What is a real-world example of Dependency Injection in a Dynamic Language?

Question

I have a thorough background in .NET but have been using Python and Ruby lately. I found myself pondering how to best provide dependencies to objects that need them in Ruby.

At first thought, I did not actually think DI and IoC frameworks would be required to interact with dependencies because of the leniency of dynamic languages (a la redefinition, mixins, stubs, etc). Then, however, I came across answers as to why DI/IoC frameworks are not needed in dynamic languages. The reasons provided don't sit too well with me. I'm hoping I can see an example that might clear things up.

Recommended suggestions that I kind of disagree with:

Reason 1: A dependent class can be changed at run time (think testing)

In Why are IOC containers unnecessary with dynamic languages we see that a dependent class (non-injected), say X, can be stubbed or mocked in a test. Sure, but that requires us to know our System Under Test is depending on something called X. If our System Under Test suddenly depends on N instead of X, we must now remember to mock N instead of X. The benefit of using DI is we'd never accidentally run a test with production dependencies because we'd always be passing in mocked dependencies.

Reason 2: Subclass or use constructor injection for testing

In everyone's favorite goto resource for all things DI + Ruby, LEGOs, Play-Doh, and Programming, we see an example of subclassing a System Under Test to mock dependencies. Alternatively, we can use constructor injection. Okay, so B depends on A. We call B.get_dependency which provides B with an instance of A. But what if A depends on N which depends on X? Must we call get_dependency on each successive object in the chain?

Reason 3: Dependencies can be mixed in or monkeypatched

Fabio mentions we can just use mixins/monkeypatch. So X is mixedin to N. But The issue is what if X depends on A which depends on B? Do we just use mixins for every dependency down the chain? I see how that can work but it could get messy and confusing quickly.

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Side note: Many users say DI frameworks are not needed in dynamic languages. However, Angular.JS has really benefited from implementing a pretty solid DI system. Angular is built on JavaScript, a dynamic language. Is this approach comparable to Ruby or Python?

Please keep in mind I'm not saying I want to force DI/IoC into Ruby, Python, etc.

Answer 1

While many think DI is not needed, I agree with you, that it's indeed needed a lot; but sometimes it gets mixed with other techniques Python provides. I suggest you to look at venusian, it may kind of verbose, but if you come from .NET you'll see the relation. In a word: venusian allows you to annotate your methods without changing their behavior. Thus, you may write venusian decorators so that your unit-testing does not get affected. Pyramid uses venusian to annotate views, for instance.