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Python / FastAPI Basics Interview Questions

How do you use class-based dependencies and sub-dependencies in FastAPI?

Dependencies can be classes with a __call__ method, enabling stateful or configurable dependencies. Sub-dependencies are automatically resolved — FastAPI builds the full dependency tree per request.

from fastapi import FastAPI, Depends, HTTPException
from typing import Annotated

app = FastAPI()

# Class-based dependency — holds configuration
class PaginationParams:
    def __init__(
        self,
        skip:  int = 0,
        limit: int = 10,
        max_limit: int = 100,
    ):
        if limit > max_limit:
            raise HTTPException(400, f"limit cannot exceed {max_limit}")
        self.skip  = skip
        self.limit = limit

# Sub-dependency tree:
# get_current_user → verify_token → oauth2_scheme
def verify_token(token: str = Depends(oauth2_scheme)) -> dict:
    if token != "valid":
        raise HTTPException(401, "Bad token")
    return {"username": "alice", "role": "admin"}

def get_current_user(
    payload: dict = Depends(verify_token),
) -> dict:
    return {"username": payload["username"]}

Pagination = Annotated[PaginationParams, Depends(PaginationParams)]
CurrentUser = Annotated[dict, Depends(get_current_user)]

@app.get("/items")
def list_items(page: Pagination, user: CurrentUser):
    return {
        "user":  user["username"],
        "skip":  page.skip,
        "limit": page.limit,
    }

# Dependency with use_cache=False — new instance per call
@app.get("/no-cache")
def no_cache(
    a: Annotated[PaginationParams, Depends(PaginationParams, use_cache=False)],
    b: Annotated[PaginationParams, Depends(PaginationParams, use_cache=False)],
):
    # a and b are separate instances (default: same instance per request)
    return {"a_skip": a.skip, "b_skip": b.skip}

By default, if the same dependency function appears multiple times in a request's dependency tree, how many times does FastAPI call it?
What does Depends(MyClass) do when MyClass has an __init__ that takes query parameters?

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