Bearer token handling in FastAPI - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the correct FastAPI security class for bearer token handling.

FastAPI

from fastapi.security import [1]

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

AHTTPBasic

BAPIKeyHeader

COAuth2PasswordBearer

DHTTPBearer

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing OAuth2PasswordBearer which is for OAuth2 flows, not simple bearer tokens.

Choosing HTTPBasic which is for basic auth, not bearer tokens.

The HTTPBearer class is used in FastAPI to handle bearer token authentication.

2fill in blank

medium

Complete the code to create an instance of the HTTPBearer security scheme.

FastAPI

security = [1]()

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

AOAuth2PasswordBearer

BHTTPBearer

CHTTPBasic

DAPIKeyHeader

Attempts:

3 left

💡 Hint

Common Mistakes

Instantiating OAuth2PasswordBearer which requires a token URL.

Using HTTPBasic which is unrelated to bearer tokens.

Creating an instance of HTTPBearer allows FastAPI to extract bearer tokens from requests.

3fill in blank

hard

Fix the error in the dependency function to correctly extract the bearer token string.

FastAPI

async def get_token(credentials: HTTPAuthorizationCredentials = Depends(security)):
    return credentials.[1]

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

Atoken

Btoken_type

Ccredentials

Daccess_token

Attempts:

3 left

💡 Hint

Common Mistakes

Using token_type which is usually 'Bearer', not the token itself.

Using access_token which is not an attribute of HTTPAuthorizationCredentials.

The HTTPAuthorizationCredentials object has a token attribute that contains the bearer token string.

4fill in blank

hard

Fill both blanks to define a FastAPI route that requires a bearer token and returns it.

FastAPI

from fastapi import FastAPI, Depends
from fastapi.security import HTTPBearer, HTTPAuthorizationCredentials

app = FastAPI()
security = HTTPBearer()

@app.get("/token")
async def read_token(credentials: [1] = Depends(security)):
    return {"token": credentials.[2]

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

AHTTPAuthorizationCredentials

BHTTPBearer

Ctoken

Dtoken_type

Attempts:

3 left

💡 Hint

Common Mistakes

Using HTTPBearer as the parameter type instead of HTTPAuthorizationCredentials.

Returning token_type instead of token.

The route depends on HTTPAuthorizationCredentials to get the credentials object, and returns the token attribute.

5fill in blank

hard

Fill all three blanks to create a dependency that validates the bearer token prefix and returns the token string.

FastAPI

from fastapi import HTTPException, status

async def verify_token(credentials: HTTPAuthorizationCredentials = Depends(security)):
    if credentials.scheme != [1]:
        raise HTTPException(status_code=[2], detail="Invalid authentication scheme")
    return credentials.[3]

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

A"Bearer"

Bstatus.HTTP_401_UNAUTHORIZED

Ctoken

D"Basic"

Attempts:

3 left

💡 Hint

Common Mistakes

Checking for "Basic" scheme instead of "Bearer".

Using wrong status code like 403 instead of 401.

Returning scheme or token_type instead of token.

The scheme must be "Bearer" to accept bearer tokens. If not, raise 401 Unauthorized. Return the token string.

Practice

1. What is the main purpose of using a Bearer token in FastAPI?

easy

A. To serve static files efficiently

B. To format JSON responses automatically

C. To speed up database queries

D. To securely identify and authorize API requests

Solution

1. Step 1: Understand Bearer token role

   Bearer tokens are used to prove the client has permission to access protected routes.

2. Step 2: Identify purpose in FastAPI

   FastAPI uses Bearer tokens to check authorization before allowing access to API endpoints.

3. Final Answer:

   To securely identify and authorize API requests -> Option D

4. Quick Check:

   Bearer token = Authorization [OK]

Hint: Bearer tokens are for security and access control [OK]

Common Mistakes:

• Confusing token with response formatting
• Thinking token speeds up database
• Assuming token serves static files

2. Which FastAPI class is used to extract a Bearer token from the Authorization header?

easy

A. OAuth2PasswordBearer

B. HTTPBasicCredentials

C. OAuth2PasswordRequestForm

D. APIKeyHeader

Solution

1. Step 1: Recall FastAPI token extraction classes

   OAuth2PasswordBearer is designed to read Bearer tokens from the Authorization header.

2. Step 2: Match class to Bearer token usage

   OAuth2PasswordRequestForm is for form data, HTTPBasicCredentials is for basic auth, APIKeyHeader is for API keys, so only OAuth2PasswordBearer fits Bearer tokens.

