Question re class variable

Antoon Pardon antoon.pardon at
Tue Sep 29 12:40:09 CEST 2015

Op 29-09-15 om 11:27 schreef plewto at
> I have a perplexing problem with Python 3 class variables. I wish to generate an unique ID each time an instance of GameClass is created. There are two versions of the __gen_id method with test run results for each listed below the code.

The problem is that in python you can't change a class variable through an instance. The moment you
try, you create an instance attribute.

> class GameObject:
>     # __instance_registry = {"":None}
>     __instance_counter = 0
>     def __init__(self, name):
>         self.__name = str(name)
>         self.__id = self.__gen_id()
>     def __gen_id(self):
>         ty = self.__class__.__name__
>         id = '%s_%d' % (ty, self.__instance_counter)
>         self.__instance_counter += 1

This last line doesn't work as expected. What happens is equivallent to
the following.

          self.__instance_counter = self.__instance_counter + 1

But the self.__instance_counter are two different things here. On the right hand
python finds that self has no __instance_counter attribute so it will fetch the
value from the class.

However on the left hand, python will create an attribute for self and assign the
value to it. Python will not rebind the class variable.

Antoon Pardon 

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