One variable two values Python

Question

Hy people. I have one variable (at least one name) but when i try to access to in in different ways it is has different value. Here is code.

class Sabirac(object):
    nums=[]
    def __init__(self):
            self.nums=[5,4]                
            for i in range (1,11):
                    self.add(randint(1, 100))
    @classmethod
    def add(self,num):
            self.nums.append(num)
    @classmethod
    def calc_sum(self):
            csum=0
            for num in self.nums:
                    csum=csum+num
            return csum
    @classmethod
    def ispis(self):
            return self.nums

Here is output.

b=Sabirac()
b.nums
[5, 4]
b.ispis()
[85, 72, 6, 42, 34, 20, 77, 89, 91, 47]
c=Sabirac()
c.ispis()
[85, 72, 6, 42, 34, 20, 77, 89, 91, 47, 36, 61, 81, 41, 60, 42, 67, 56, 40, 99]
c.nums
[5, 4]

So I made class Sabirac and the acces directly to variable nums and have output

[5, 4]

After that I access same variable through method ispis(). However it give me completely different values

[85, 72, 6, 42, 34, 20, 77, 89, 91, 47]

Then if I create new instance of class c=Sabirac() and call c.ispis() my output will be:

[85, 72, 6, 42, 34, 20, 77, 89, 91, 47, 36, 61, 81, 41, 60, 42, 67, 56, 40, 99]

note that is just append 10 values to b.ispis().

Could someone please tell me what is going on here??

Can you explain why you are using @classmethod? (BTW, if you really do want class methods, the first argument should be called cls, not self, by convention and to make things clearer.) — Wooble
– Wooble, Commented Apr 4, 2014 at 17:50

Raydel Miranda · Accepted Answer · 2014-04-04 17:52:47Z

1

nums is a class variable. Not an instance variable. If you want nums to be an object variable you have to declare into the __init__ method of the class(or some other method).

For instance:

class X:
    class_var = 3

    def __init__(self):
        X.class_var = X.class_var + 1
        self.instance_var =  3



>>> a = X()
>>> print(a.class_var)
>>> 4
>>> print(a.instance_var)
>>> 3

>>> b = X()
>>> print(b.class_var)
>>> 5
>>> print(a.instance_var)
>>> 3

edited Apr 4, 2014 at 17:52

answered Apr 4, 2014 at 17:46

Raydel Miranda

14.4k3 gold badges48 silver badges66 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

Two-Bit Alchemist Over a year ago

That's not a very helpful answer to someone who doesn't understand this situation.

Wooble Over a year ago

self.nums is being assigned in OP's __init__.

Raydel Miranda Over a year ago

@Wooble yes but assigned is not the same as declared that is what I said in my answer.

Wooble Over a year ago

Considering Python doesn't have variable declaration, I don't see the distinction you're trying to make here.

Joran Beasley · Accepted Answer · 2014-04-04 17:51:52Z

class Sabirac(object):
    nums=[]
    def __init__(self):
            self.nums=[5,4]                
            for i in range (1,11):
                    self.add(randint(1, 100))

    def add(self,num):
            self.nums.append(num)

    def calc_sum(self):
            csum=0
            for num in self.nums:
                    csum=csum+num
            return csum

    def ispis(self):
            return self.nums

dont make your methods class methods ... it is clear you are lacking some of the knowledge about what these are

for example

class People:
     my_people = {}
     def __init__(self,name,age):
         self.name = name
         self.age = age
         People.my_people[name] = self    
     def birthday(self):
         #a birthday happens to an instance (or a person, not to people in general)
         self.age += 1
     @classmethod
     def get_person(cls,name):
         #this is asking the People class for an instance of a person
         return cls.my_people[name]

  billy = People("billy",8)
  billy.birthday()
  susan = People("susan",12)

  #somewhere else
  susan = People.get_person("susan") #call the classmethod
  susan.birthday()

glglgl · Accepted Answer · 2014-04-04 17:54:27Z

0

On first glance, it seems irrelevant that we have a nums at class level:

it gets shadowed at class instantiation in __init__().

So each object has an instance variable with 5 and 4 in it, which you get with b.nums resp. c.nums.

But all your methods operate on the class, as you use @classmethod.

Thus, self should be replaced by cls, as it really represents the class.

And they modify the class attribute nums, which is shared by all instances.

answered Apr 4, 2014 at 17:54

glglgl

91.5k13 gold badges157 silver badges230 bronze badges

Collectives™ on Stack Overflow

One variable two values Python

3 Answers 3

4 Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

4 Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related