Python 创建派生实例 class

Question

I want to have abstract class Task and some derived classes like TaskA, TaskB, ... I need static method in Task fetching all the tasks and returning list of them. But problem is that I have to fetch every task differently. I want Task to be universal so when I create new class for example TaskC it should work without changing class Task. Which design pattern should I use? Let's say every derived Task will have decorator with its unique id, I am looking for function that would find class by id and create instance of it. How to do it in python?

Answer 1

There are a couple of ways you could achieve this.

the first and most simple is using the __new__ method as a factory to decide what subclass should be returned.

class Base:
    UUID = "0"
    def __new__(cls, *args, **kwargs):
        if args == "some condition":
            return A(*args, **kwargs)
        elif args == "another condition":
            return B(*args, **kwargs)

class A(Base):
    UUID = "1"

class B(Base):
    UUID = "2"
    
instance = Base("some", "args", "for", "the", condition=True)

in this example, if you wanted to make sure that the class is selected by uuid. you can replace the if condition to read something like

if a.UUID == "an argument you passed":
    return A

but it's not really useful. since you have knowledge of the specific UUID, you might as well not bother going through the interface.

since I don't know what you want the decorator for, I can't think of a way to integrate it.

EDIT TO ADDRESS THE NOTE:

you don't need to have update it every time, if you do your expressions smartly.

let's say that the defining factor comes from a config file, that says "use class B"

for sub_classs in self.__subclasses__():
    if sub_class.UUID == config.uuid:
        return sub_class(*args, **kwargs) # make an instance and return it

the problem with that is that uuid is not useful to us as people. it would be easier to understand if instead we used a config.name to replace every place we have uuid in the example

Answer 2

I was fighting with this a lot of time and this is exactly what I wanted:

def class_id(id:int):
    def func(cls):
        cls.class_id = lambda : id
        return cls
    return func

def find_subclass_by_id(cls:type, id:int) -> type:
    for t in cls.__subclasses__():
        if getattr(t, "class_id")() == id:
            return t


def get_class_id(obj)->int:
    return getattr(type(obj), "class_id")()



class Task():
    def load(self, dict:Dict) -> None:
        pass

    @staticmethod
    def from_dict(dict:Dict) -> 'Task':
        task_type = int(dict['task_type'])
        t = find_subclass_by_id(Task, task_type)
        obj:Task = t()
        obj.load(dict)
        return obj


    @staticmethod
    def fetch(filter: Dict):
        return [Task.from_dict(doc) for doc in list_of_dicts]

@class_id(1)
class TaskA(Task):
     def load(self, dict:Dict) -> None:
        ...
     ...