Bearer token handling in FastAPI - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a Bearer token in API authentication?

A Bearer token is a security token that a client sends to a server to prove its identity. It is called 'Bearer' because whoever holds the token can access the protected resource.

beginner

How does FastAPI extract a Bearer token from an HTTP request?

FastAPI uses the OAuth2PasswordBearer class to declare a dependency that extracts the Bearer token from the Authorization header automatically.

intermediate

Show a simple FastAPI dependency that reads a Bearer token from the Authorization header.

from fastapi import Depends
from fastapi.security import OAuth2PasswordBearer
oauth2_scheme = OAuth2PasswordBearer(tokenUrl="token")

async def get_token(token: str = Depends(oauth2_scheme)):
    return token

intermediate

Why should you validate the Bearer token after extracting it in FastAPI?

Extracting the token only gets the string. You must validate it to check if it is correct, not expired, and belongs to an authorized user before allowing access.

beginner

What HTTP header carries the Bearer token in a request?

The Authorization header carries the Bearer token, formatted as: Authorization: Bearer <token>

Which FastAPI class helps extract Bearer tokens from requests?

A Depends

B HTTPBearer

C APIKeyHeader

D OAuth2PasswordBearer

OAuth2PasswordBearer is designed to extract Bearer tokens from the Authorization header in FastAPI.

What prefix must the Authorization header contain for a Bearer token?

A Token

B Bearer

C Basic

D APIKey

Bearer tokens are sent with the prefix 'Bearer' in the Authorization header.

After extracting a Bearer token, what is the next important step?

A Validate the token's authenticity and expiry

B Send it back to the client

C Store it in a database

D Ignore it

Validation ensures the token is valid and the user is authorized.

In FastAPI, how do you declare a dependency to get the Bearer token?

A Use Depends with APIKeyQuery

B Use Depends with HTTPBasic

C Use Depends with OAuth2PasswordBearer instance

D Use Depends with Cookie

OAuth2PasswordBearer is used with Depends to extract Bearer tokens.

What HTTP status code should you return if the Bearer token is missing or invalid?

A 401 Unauthorized

B 200 OK

C 403 Forbidden

D 404 Not Found

401 Unauthorized indicates the client must authenticate with a valid token.

Explain how FastAPI handles Bearer token extraction and validation in a simple API endpoint.

Think about the Authorization header and FastAPI's dependency system.

Describe the role of the Authorization header and the Bearer token format in API security.

Focus on how the token is sent and why it matters.

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