How to set an instance attribute for all instances of a class

Question

I have below Car class. I would like to be able to reset the mileage for all car class instances to 0. How can I do this?

class Car():

        carpark = []

        # initialising instance

        def __init__(self, brand, color, fuel, mileage):
            self.brand = brand
            self.color = color
            self.fuel = fuel
            self.mileage = mileage
            self.drives = []

            Car.carpark.append(self)

        @classmethod
        def purchase(cls, brand, color):
            cls(brand, color, "Diesel", 0)

Since I have the carpark list, i could do something like:

@classmethod
def reset_mileage(cls):
for car in cls.carpark:
    car.mileage = 0

This works but I am wondering if there is a better, cleaner way to do this?

You could create a reset_mileage function in the Car class which sets its mileage to 0. And then call that for each car in the collection. It's usually not a good idea for a class instance to control/modify the collection it is a part of. — Karl
– Karl, Commented Aug 24, 2018 at 17:38
What you have for mileage resetting seems fine to me. Generally, for questions regarding the style of working code, you might consider asking them at Code Review. — user2390182
– user2390182, Commented Aug 24, 2018 at 17:45

jsbueno · Accepted Answer · 2018-08-24 19:23:24Z

It is possible to use Python's property feature to update the attributes lazily - just keep a tick counter on the class, and the corresponding tick on each instance:

class Car:
   cls_mileage = 0 
   cls_mileage_tick = 0

   @classmethod
   def reset_mileage(cls, value=0):
       cls.cls_mileage = value
       cls.cls_mileage_tick = value


   def __init__(self):
       self.mileage_tick = self.cls_mileage_tick
       self.mileage = 0

   @property 
   def mileage(self):
       if self._mileage_tick < self.cls_mileage_tick:
           self._mileage_tick = self.cls_mileage_tick
           self._mileage = self.cls_mileage
       return self._mileage

   @mileage.setter
   def mileage(self, value):
       self._mileage_tick = self.cls_mileage_tick
       self._mileage = value

Here is some explanation on the @property decorator. This is some sort of "reactive pattern" on top of that. - https://www.programiz.com/python-programming/property

Of course, if you need this for a lot of attributes, this pattern requires a lot of code for each one, there are ways to make this generic using the __getattribute__ and __setattr__ magic methods.

Also, if you can be sure of whether you are getting and setting an attribute on the instance or the class itself , there is no need for the extra cls_ prefix on both class attributes: the names can be re-used.

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