OAuth2 password flow in FastAPI - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the OAuth2PasswordBearer class from FastAPI.

FastAPI

from fastapi.security import [1]

Drag options to blanks, or click blank then click option'

AAPIKeyHeader

BOAuth2PasswordRequestForm

COAuth2PasswordBearer

DHTTPBasic

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing OAuth2PasswordBearer with OAuth2PasswordRequestForm.

Importing HTTPBasic instead of OAuth2PasswordBearer.

The OAuth2PasswordBearer class is used to define the OAuth2 password flow token URL in FastAPI.

2fill in blank

medium

Complete the code to create an OAuth2PasswordBearer instance with the token URL '/token'.

FastAPI

oauth2_scheme = OAuth2PasswordBearer(tokenUrl=[1])

Drag options to blanks, or click blank then click option'

A"/login"

B"/access"

C"/auth"

D"/token"

Attempts:

3 left

💡 Hint

Common Mistakes

Using '/login' or other URLs that do not match the token endpoint.

Forgetting to put quotes around the URL string.

The tokenUrl parameter should point to the URL where the client sends username and password to get the token. Commonly, this is '/token'.

3fill in blank

hard

Fix the error in the dependency injection to get the token from the OAuth2 scheme.

FastAPI

async def read_items(token: str = [1]):

Drag options to blanks, or click blank then click option'

Aoauth2_scheme()

BDepends(oauth2_scheme)

CDepends(oauth2_scheme())

Doauth2_scheme

Attempts:

3 left

💡 Hint

Common Mistakes

Calling the oauth2_scheme inside Depends, which causes an error.

Not using Depends at all.

When using FastAPI dependencies, you pass the callable without parentheses to Depends. So it should be Depends(oauth2_scheme).

4fill in blank

hard

Fill both blanks to define a token endpoint that uses OAuth2PasswordRequestForm and returns a token.

FastAPI

from fastapi import FastAPI, Depends
from fastapi.security import OAuth2PasswordRequestForm

app = FastAPI()

@app.post("/token")
async def login(form_data: [1] = Depends()):
    return {"access_token": form_data.[2], "token_type": "bearer"}

Drag options to blanks, or click blank then click option'

AOAuth2PasswordRequestForm

Busername

Cpassword

DOAuth2PasswordBearer

Attempts:

3 left

💡 Hint

Common Mistakes

Using OAuth2PasswordBearer instead of OAuth2PasswordRequestForm.

Accessing form_data.password instead of username for the token.

The OAuth2PasswordRequestForm dependency extracts form data including the username and password. The token response usually includes the username as the access token for demonstration.

5fill in blank

hard

Fill all three blanks to create a protected route that extracts the token and returns a welcome message.

FastAPI

from fastapi import FastAPI, Depends, HTTPException
from fastapi.security import OAuth2PasswordBearer

app = FastAPI()
oauth2_scheme = OAuth2PasswordBearer(tokenUrl="/token")

@app.get("/users/me")
async def read_users_me(token: str = Depends([1])):
    if not token:
        raise HTTPException(status_code=401, detail="Invalid authentication")
    return {"message": f"Welcome, [2]! Your token is [3]."}

Drag options to blanks, or click blank then click option'

Aoauth2_scheme

Buser

Ctoken

DtokenUrl

Attempts:

3 left

💡 Hint

Common Mistakes

Using Depends(oauth2_scheme()) instead of Depends(oauth2_scheme).

Using tokenUrl instead of oauth2_scheme.

Using undefined variables in the message.

The dependency oauth2_scheme extracts the token. The welcome message uses a placeholder user (which could be replaced by username in real apps) and the token variable.

Practice

1. What is the main purpose of the OAuth2 password flow in FastAPI?

easy

A. To allow users to log in by sending their username and password directly to the app.

B. To register new users automatically without credentials.

C. To refresh access tokens without user interaction.

D. To encrypt user passwords before storing them.

Solution

1. Step 1: Understand OAuth2 password flow purpose

   This flow lets users send their username and password to the app to get an access token.

2. Step 2: Compare options with flow purpose

   Only To allow users to log in by sending their username and password directly to the app. describes this direct login method; others describe different features.

3. Final Answer:

   To allow users to log in by sending their username and password directly to the app. -> Option A

4. Quick Check:

   OAuth2 password flow = direct login [OK]

Hint: Password flow means user sends username and password [OK]

Common Mistakes:

• Confusing password flow with token refresh
• Thinking it registers users automatically
• Assuming it encrypts passwords by itself

2. Which FastAPI import is used to handle OAuth2 password flow form data?

easy

A. from fastapi.security import OAuth2PasswordBearer

B. from fastapi.security import OAuth2PasswordRequestForm

C. from fastapi.security import HTTPBasicCredentials

D. from fastapi.security import APIKeyHeader

Solution

1. Step 1: Identify form class for password flow

   FastAPI uses OAuth2PasswordRequestForm to parse username and password from form data.

