How to change all objects of a class at once?

Question

I am rediscovering OOP through a GUI application I am writing and would like to understand if it is possible to change a characteristic of all objects instantiated from a class. I have the following code:

class myclass():
    def __init__(self):
        self.a = 1

x = myclass()
y = myclass()
print x.a, y.a  # outputs 1 1
# the instruction I am looking for, assigns 2 to a in each object at once
print x.a, y.a  # outputs 2 2

What I am looking for is a way to change x and y at once by manipulating the parent class. I know that I can have a method which modifies self.a - I do not want to use that because it means I have to call it separately for each object.

My gut feeling is that this is not possible and that I have to cleverly handle my objects to simplify such activities (for instance by storing them in a list I would loop over, applying a method). I just do not want to miss a mechanism which I am not aware of and which would greatly simplify my code.

Wouldn't that be a situation where you'd use class variables instead of instance variables? — Tim Pietzcker
– Tim Pietzcker, Commented Jul 18, 2014 at 7:22

Tim Pietzcker · Accepted Answer · 2014-07-18 07:27:07Z

10

You can have all instances share a variable if you define it as a class variable:

>>> class myclass():
...     a = 1
...     def __init__(self):
...         self.b = 2
...
>>> x = myclass()
>>> y = myclass()
>>> x.a
1
>>> myclass.a = 2   # modify the class variable
>>> x.a
2
>>> y.a
2
>>> x.b = 3         # modify the instance variable
>>> x.b
3
>>> y.b
2
>>> x.a = 4         # create new local instance variable a
>>> x.a
4
>>> y.a
2

Note that now if you change myclass.a, you won't see the change in x because the instance variable will be looked up before the class variable - until you remove the instance variable using del:

>>> myclass.a = 3
>>> x.a
4
>>> y.a
3
>>> del x.a
>>> x.a
3

answered Jul 18, 2014 at 7:27

Tim Pietzcker

337k59 gold badges520 silver badges572 bronze badges

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

Comments

RomanHotsiy · Accepted Answer · 2014-07-18 09:16:53Z

3

No, you can't. Object data can't be changed without pointer to the object. Possibly you need class variables and classmethod:

class myclass:
  a = 1
  @classmethod
  def set_a(cls, val):
    cls.a = val

x = myclass()
y = myclass()
print x.a, y.a  # outputs 1 1
myclass.set_a(2)
print x.a, y.a  # outputs 2 2

But in such case you cant change a value of objects separately. If you want such behavior you can save all instances of objects in classdata such follows:

class myclass:

  objects = []

  def __init__(self):
    self.a = 1
    # add instantiated object to object list 
    self.__class__.objects.append(self)

  @classmethod
  def set_all(cls, val):
    # go through list and change each object
    for obj in cls.objects:
      obj.a = 2

x = myclass()
y = myclass()
print x.a, y.a  # outputs 1 1
myclass.set_all(2)
print x.a, y.a  # outputs 2 2
# and you cat change data of each object separately:
x.a = 3
print x.a, y.a  # outputs 3 2

edited Jul 18, 2014 at 9:16

answered Jul 18, 2014 at 7:27

RomanHotsiy

5,1482 gold badges30 silver badges37 bronze badges

4 Comments

bruno desthuilliers Over a year ago

You may want to learn about classmethod stackoverflow.com/questions/12179271/python-classmethod-and-staticmethod-for-beginner

RomanHotsiy Over a year ago

I know about classmethod. Would using classmethod change this answer dramatically?

bruno desthuilliers Over a year ago

Using a staticmethod and hardcoding the class name is bad practice. Since a lot of Python newbies might read your snippet, it would be better for them (and everyone that will have to maintain their code) to point them to the correct solution.

RomanHotsiy Over a year ago

Agree. Updated answer.

sundar nataraj · Accepted Answer · 2014-07-18 07:27:48Z

0

class myclass():
    a=10
    def __init__(self):
        self.b=0

a=myclass()
b=myclass()
print a.a,b.a #output 10,10
myclass.a=2
a=myclass()
b=myclass()


print a.a,b.a #output 2,2

answered Jul 18, 2014 at 7:27

sundar nataraj

8,7102 gold badges36 silver badges48 bronze badges

Collectives™ on Stack Overflow

How to change all objects of a class at once?

3 Answers 3

Comments

4 Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

Comments

4 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related