Prevent two child classes to set an attribute to the same value in Python

Question

I have several child classes in my code, and there is a specific attribute for which I don't want any child classes to have the same value. Let's call this attribute command_name

I tried to implement that using init_subclasses :

class Parent:
    list_of_unique_values_command_name = list()
    def __init_subclass__(cls, **kwargs):
        super().__init_subclass__(**kwargs)
        if hasattr(cls, "command_name"):
            if cls.command_name in Parent.list_of_unique_values_command_name:
                raise ValueError("cant have the same attribute value twice")
            Parent.list_of_unique_values_command_name.append(cls.command_name)

Child1

class ChildOne(Parent):
    command_name = "blabla"

    def __init__():
        print("hello1")

Child2

class ChildTwo(Parent):
    command_name = "blabla"

    def __init__():
        print("hello2")

This code works as expected when all classes are parsed by the Python interpreter. However, let's say the child classes are on different modules, and I run a command which only use one of them, Python won't detect that two child classes have the command_name attribute with the same value.

Is there any solution to fix this?

Edit1: I'm not searching for a solution such as a Singleton. The goal is that even if two child classes run in two different processes, they still can't have the same command_name attribute. (And it must be stateless)

Use UUIDs for the command names. That's the only way to guarantee they can't be the same if none of the subclasses can see each other or any shared state. — Samwise, Apr 27 '22 at 15:42
"However, let's say the child classes are on different modules, and I run a command which only use one of them, Python won't detect that two child classes have the command_name attribute with the same value." That is absolutely not true. You seem to mean *on two different processes*. Which is an *entirely different circumstance*. Is that correct? — juanpa.arrivillaga, May 24 '23 at 20:13

score 1 · Answer 1 · answered May 24 '23 at 19:58

If you have different modules on the same process you are good to go as you are: every class in the same process which inherits from your Parent class will share the same registry in the class, the list_of_unique_values_command_name attribute.

(The only remark is that it can be a set instead of a list, but there is little practical difference).

Now, if you have serveral processes as you mention en passant, each running a different set of derived classes, and you want a single registry, that is more complex: You need to have a resource that is shared across all processes that would allow you to check for the uniqueness of the attributes you want.

I'd suggest using redis for that - if you can make all your processes connect to a single redis instance, it is trivial to keep the list of used names in the redis DB, instead of in a particular running process, as a class attribute. https://redis.io/docs/clients/python/

score -3 · Answer 2 · answered Apr 27 '22 at 15:15

-3

Use a singleton class to create the Parent class. That should make it so that only one instance of Parent exists and that can store a global state.

Other than that, you could use something like SQLite to store information if that is simpler for you so that you can create a one-to-one mapping for classes and their names.

answered Apr 27 '22 at 15:15

Niteya Shah

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that would not work. I updated my question – nolwww Apr 27 '22 at 15:19
1

You want a stateless solution but want to store state. Think about your problem first and think if it is possible. – Niteya Shah Apr 27 '22 at 15:32
where did I say I want to store state ? – nolwww Apr 27 '22 at 15:56
How else can you keep track of all used names? – Niteya Shah Apr 27 '22 at 16:19
That is the point of my question => Knowing if two child classes have the same attribute name. – nolwww Apr 28 '22 at 09:56

Prevent two child classes to set an attribute to the same value in Python

2 Answers2