Changing variables in multiple Python instances

Question

Is there anyway to set the variables of all instances of a class at the same time? I've got a simplified example below:

class Object():
   def __init__(self):
       self.speed=0

instance0=Object()
instance1=Object()
instance2=Object()

#Object.speed=5 doesn't work of course

I can see it would be possible by adding all new instances to a list and iterating with isinstance(), but that's not desirable.

Answer 1

One, simpler way, as the other answers put it, is to keep your attribute always as a class attribute. If it is set on the class body, and all write access to the attribute is via the class name, not an instance, that would work:

>>> class Object(object):
...     speed = 0
... 
>>> a = Object()
>>> b = Object()
>>> c = Object()
>>> 
>>> Object.speed = 5
>>> print a.speed
5
>>> 

However, if you ever set the attribute in a single instance doing it this way, the instance will have its own attribute and it will no longer change along with the other instance's:

>>> a.speed = 10
>>> Object.speed = 20
>>> print b.speed
20
>>> print a.speed
10
>>>

To overcome that, so that whenever the attribute is set in any instance, the class attribute itself is changed, the easier way is to have the object as a property - whose setter sets the class attribute instead:

class Object(object):
  _speed = 0
  @property
  def speed(self):
     return self.__class__._speed
  @speed.setter
  def speed(self, value):
     self.__class__._speed = value

Which works:

>>> 
>>> a = Object()
>>> b = Object()
>>> a.speed, b.speed
(0, 0)
>>> a.speed = 10
>>> a.speed, b.speed
(10, 10)

If you want to have independent attribute on the instances, but a special "set_all" method that would set the attribute in all instances, the way to go is to use the gc (Garbage Collector) module in standard librayr, to find and loop through all instances of the class, and set their instance attributes:

import gc

class Object(object):
    def __init__(self):
        self.speed = 0
        
    def set_all_speed(self, value):
        for instance in (obj for obj in gc.get_referrers(self.__class__):
            if isinstance(obj, self.__class__)):
                instance.speed = value

Which results in:

>>> a  =Object()
>>> b = Object()
>>> a.speed = 5
>>> b.speed = 10
>>> a.speed, b.speed
(5, 10)
>>> a.set_all_speed(20)
>>> a.speed, b.speed
(20, 20)