Consider mentioning SRP style of classes together with DI

[sobolevn]

This chapter describes a lot of ways to handle Dependency Injection in Python. (It even mentions dependencies 🎉 )

But, it does not mention SRP-styled classes. They are originally invented in Ruby (dry-system), but in my practice they are perfectly usable in Python (providing type-safe DI and declarative style):

from typing import final
from attr import dataclass

@final
@dataclass(frozen=True, slots=True)
class SendTodaysPostcardsUsecase(object):
    _repository: PostcardsForToday
    _email: SendPostcardsByEmail
    _analytics: CountPostcardInAnalytics

    def __call__(self, today: datetime) -> None:
        postcards = self._repository(today)
        self._email(postcards)
        self._analytics(postcards)

This way we don't have an explicit constructor. We only have a single public method with possibly other protected ones.

What do you think?

Original article (using punq as an implementation): https://sobolevn.me/2019/03/enforcing-srp
dry-rb/dry-system docs here: https://dry-rb.org/

[simkimsia]

Adding a +1 because I would also like to know @hjwp or @bobthemighty thoughts on this

[Bryant-Yang]

+1 for the article, broadens my mind.

[bobthemighty]

I don't have a strong opinion here. Using a dataclass instead of an ordinary Python class saves a little bit of boiler plate, but I think it's a question of taste. I don't think it's wrong in any way, and there's something nice about having service classes be immutable, because it saves you from cheating with state.

I would, though, like to challenge the assertion in the article that "combin[ing] data and behavior together ... is a clear violation of SRP" since the SRP is specifically a principle of object-oriented software, where the combination of data and behaviour is axiomatic.

Even in the class listed above, the UseCase object has data, it's just immutable, so I don't think it has any fewer responsibilities than any other way of building a use case.

Neat idea, though, and of course you get bonus points for using punq 😄 ❤️

[sobolevn]

object-oriented software, where the combination of data and behaviour is axiomatic

Please, don't take it seriously from a functional programmer 😆
It was more like a pun!