JWT token creation in FastAPI - Mini Project: Build & Apply

JWT Token Creation with FastAPI

📖 Scenario: You are building a simple API that needs to create secure tokens for users after they log in. These tokens will help the API know who the user is without asking for their password every time.

🎯 Goal: Build a FastAPI app that creates a JWT token using a secret key and user data.

📋 What You'll Learn

Create a dictionary with user data

Add a secret key variable for signing the token

Use the jwt.encode function to create the token

Return the token in a FastAPI response

💡 Why This Matters

🌍 Real World

APIs use JWT tokens to securely identify users without sending passwords repeatedly. This keeps user sessions safe and efficient.

💼 Career

Understanding JWT token creation is essential for backend developers working on authentication and security in web applications.

1

Create user data dictionary

Create a dictionary called user_data with these exact entries: "sub": "user123" and "role": "admin".

FastAPI

# Create a dictionary called user_data with 'sub' and 'role' keys
# Your code here

Hint

Think of user_data as a small card with user info that will go inside the token.

2

Add secret key variable

Add a variable called SECRET_KEY and set it to the string "mysecretkey123".

FastAPI

user_data = {"sub": "user123", "role": "admin"}
# Add a secret key variable called SECRET_KEY
# Your code here

Hint

The secret key is like a password that only your API knows to keep tokens safe.

3

Create JWT token using jwt.encode

Import jwt from jose and create a variable called token by encoding user_data with SECRET_KEY using the algorithm HS256.

FastAPI

from jose import jwt

user_data = {"sub": "user123", "role": "admin"}
SECRET_KEY = "mysecretkey123"
# Create a JWT token called token using jwt.encode with user_data, SECRET_KEY, and algorithm 'HS256'
# Your code here

Hint

Use jwt.encode to turn the user data into a secure token string.

4

Return token in FastAPI response

Import FastAPI and JSONResponse from fastapi. Create a FastAPI app called app. Add a GET route /token that returns a JSON response with the key "access_token" and the value token.

FastAPI

from jose import jwt
from fastapi import FastAPI
from fastapi.responses import JSONResponse

user_data = {"sub": "user123", "role": "admin"}
SECRET_KEY = "mysecretkey123"
token = jwt.encode(user_data, SECRET_KEY, algorithm="HS256")

# Create FastAPI app called app
# Add GET route '/token' that returns JSONResponse with {'access_token': token}
# Your code here

Hint

This step makes your API give out the token when someone visits the /token URL.

Practice

1. What is the main purpose of creating a JWT token in FastAPI?

easy

A. To style the user interface

B. To send emails automatically

C. To connect to a database

D. To securely store user information for authentication

Solution

1. Step 1: Understand JWT token purpose

   JWT tokens are used to safely carry user data for authentication.

2. Step 2: Identify correct use in FastAPI

   FastAPI uses JWT tokens to verify user identity securely.

3. Final Answer:

   To securely store user information for authentication -> Option D

4. Quick Check:

   JWT purpose = secure user info [OK]

Hint: JWT tokens are for secure user identity, not UI or DB [OK]

Common Mistakes:

• Confusing JWT with UI styling or database connection
• Thinking JWT sends emails
• Assuming JWT stores passwords directly

2. Which of the following is the correct way to import the function to create JWT tokens in FastAPI using PyJWT?

easy

A. import jwt.encode

B. from fastapi import jwt_encode

C. from jwt import encode

D. from fastapi.security import create_jwt

Solution

1. Step 1: Identify the JWT library used

   PyJWT is commonly used and provides an encode function imported as 'from jwt import encode'.

2. Step 2: Check FastAPI imports

   FastAPI itself does not provide jwt_encode or create_jwt functions directly.

3. Final Answer:

   from jwt import encode -> Option C

4. Quick Check:

   PyJWT encode import = from jwt import encode [OK]

Hint: PyJWT encode is imported from jwt, not fastapi [OK]

Common Mistakes:

• Trying to import JWT functions directly from FastAPI
• Using incorrect import syntax
• Confusing module names

3. Given this code snippet, what will be the output of the print(token) statement?

from jwt import encode

payload = {"user_id": 123}
secret = "mysecret"
algorithm = "HS256"
token = encode(payload, secret, algorithm=algorithm)
print(token)

medium

A. A JWT token string encoded with user_id 123

B. An error because algorithm parameter is missing

C. The original payload dictionary printed

D. None, because encode returns nothing

Solution

1. Step 1: Understand encode function behavior

   The encode function creates a JWT token string from the payload using the secret and algorithm.

2. Step 2: Analyze the code snippet

   Payload and secret are provided correctly, algorithm is set to HS256, so encode returns a JWT token string.

3. Final Answer:

   A JWT token string encoded with user_id 123 -> Option A

4. Quick Check:

   encode returns JWT string [OK]

Hint: encode returns a token string, not the original data [OK]

Common Mistakes:

• Expecting encode to print the payload
• Missing algorithm causes error (not true here)
• Thinking encode returns None

4. Identify the error in this JWT token creation code snippet:

from jwt import encode

payload = {"user_id": 42}
secret = "secretkey"
token = encode(payload, secret)
print(token)

medium

A. Missing algorithm parameter causes an error

B. No error; code runs correctly

C. Secret key should be bytes, not string

D. Payload must be a string, not a dictionary

Solution

1. Step 1: Check encode function requirements

   PyJWT's encode has a default algorithm='HS256', so it is not strictly required.

2. Step 2: Analyze the code snippet

   The code calls encode with payload and secret; algorithm defaults to HS256, so it runs correctly and produces a token.

3. Final Answer:

   No error; code runs correctly -> Option B

4. Quick Check:

   Algorithm defaults to HS256 = no error [OK]

Hint: PyJWT encode defaults to HS256 algorithm [OK]

Common Mistakes:

• Assuming algorithm defaults to HS256
• Thinking payload must be string
• Believing secret must be bytes

5. You want to create a JWT token in FastAPI that expires in 30 minutes. Which code snippet correctly adds the expiration time to the payload before encoding?

hard

A. payload = {"user_id": 1, "exp": datetime.utcnow() + timedelta(minutes=30)}

B. payload = {"user_id": 1, "exp": str(datetime.utcnow() + timedelta(minutes=30))}

C. payload = {"user_id": 1, "exp": time.time() + 1800}

D. payload = {"user_id": 1, "exp": datetime.now() + timedelta(minutes=30)}

Solution

1. Step 1: Understand JWT expiration format

   The 'exp' claim must be a UTC datetime or a timestamp representing expiration time.

2. Step 2: Evaluate each option

   payload = {"user_id": 1, "exp": datetime.utcnow() + timedelta(minutes=30)} uses datetime.utcnow() + timedelta for 30 minutes, which is correct. payload = {"user_id": 1, "exp": str(datetime.utcnow() + timedelta(minutes=30))} converts datetime to string, which is invalid. payload = {"user_id": 1, "exp": time.time() + 1800} uses time.time() but JWT expects datetime or timestamp as int, so this may cause issues. payload = {"user_id": 1, "exp": datetime.now() + timedelta(minutes=30)} uses datetime.now() which is local time, not UTC, causing potential errors.

3. Final Answer:

   payload = {"user_id": 1, "exp": datetime.utcnow() + timedelta(minutes=30)} -> Option A

4. Quick Check:

   Use UTC datetime for 'exp' claim [OK]

Hint: Use datetime.utcnow() + timedelta for expiration [OK]

Common Mistakes:

• Using local time instead of UTC
• Converting datetime to string for 'exp'
• Using wrong time functions like time.time() without conversion