.__element1 = 123
    self.__element2 = "this is Africa"

哪个是初始化类属性的正确方法？

Answer 1

这两种方式都不一定正确或不正确,它们只是两种不同的类元素:

• __init__方法之外的元素是静态元素; 他们属于这个班级.
• 内部元件__init__方法是对象的元件(self); 他们不属于班级.

您可以通过一些代码更清楚地看到它:

class MyClass:
    static_elem = 123

    def __init__(self):
        self.object_elem = 456

c1 = MyClass()
c2 = MyClass()

# Initial values of both elements
>>> print c1.static_elem, c1.object_elem 
123 456
>>> print c2.static_elem, c2.object_elem
123 456

# Nothing new so far ...

# Let's try changing the static element
MyClass.static_elem = 999

>>> print c1.static_elem, c1.object_elem
999 456
>>> print c2.static_elem, c2.object_elem
999 456

# Now, let's try changing the object element
c1.object_elem = 888

>>> print c1.static_elem, c1.object_elem
999 888
>>> print c2.static_elem, c2.object_elem
999 456

如您所见,当我们更改了class元素时,它对两个对象都进行了更改.但是,当我们更改了对象元素时,另一个对象保持不变.

Answer 2

我想这个样本解释了样式之间的区别:

james@bodacious-wired:~$cat test.py 
#!/usr/bin/env python

class MyClass:
    element1 = "Hello"

    def __init__(self):
        self.element2 = "World"

obj = MyClass()

print dir(MyClass)
print "--"
print dir(obj)
print "--"
print obj.element1 
print obj.element2
print MyClass.element1 + " " + MyClass.element2
james@bodacious-wired:~$./test.py 
['__doc__', '__init__', '__module__', 'element1']
--
['__doc__', '__init__', '__module__', 'element1', 'element2']
--
Hello World
Hello
Traceback (most recent call last):
  File "./test.py", line 17, in <module>
    print MyClass.element2
AttributeError: class MyClass has no attribute 'element2'

element1绑定到类,element2绑定到类的实例.