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.

Practice

1. What is the main purpose of the Depends function in FastAPI?

easy

A. To create HTML templates

B. To define database models

C. To inject dependencies automatically into path operation functions

D. To handle HTTP status codes

Solution

1. Step 1: Understand what Depends does

   Depends is used to declare dependencies that FastAPI will automatically provide to your route functions.

2. Step 2: Identify the main use case

   It helps inject reusable code like authentication, database sessions, or other shared logic into routes.

3. Final Answer:

   To inject dependencies automatically into path operation functions -> Option C

4. Quick Check:

   Depends injects dependencies = C [OK]

Hint: Depends injects reusable code into routes automatically [OK]

Common Mistakes:

• Confusing Depends with database or template functions
• Thinking Depends handles HTTP status codes
• Assuming Depends creates models

2. Which of the following is the correct way to declare a dependency in a FastAPI route using Depends?

easy

A. def read_items(db=Depends(get_db)): pass

B. def read_items(db: Depends(get_db)): pass

C. def read_items(db: Depends = get_db): pass

D. def read_items(db=Depends): pass

Solution

1. Step 1: Recall Depends syntax

   The correct syntax is to assign the parameter a default value of Depends with the dependency function inside.

2. Step 2: Match the correct option

   def read_items(db=Depends(get_db)): pass uses db=Depends(get_db), which is the proper way to declare a dependency.

3. Final Answer:

   def read_items(db=Depends(get_db)): pass -> Option A

4. Quick Check:

   Depends usage = parameter=Depends(function) [OK]

Hint: Use parameter=Depends(function) to declare dependencies [OK]

Common Mistakes:

• Using type annotation instead of default value for Depends
• Passing Depends without parentheses
• Assigning Depends without a function

3. Given the code below, what will be the output when accessing the /items/ endpoint?

from fastapi import FastAPI, Depends

app = FastAPI()

def get_number():
    return 42

@app.get('/items/')
def read_items(number: int = Depends(get_number)):
    return {"number": number}

medium

A. {"number": "get_number"}

B. {"number": 42}

C. Error: missing required parameter

D. {"number": null}

Solution

1. Step 1: Understand dependency injection

   The get_number function returns 42, and FastAPI injects this value into the number parameter.

2. Step 2: Check the returned response

   The route returns a dictionary with key "number" and value 42, so the output is {"number": 42}.

3. Final Answer:

   {"number": 42} -> Option B

4. Quick Check:

   Depends injects 42 = {"number": 42} [OK]

Hint: Depends calls function and injects return value [OK]

Common Mistakes:

• Expecting the function name instead of its return value
• Thinking parameter is missing if not passed explicitly
• Assuming null is returned if no argument given

4. What is wrong with the following FastAPI code using Depends? How to fix it?

from fastapi import FastAPI, Depends

app = FastAPI()

def get_user():
    return "Alice"

@app.get('/user/')
def read_user(user: str = Depends):
    return {"user": user}

medium

A. Depends is missing the dependency function; fix by using Depends(get_user)

B. The route path is invalid; fix by changing '/user/' to '/users/'

C. The return type is wrong; fix by returning a list instead of dict

D. The function get_user should accept parameters; fix by adding parameters

Solution

1. Step 1: Identify the Depends usage error

   The parameter uses Depends without specifying the dependency function, which is incorrect.

2. Step 2: Correct the Depends syntax

   It should be Depends(get_user) to tell FastAPI which function to call for the dependency.

3. Final Answer:

   Depends is missing the dependency function; fix by using Depends(get_user) -> Option A

4. Quick Check:

   Depends needs function argument = Depends(get_user) [OK]

Hint: Always pass the dependency function inside Depends() [OK]

Common Mistakes:

• Using Depends without parentheses or function
• Changing route path unnecessarily
• Changing return type without reason

5. How can you use Depends to share a database session across multiple routes without repeating code? Choose the best approach.

hard

A. Use Depends without any function to automatically get the session

B. Pass the session as a global variable to all routes

C. Manually create a session inside each route function

D. Create a function that returns the session and use Depends on it in each route

Solution

1. Step 1: Understand code reuse with Depends

   Depends allows you to write a function that creates or yields a database session once and injects it wherever needed.

2. Step 2: Identify the best practice

   Creating a session function and using Depends on it in routes avoids repetition and manages session lifecycle cleanly.

3. Final Answer:

   Create a function that returns the session and use Depends on it in each route -> Option D

4. Quick Check:

   Use Depends with session function for reuse = D [OK]

Hint: Use Depends with a session function to share DB session [OK]

Common Mistakes:

• Using global variables for sessions (not safe)
• Creating sessions manually in every route (repetitive)
• Using Depends without specifying a function