类变量Vs。 python中的实例变量的int值

Question

I am new to python and i am not sure how this is working. Code is as below:

class test():
    d=0
    def __init__(self):
       self.d=self.d+1;

D=test()
print D.d
D1=test()
print D1.d
D2=test()
print D2.d

Output is

1,1,1  # This should not be

Now using this :

class test():
    d=[]
    def __init__(self):
       self.d.apend("1");

 D=test()
 print D.d
 D1=test()
 print D1.d
 D2=test()
 print D2.d

Result is (This should be)

['1']
['1', '1']
['1', '1', '1']

So i am not sure why integer value is not being treated as class variable while list is being treated.

Answer 1

In the first example,

self.d = self.d + 1

rebinds self.d , making it independent of test.d .

In the second example,

   self.d.append("1")

modifies test.d .

To see that for yourself, print id(self.d) at the end of both constructors.

If you modified the second example to match the first:

   self.d = self.d + ["1"]

you'd see that the behaviour would also change to match.

Answer 2

If you want to modify a class variable, do:

class test(object):
    d=0
    def __init__(self):
       type(self).d=self.d+1;

D=test()
print D.d
D1=test()
print D1.d
D2=test()
print D2.d

You don't need the type on the right hand side of the assignment, because this way you never create an instance variable d . Note that new-style classes are necessary to this.

type is a function (actually a callable - it is also a class; but don't worry about that for now) which returns the class of its argument. So, type(self) returns the class of self . Classes are first class objects in Python.

Demo here: http://ideone.com/JdNpiV

Update: An alternative would be to use a classmethod .

Answer 3

To address a class variable use class_name.variable_name, giving :

class test(object):
    d=0
    def __init__(self):
       test.d = test.d + 1;

Answer 4

NPE's answer tells you what is going wrong with your code. However, I'm not sure that it really tells you how to solve the issue properly.

Here's what I think you want, if each test instance should have a different d value in an instance variable:

class test(object): # new style class, since we inherit from "object"
    _d = 0 # this is a class variable, which I've named _d to avoid confusion

    def __init__(self):
        self.d = test._d # assign current value of class variable to an instance variable
        test._d += 1     # increment the class variable

Now, you can create multiple instances and each one will get a unique value for d :

>>> D0 = test()
>>> D1 = test()
>>> D2 = test()
>>> print D0.d
0
>>> print D1.d
1
>>> print D2.d
2