2. Step 2: Check other imports

   OAuth2PasswordBearer is for token extraction, HTTPBasicCredentials is for basic auth, APIKeyHeader is for API keys.

3. Final Answer:

   from fastapi.security import OAuth2PasswordRequestForm -> Option B

4. Quick Check:

   Form data handler = OAuth2PasswordRequestForm [OK]

Hint: Password flow form uses OAuth2PasswordRequestForm [OK]

Common Mistakes:

• Using OAuth2PasswordBearer instead of RequestForm
• Confusing HTTPBasicCredentials with OAuth2 forms
• Importing unrelated security classes

3. Given this FastAPI endpoint using OAuth2 password flow, what will be the response if username is 'alice' and password is 'secret'?

from fastapi import FastAPI, Depends
from fastapi.security import OAuth2PasswordRequestForm

app = FastAPI()

@app.post('/token')
async def login(form_data: OAuth2PasswordRequestForm = Depends()):
    if form_data.username == 'alice' and form_data.password == 'secret':
        return {'access_token': 'token123', 'token_type': 'bearer'}
    return {'error': 'Invalid credentials'}

medium

A. {'access_token': 'token123', 'token_type': 'bearer'}

B. {'error': 'Invalid credentials'}

C. HTTP 422 Unprocessable Entity error

D. Empty response with status 204

Solution

1. Step 1: Check input credentials against condition

   The code checks if username is 'alice' and password is 'secret'. Given inputs match this.

2. Step 2: Determine returned response

   Since condition is true, it returns the access token dictionary with 'token123' and 'bearer'.

3. Final Answer:

   {'access_token': 'token123', 'token_type': 'bearer'} -> Option A

4. Quick Check:

   Correct credentials = access token response [OK]

Hint: Match username and password to get token response [OK]

Common Mistakes:

• Assuming error response for correct credentials
• Confusing HTTP errors with normal returns
• Ignoring the if condition logic

4. What is wrong with this FastAPI OAuth2 password flow code snippet?

from fastapi import FastAPI, Depends
from fastapi.security import OAuth2PasswordRequestForm

app = FastAPI()

@app.post('/token')
async def login(form_data: OAuth2PasswordRequestForm):
    if form_data.username == 'bob' and form_data.password == 'pass':
        return {'access_token': 'abc', 'token_type': 'bearer'}
    return {'error': 'Invalid'}

medium

A. Endpoint should use GET method instead of POST

B. Incorrect import of OAuth2PasswordRequestForm

C. Return type should be a string, not dict

D. Missing Depends() in function parameter for form_data

Solution

1. Step 1: Check function parameter for dependency injection

   OAuth2PasswordRequestForm must be wrapped with Depends() to extract form data properly.

2. Step 2: Verify other parts

   Imports are correct, return type as dict is valid JSON response, POST method is correct for token requests.

3. Final Answer:

   Missing Depends() in function parameter for form_data -> Option D

4. Quick Check:

   Use Depends() to get form data [OK]

Hint: Always wrap OAuth2PasswordRequestForm with Depends() [OK]

Common Mistakes:

• Forgetting Depends() causes runtime errors
• Using GET instead of POST for token endpoint
• Thinking return must be string, not dict

5. You want to secure a FastAPI endpoint so only users with a valid OAuth2 password flow token can access it. Which approach correctly uses OAuth2PasswordBearer and token verification?

from fastapi import FastAPI, Depends, HTTPException
from fastapi.security import OAuth2PasswordBearer

app = FastAPI()
oauth2_scheme = OAuth2PasswordBearer(tokenUrl='token')

def verify_token(token: str):
    if token != 'validtoken':
        raise HTTPException(status_code=401, detail='Invalid token')

@app.get('/secure-data')
async def secure_data(token: str = Depends(oauth2_scheme)):
    verify_token(token)
    return {'data': 'secret info'}

hard

A. Incorrect: verify_token should return True/False, not raise exceptions.

B. Incorrect: tokenUrl should be '/secure-data' not 'token'.

C. Correct: uses OAuth2PasswordBearer and verifies token before returning data.

D. Incorrect: OAuth2PasswordBearer cannot be used with GET endpoints.

Solution

1. Step 1: Check OAuth2PasswordBearer usage

   oauth2_scheme is created with tokenUrl='token', which is correct for password flow token endpoint.

2. Step 2: Verify token validation logic

   verify_token raises HTTPException on invalid token, which is proper for access control.

3. Step 3: Confirm endpoint dependency and response

   secure_data depends on oauth2_scheme to get token, verifies it, then returns protected data.

4. Final Answer:

   Correct: uses OAuth2PasswordBearer and verifies token before returning data. -> Option C

5. Quick Check:

   Use OAuth2PasswordBearer + verify token = secure endpoint [OK]

Hint: Use OAuth2PasswordBearer with tokenUrl and verify token [OK]

Common Mistakes:

• Setting wrong tokenUrl in OAuth2PasswordBearer
• Not raising exceptions on invalid token
• Thinking OAuth2PasswordBearer can't be used with GET