Depends function basics in FastAPI - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this FastAPI endpoint using Depends?

Consider this FastAPI code snippet:

from fastapi import FastAPI, Depends

app = FastAPI()

def get_query():
    return "query_value"

@app.get("/items")
async def read_items(q: str = Depends(get_query)):
    return {"q": q}

What will be the JSON response when calling /items?

FastAPI

from fastapi import FastAPI, Depends

app = FastAPI()

def get_query():
    return "query_value"

@app.get("/items")
async def read_items(q: str = Depends(get_query)):
    return {"q": q}

A {"q": "query_value"}

B {"q": null}

C HTTP 422 Unprocessable Entity error

D {"q": ""}

💡 Hint

Think about what the Depends function does when used as a default value.

The Depends function calls the dependency function get_query and uses its return value as the parameter q. So the endpoint returns {"q": "query_value"}.

❓ state_output

intermediate

What is the value of 'user' after this dependency injection?

Given this FastAPI code:

from fastapi import Depends, FastAPI

app = FastAPI()

def get_user():
    return {"name": "Alice"}

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

What will be the output JSON when calling /profile?

FastAPI

from fastapi import Depends, FastAPI

app = FastAPI()

def get_user():
    return {"name": "Alice"}

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

A {}

B {"user": "Alice"}

C {"name": "Alice"}

D HTTP 500 Internal Server Error

💡 Hint

Check what the dependency function returns and how it is used in the endpoint.

The dependency get_user returns a dictionary with key "name" and value "Alice". The endpoint returns this dictionary directly.

📝 Syntax

advanced

Which option causes a syntax error in Depends usage?

Identify which code snippet will cause a syntax error when using Depends in FastAPI.

A

async def endpoint(dep Depends(get_dep)):
    return dep

B

async def endpoint(dep: str = Depends(get_dep)):
    return dep

C

async def endpoint(dep=Depends(get_dep)):
    return dep

D

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💡 Hint

Look carefully at the function parameter syntax.

Option A is missing the equals sign (=) between the parameter name and the default value, causing a syntax error.

🔧 Debug

advanced

Why does this FastAPI endpoint raise a runtime error?

Examine this code:

from fastapi import FastAPI, Depends

app = FastAPI()

def get_number():
    return 42

@app.get("/number")
async def read_number(num: int = Depends(get_number)):
    return {"num": num + 1}

When calling /number, a runtime error occurs. Why?

FastAPI

from fastapi import FastAPI, Depends

app = FastAPI()

def get_number():
    return 42

@app.get("/number")
async def read_number(num: int = Depends(get_number)):
    return {"num": num + 1}

A Because get_number returns int but FastAPI expects a str for query parameters

B No runtime error occurs; the output is {"num": 43}

C Because num is None and cannot be added to 1

D Because Depends does not support functions returning int

💡 Hint

Consider the type returned by the dependency and how it is used.

The dependency returns 42 (int). The endpoint adds 1 to it, resulting in 43. No error occurs.

🧠 Conceptual

expert

Which statement best describes how Depends works in FastAPI?

Choose the most accurate description of the Depends function behavior in FastAPI.

A Depends automatically converts all dependency return values to strings before injection.

B Depends caches the dependency function result globally and reuses it for all requests.

C Depends requires the dependency function to be async and returns a coroutine object.

D Depends calls the dependency function once per request and injects its return value into the endpoint parameter.

💡 Hint

Think about how dependencies are executed and their scope.

Depends calls the dependency function for each request and injects its result. It does not cache globally, does not require async, and does not convert return values to strings.