Class Variable Vs. Instance Variable In Python For Int Value

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() prin

Solution 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.

Solution 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.

Solution 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;

Solution 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