Differentiating between multiple providers

[cswartzvi]

Hi! First off, this is a fantastic library - thank you for all your efforts. If I define multiple providers for the same type and then pass both providers to the Injector, the "last" provider takes precedence (see below). While I am pretty sure this is intentional - and aligns nicely with how python handles similarly named functions - I was wondering if there was a way to a) differentiate between the providers list the multiple providers for a type or b) raise an exception if multiple providers have been defined for the same type. I would be happy to contribute a PR if you think there are any worthwhile features to be added. Thanks again!

from dataclasses import dataclass
from injector import Module, singleton, provider, Injector


@dataclass
class Output:
    value: str


class Provider01(Module):
    @singleton
    @provider
    def provide_output(self) -> Output:
        return Output("Using Provider01")


class Provider02(Module):
    @singleton
    @provider
    def provide_output(self) -> Output:
        return Output("Using Provider02")


injector = Injector([Provider01(), Provider02()])
output = injector.get(Output)
print(output.value)  # Using Provider02

[davidparsson]

What would you want to achieve in option a? I'm not sure it matches your use case, but I have previously used parent/child injectors to override providers.

[cswartzvi]

Ah, I think I choose my words poorly in this case - my apologies. @davidparsson, your suggestion about using parent/child injectors is brilliant and I will absolutely keep that in mind when I have to, knowingly, override a provider. However, what I meant to ask in (a) was more along the lines of 'is there a way to detect that multiple providers have been bound to the same type'? I will edit the original question.

[davidparsson]

Thanks for clarifying. That makes sense, but I'm not aware of any way to achieve that in a general way. It's probably possible to explicitly look for previously bound classes in the provider, but that's in no way convenient.

I think it makes sense to be able to detect that, and raising an exception is probably the most intuitive way of feedback, but it's also a breaking change that I'm not sure how to introduce.

@jstasiak, what are your thoughts here?

[jstasiak]

I used the "last processed module overrides whatever previous modules defined if there is an overlap" behavior in tests in the past.

That said, it seems like it'd be a good idea to at least provide an option to disallow that (by raising an exception) – using a switch in the Injector constructor, possibly?