calling a callable stored in a class variable

Question

Looks like I'm either missing something basic or something complicated. I want to store and use a callable as a class variable in a base class to redefine it in subclasses.

from core.singleton import Singleton

import yaml


class Item:
    def __init__(self, arg1, arg2):
        self.a = arg1
        self.b = arg2

    @staticmethod
    def from_yaml(id, data):
        # some complicated logic to calculate arguments
        return Item(arg1, arg2)


class BaseDatabase(metaclass=Singleton):
    filename = None
    item_class_factory = None

    def __init__(self):
        # do some checks
        with open(self.filename) as f:
            self._data = yaml.load(f, yaml.Loader)

    def find(self, id):
        if id in self._data:
            # the line below generates ```TypeError: from_yaml() takes 2 positional arguments but 3 were given```
            return self.item_class_factory(id, self._data[id])  


class ItemDatabase(BaseDatabase):
    filename = 'data/itemsA.yaml'
    item_class_factory = Item.from_yaml


class AnotherDatabase(BaseDatabase):
    filename = 'data/itemsB.yaml'
    item_class_factory = Item.from_yaml

    def find_by_name(self, name):
        # some extended logic

What am I doing wrong? When I try to execute this code I get Type Error about one extra argument passed to the factory method I'm trying to store in a class variable. Evidently, self is being passed to. How to avoid it?

Not sure if directly related to the problem, but typically `Item.from_yaml` would be a `classmethod`. — mkrieger1, Mar 01 '21 at 22:00
I think this may be a duplicate of https://stackoverflow.com/questions/28415595/how-to-store-function-in-class-attribute — Barmar, Mar 01 '21 at 22:12
Yes because `item_class_factory = Item.from_yaml` puts a function object into the class namespace, which will be treated like an instance method... just the same as `def item_class_factory(...): ...` — juanpa.arrivillaga, Mar 01 '21 at 22:17
One quick solution, use `item_class_factory = staticmethod(Item.from_yaml)` — juanpa.arrivillaga, Mar 01 '21 at 22:26
@juanpa.arrivillaga Can you submit an answer so I can accept it? — Oleg Titov, Mar 01 '21 at 22:32
```core.singleton``` is https://stackoverflow.com/questions/6760685/creating-a-singleton-in-python (Method 3) It's not relevant to the problem though. — Oleg Titov, Mar 02 '21 at 01:48

calling a callable stored in a class variable

0 Answers0