Path operation dependencies in FastAPI - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this FastAPI path operation with dependencies?

Consider this FastAPI code snippet:

from fastapi import FastAPI, Depends

app = FastAPI()

def common_parameters(q: str | None = None):
    return {"q": q}

@app.get("/items/")
async def read_items(commons: dict = Depends(common_parameters)):
    return commons

What will be the JSON response when a client requests /items/?q=hello?

FastAPI

from fastapi import FastAPI, Depends

app = FastAPI()

def common_parameters(q: str | None = None):
    return {"q": q}

@app.get("/items/")
async def read_items(commons: dict = Depends(common_parameters)):
    return commons

A {"q": "hello"}

B HTTP 422 Unprocessable Entity error

C {}

D {"q": null}

💡 Hint

Think about how query parameters are passed and how the dependency function returns them.

The dependency common_parameters receives the query parameter q with value "hello" and returns it in a dictionary. The path operation returns this dictionary as JSON.

❓ state_output

intermediate

What is the value of 'user' inside the path operation?

Given this FastAPI code:

from fastapi import FastAPI, Depends

app = FastAPI()

def get_user(token: str):
    if token == "secret":
        return "admin"
    return "guest"

@app.get("/profile")
async def profile(user: str = Depends(get_user)):
    return {"user": user}

What will be the response when a client calls /profile?token=secret?

FastAPI

from fastapi import FastAPI, Depends

app = FastAPI()

def get_user(token: str):
    if token == "secret":
        return "admin"
    return "guest"

@app.get("/profile")
async def profile(user: str = Depends(get_user)):
    return {"user": user}

A {"user": "guest"}

B {"user": "admin"}

C HTTP 422 Unprocessable Entity error

D {"user": null}

💡 Hint

Check how the token query parameter is passed to the dependency.

The dependency get_user receives the token "secret" and returns "admin". The path operation returns this as the user.

📝 Syntax

advanced

Which option causes a syntax error in FastAPI dependency declaration?

Which of the following FastAPI dependency declarations will cause a syntax error?

A def dep(q: str = Depends()): return q

B q nruter :))(sdnepeD = rts :q(ped fed

C q nruter :)sdnepeD = rts :q(ped fed

D def dep(q: str = Depends): return q

💡 Hint

Check how Depends is used as a callable.

Depends must be called as Depends() to create a dependency instance. Using Depends without parentheses is a syntax error in this context.

❓ lifecycle

advanced

How many times is the dependency function called per request?

Given this FastAPI code:

from fastapi import FastAPI, Depends

app = FastAPI()

call_count = 0

def dep():
    global call_count
    call_count += 1
    return call_count

@app.get("/count")
async def count_calls(a: int = Depends(dep), b: int = Depends(dep)):
    return {"a": a, "b": b}

How many times is dep called when a client requests /count?

FastAPI

from fastapi import FastAPI, Depends

app = FastAPI()

call_count = 0

def dep():
    global call_count
    call_count += 1
    return call_count

@app.get("/count")
async def count_calls(a: int = Depends(dep), b: int = Depends(dep)):
    return {"a": a, "b": b}

A 2

B 1

C 0

D Depends on caching settings

💡 Hint

Each Depends call creates a separate dependency instance.

Each parameter uses Depends(dep) separately, so the function dep is called twice per request.

🔧 Debug

expert

Why does this FastAPI dependency cause a runtime error?

Examine this FastAPI code snippet:

from fastapi import FastAPI, Depends

app = FastAPI()

def get_token(token: str):
    return token

@app.get("/secure")
async def secure_route(token: str = Depends(get_token)):
    return {"token": token}

When calling /secure without query parameters, what error occurs and why?

FastAPI

from fastapi import FastAPI, Depends

app = FastAPI()

def get_token(token: str):
    return token

@app.get("/secure")
async def secure_route(token: str = Depends(get_token)):
    return {"token": token}

A Returns {"token": null}

B HTTP 500 Internal Server Error due to missing dependency

C HTTP 422 Unprocessable Entity because 'token' is missing

D Returns {"token": ""} (empty string)

💡 Hint

Consider how FastAPI handles required query parameters in dependencies.

The dependency get_token requires a token query parameter. If missing, FastAPI returns HTTP 422 error indicating a validation failure.