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Answer: FastAPI has a very powerful but intuitive Dependency Injection system. Although you use Depends in the parameters of your function the same way you use Body, Query, etc, Depends works a bit differently. (S1)

Shared code

• This way you write shared code once and FastAPI takes care of calling it for your path operations. (S1)
• You don't have to create a special class and pass it somewhere to FastAPI to "register" it or anything similar. (S1)
• You just pass it to Depends and FastAPI knows how to do the rest. (S1)

Any callable works

• A "callable" in Python is anything that Python can "call" like a function. (S2)
• In FastAPI, you could use a Python class as a dependency. (S2)
• What FastAPI actually checks is that it is a "callable" (function, class or anything else) and the parameters defined. (S2)
• If you pass a "callable" as a dependency in FastAPI, it will analyze the parameters for that "callable", and process them in the same way as the parameters for a path operation function. (S2)

Claims

• id: C1 claim: FastAPI has a dependency injection system. source: S1 evidence: "FastAPI has a very powerful but intuitive Dependency Injection system."
• id: C2 claim: Shared code is written once. source: S1 evidence: "and then FastAPI takes care of calling the function with the correct parameters"
• id: C3 claim: Classes can be dependencies. source: S2 evidence: "you could use a Python class as a dependency."
• id: C4 claim: FastAPI checks for a callable. source: S2 evidence: "...is what FastAPI will actually use to know what is the dependency."

Sources

• [S1] Dependencies - FastAPI. FastAPI docs. https://fastapi.tiangolo.com/tutorial/dependencies/ . License: MIT (fastapi/fastapi LICENSE; Copyright (c) 2018 Sebastián Ramírez; https://github.com/fastapi/fastapi/blob/master/LICENSE). Accessed 2026-11-04.
• [S2] Classes as Dependencies - FastAPI. FastAPI docs. https://fastapi.tiangolo.com/tutorial/dependencies/classes-as-dependencies/ . License: MIT (fastapi/fastapi LICENSE; Copyright (c) 2018 Sebastián Ramírez; https://github.com/fastapi/fastapi/blob/master/LICENSE). Accessed 2026-11-04.

License: this note is CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Summary by iinventedlight (AI-assisted).

@misc{iil_note_8t8grmwz,
  author = {{iinventedlight}},
  title = {{Dependency injection with Depends in FastAPI}},
  howpublished = {iinventedlight Library},
  year = {2026},
  date = {2026-11-04},
  url = {https://iinventedlight.com/library/notes/fastapi-dependencies/},
  note = {Permanent ID iil:note:8t8grmwz. License: CC-BY-4.0.},
}

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