3. Final Answer:

   OAuth2PasswordBearer -> Option A

4. Quick Check:

   Bearer token extractor = OAuth2PasswordBearer [OK]

Hint: Bearer token uses OAuth2PasswordBearer class [OK]

Common Mistakes:

• Using OAuth2PasswordRequestForm for token extraction
• Confusing basic auth with Bearer token
• Choosing APIKeyHeader for Bearer tokens

3. Given this FastAPI dependency:

oauth2_scheme = OAuth2PasswordBearer(tokenUrl="token")

@app.get("/users/me")
async def read_users_me(token: str = Depends(oauth2_scheme)):
    return {"token": token}

What will be the output if the client sends a request with header Authorization: Bearer abc123?

medium

A. {"token": "Bearer abc123"}

B. HTTP 401 Unauthorized error

C. {"token": "abc123"}

D. {"token": null}

Solution

1. Step 1: Understand OAuth2PasswordBearer behavior

   This class extracts only the token string after 'Bearer ' from the Authorization header.

2. Step 2: Analyze the returned value

   The function returns a JSON with the token string, so it will return {"token": "abc123"}.

3. Final Answer:

   {"token": "abc123"} -> Option C

4. Quick Check:

   Bearer token string extracted = "abc123" [OK]

Hint: OAuth2PasswordBearer strips 'Bearer ' prefix automatically [OK]

Common Mistakes:

• Expecting full 'Bearer abc123' string returned
• Assuming 401 error without token validation
• Thinking token is null if present

4. What is wrong with this FastAPI code snippet for Bearer token validation?

oauth2_scheme = OAuth2PasswordBearer(tokenUrl="token")

@app.get("/items/")
async def read_items(token: str = Depends(oauth2_scheme)):
    if token == None:
        raise HTTPException(status_code=401, detail="Invalid token")
    return {"token": token}

medium

A. The token check should use 'if not token' instead of 'if token == None'

B. OAuth2PasswordBearer does not extract tokens from headers

C. The route path should not end with a slash

D. HTTPException requires a status_code of 403 for invalid tokens

Solution

1. Step 1: Check token validation logic

   OAuth2PasswordBearer returns a string or raises an error if missing; token is never None but can be empty or missing.

2. Step 2: Correct token presence check

   Using 'if not token' is safer to catch empty strings or missing tokens rather than 'token == None'.

3. Final Answer:

   The token check should use 'if not token' instead of 'if token == None' -> Option A

4. Quick Check:

   Token presence check = 'if not token' [OK]

Hint: Check token presence with 'if not token' for safety [OK]

Common Mistakes:

• Using 'token == None' which misses empty strings
• Thinking OAuth2PasswordBearer doesn't extract tokens
• Confusing HTTP status codes for auth errors

5. You want to protect a FastAPI route so only requests with the exact Bearer token "secret123" are allowed. Which code snippet correctly implements this?

hard

A. async def protected_route(token: str = Depends(oauth2_scheme)):
if token == None:
return {"message": "Access granted"}
raise HTTPException(status_code=401, detail="Unauthorized")

B. async def protected_route(token: str = Depends(oauth2_scheme)):
if token != "secret123":
raise HTTPException(status_code=401, detail="Unauthorized")
return {"message": "Access granted"}

C. async def protected_route(token: str):
if token == "secret123":
return {"message": "Access granted"}
raise HTTPException(status_code=403, detail="Forbidden")

D. async def protected_route(token: str = Depends(oauth2_scheme)):
if token == "Bearer secret123":
return {"message": "Access granted"}
raise HTTPException(status_code=401, detail="Unauthorized")

Solution

1. Step 1: Use OAuth2PasswordBearer dependency

   We must use Depends(oauth2_scheme) to extract the token from the Authorization header.

2. Step 2: Check token value correctly

   The token string is just the token without 'Bearer ' prefix, so compare directly to "secret123" and raise 401 if not matching.

3. Final Answer:

   async def protected_route(token: str = Depends(oauth2_scheme)): if token != "secret123": raise HTTPException(status_code=401, detail="Unauthorized") return {"message": "Access granted"} -> Option B

4. Quick Check:

   Compare token string directly to "secret123" [OK]

Hint: Compare token string directly, no 'Bearer ' prefix included [OK]

Common Mistakes:

• Comparing token to 'Bearer secret123' including prefix
• Not using Depends(oauth2_scheme) to get token
• Returning access granted when token is None