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Answer: The value of the path parameter item_id will be passed to your function as the argument item_id. When you declare other function parameters that are not part of the path parameters, they are automatically interpreted as "query" parameters. (S1, S2)

Types give validation

• With the same Python type declaration, FastAPI gives you data validation. (S1)
• Notice that the error also clearly states exactly the point where the validation didn't pass. (S1)
• This will give you editor support inside of your function, with error checks, completion, etc. (S1)

Query parameters

• The query is the set of key-value pairs that go after the ? in a URL, separated by & characters. (S2)
• As they are part of the URL, they are "naturally" strings. (S2)
• When you declare them with Python types, they are converted to that type and validated against it. (S2)
• As query parameters are not a fixed part of a path, they can be optional and can have default values. (S2)

Claims

• id: C1 claim: Path values are passed as function arguments. source: S1 evidence: "item_id is declared to be an int."
• id: C2 claim: Type declarations give validation. source: S1 evidence: "FastAPI gives you data validation."
• id: C3 claim: Non-path parameters are query parameters. source: S2 evidence: "As query parameters are not a fixed part of a path"
• id: C4 claim: Query parameters can be optional. source: S2 evidence: "they can be optional and can have default values."

Sources

• [S1] Path Parameters - FastAPI. FastAPI docs. https://fastapi.tiangolo.com/tutorial/path-params/ . License: MIT (fastapi/fastapi LICENSE; Copyright (c) 2018 Sebastián Ramírez; https://github.com/fastapi/fastapi/blob/master/LICENSE). Accessed 2026-11-04.
• [S2] Query Parameters - FastAPI. FastAPI docs. https://fastapi.tiangolo.com/tutorial/query-params/ . 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_gbmmqzdy,
  author = {{iinventedlight}},
  title = {{Path and query parameters in FastAPI}},
  howpublished = {iinventedlight Library},
  year = {2026},
  date = {2026-11-04},
  url = {https://iinventedlight.com/library/notes/fastapi-path-query-params/},
  note = {Permanent ID iil:note:gbmmqzdy. License: CC-BY-4.0.},
}

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