examples.md

Dependency Injection — where it lives outside this repo

Cited, real implementations to study (or point an agent at) when designing or reviewing injection seams.

Python standard library

• sorted(key=...) / min / max. The ordering policy is injected as a callable — micro-DI so idiomatic nobody calls it a pattern. docs.python.org/3/library/functions.html#sorted
• json.dumps(cls=...). The encoder is a constructor-injected collaborator with a production default (JSONEncoder). docs.python.org/3/library/json.html
• unittest.mock. The other half of the pattern: the fakes that exist to be injected through the seams you left. docs.python.org/3/library/unittest.mock.html

Major ecosystems

• pytest fixtures. Injection driven by argument name: declaring a parameter called tmp_path is asking the framework to construct and pass one — a composition root run per test. docs.pytest.org/en/stable/how-to/fixtures.html
• FastAPI Depends. Request-scoped DI as a framework feature; the declared dependency graph is resolved per call, with overrides for tests. fastapi.tiangolo.com/tutorial/dependencies/
• Fowler's taxonomy. Constructor vs setter vs interface injection, and why containers exist at all — the vocabulary the industry still uses. martinfowler.com/articles/injection.html

What to notice across all of them

None of the Python examples involve a container: the language's keyword arguments and structural typing carry the whole pattern. When reviewing, look for the two failure directions — a seam that is missing (tests patch internals) and seams that are gratuitous (constructors as wiring diagrams).