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Concerns

Python implementation of the
Ruby’s
ActiveSupport::Concern
behavior.

Use examples

Note: the class context is the fourth argument of the
metaclass __init__ method,
formally “namespace”:

def __init__(cls, name: str, bases: tuple, namespace: dict) -> None:
    ...

It is a dictionary representation of the class body.

Using a class as mix-in

If the aspect is a class, its context will be merged to the target context on
its factoring.

from typing import NamedTuple
from concerns import concern


class SerialMixin:

    def serialize(self) -> str:
        return json.dumps(self._asdict(), sort_keys=True)

    @classmethod
    def deserialize(cls, data: str):
        return cls(**json.loads(data))


class PersonBase(NamedTuple):
    name: str
    surname: str


class Person(PersonBase):
    concern(SerialMixin)

Using a mapping as default values map

If the aspect is a mapping (a dictionary, by example), it itself will be merged
to the target context on its factoring.

from concerns import concern

class Animal:
    # Too simple use, avoid doing it
    concern({voice='grrr'})

    def __init__(self, *, voice: str = None):
        if voice:
            self.voice = voice

    def say(self) -> None:
        print(self.voice)

Function to change behavior

If the aspect is a callable, the target class context will be passed as
parameter to the aspect for processing before its factoring.

from concerns import concern
from collections.abc import MutableMapping
from datetime import datetime

def make_timed(context: MutableMapping) -> None:
    """
    Reusable aspect function to add creating time to objects
    """

    def set_created_attr(attr: str) -> None:
        context[attr] = None
        def new(cls, name: str, bases: tuple, _dict: dict):
            self = super().__new__(cls, name, bases, _dict)
            setattr(self, attr) = datetime.now()
            return self
        context['__new__'] = new
    context['set_created_attr'] = set_created_attr


class Register:
    concern(make_timed)

    ...  # more class attributes

    set_created_attr('created_at')