Python3 inherit class variable without inheriting value

Question

I have a set of classes which should all keep track of which instance they are. For this purpose I have a class variable in the parent class, which is copied and incremented at initialization of each object. Here's a minimal example which seems to work just fine:

class Parent:
    counter = 0
    def __init__(self):
        self.name = self.__class__.__name__ + "_" + str(self.__class__.counter)
        self.__class__.counter += 1

    def __repr__(self):
        return self.name

mother = Parent()
father = Parent()
print(mother)
print(father)

Parent_0

Parent_1

So far so good. Now I'd like to use make some subclasses that have the same behavior without rewriting the code. So I do this:

class Child(Parent):
    # counter = 0
    pass

son = Child()
daughter = Child()
stepfather = Parent()

print(son)
print(daughter)
print(stepfather)

Child_2

Child_3

Parent_2

This behavior is not what I intended. When the first instance of a child is created it inherits whichever value the parent counter has at that moment, however after this the classes each maintain their own counter. I can avoid this by putting the line counter = 0 into every single subclass, but this strikes me as redundant. Is there some more elegant way that I can inherit the behavior from the parent without having to redeclare the same variable every time?

not sure if I got you correctly - do you want each subclass to have its own 0 based counter or do you want a running counter over all base/derived classes? — Patrick Artner
– Patrick Artner, Commented Mar 5, 2018 at 12:49
What is your expected output? Do all objects share a counter, or a separate one for children and parents? — kabanus
– kabanus, Commented Mar 5, 2018 at 12:50
Sorry for being unclear. I want each class and subclass to have its own counter starting at 0. Uncommenting the single commented line in my code example gives my desired behavior. — James
– James, Commented Mar 5, 2018 at 12:54

Patrick Artner · Accepted Answer · 2018-03-05 12:55:51Z

2

No idea why you want this behavior, but ... instead of an counter int, create a counter dict:str->int in Parent. Use a key of self.__class__.__name__ to acces the running counter of each seperate class in it. With assign it with dict.setdefault(self.__class__.__name__,0) and increment it inside __init__.

 class Parent:
    counter = dict()

    def __init__(self):
        Parent.counter.setdefault(self.__class__.__name__,0)
        self.name = f'{self.__class__.__name__}_{Parent.counter[self.__class__.__name__]}' 
        Parent.counter[self.__class__.__name__] += 1

    def __repr__(self):
        return self.name

mother = Parent()
father = Parent()
print(mother)
print(father)

class Child(Parent):
    # counter = 0
    pass

son = Child()
daughter = Child()
stepfather = Parent()

print(son)
print(daughter)
print(stepfather)

Output:

Parent_0
Parent_1
Child_0
Child_1
Parent_2

edited Mar 5, 2018 at 12:55

answered Mar 5, 2018 at 12:46

Patrick Artner

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3 Comments

James Over a year ago

This seems to accomplish what I am looking for. I have changed the 0 to an empty dict and the init to cl = self.__class__ ; self.name = cl.__name__ + "_" + str(cl.counter.setdefault(cl.__name__, 0)) ; cl.counter[cl.__name__] += 1 . Could you elaborate why this behavior is weird / unexpected for you?

Patrick Artner Over a year ago

I never hat the need to auto-name and "count" the "instances" of my classes :) so no need to do this. I am not quite getting why you would need a dict of dicts to hold the counters (or are you talking of your code, not the one I added ?) Mine uses the f''{} format syntax. @James

James Over a year ago

Basically I want a unique string describing each object, because the objects are wrappers for tensorflow subgraphs, and tensorflow allows you to give your variables names in the form of python strings. That's why I wanted to provide easily interpretable default values automatically. The counters are there so it doesn't accidentally lump together different objects. The {} formatting I know, I just forget the rules for it sometimes :) - anyway thanks for your help.

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