HTTP status codes for APIs in Flask - Mini Project: Build & Apply

HTTP status codes for APIs

📖 Scenario: You are building a simple web API using Flask. This API will respond to client requests with data and appropriate HTTP status codes to indicate success or errors.

🎯 Goal: Create a Flask API with one route that returns JSON data and uses HTTP status codes to show if the request was successful or if there was an error.

📋 What You'll Learn

Create a Flask app with one route /api/data

Return JSON data with a success message and HTTP status code 200

Add a condition to return an error message with HTTP status code 404 if data is missing

Use Flask's jsonify and make_response to send responses

💡 Why This Matters

🌍 Real World

APIs use HTTP status codes to tell clients if their request worked or if there was a problem. This helps apps communicate clearly over the web.

💼 Career

Knowing how to use HTTP status codes in APIs is essential for backend developers, web developers, and anyone building web services.

1

Set up Flask app and data

Import Flask and jsonify from flask. Create a Flask app called app. Create a dictionary called data with the entry 'message': 'Hello, API!'.

Flask

from flask import Flask, jsonify

# Create Flask app
app = Flask(__name__)

# Create data dictionary
# Your code here

Hint

Use data = {'message': 'Hello, API!'} exactly to create the data dictionary.

2

Add a config variable for data availability

Create a variable called data_available and set it to True. This will simulate whether the data is available or not.

Flask

from flask import Flask, jsonify

app = Flask(__name__)

data = {'message': 'Hello, API!'}

# Create data_available variable
# Your code here

Hint

Set data_available = True exactly as shown.

3

Create the API route with status codes

Define a route /api/data using @app.route. Create a function called get_data. Inside it, use an if statement to check if data_available is True. If yes, return jsonify(data) with status code 200. Otherwise, return jsonify({'error': 'Data not found'}) with status code 404. Use make_response to set the status code.

Flask

from flask import Flask, jsonify, make_response

app = Flask(__name__)

data = {'message': 'Hello, API!'}

data_available = True

# Create /api/data route
# Your code here

Hint

Use @app.route('/api/data') and define get_data(). Use make_response with jsonify and status codes 200 or 404.

4

Add the app run command

Add the code to run the Flask app only if the script is run directly. Use if __name__ == '__main__': and inside it call app.run(debug=True).

Flask

from flask import Flask, jsonify, make_response

app = Flask(__name__)

data = {'message': 'Hello, API!'}

data_available = True

@app.route('/api/data')
def get_data():
    if data_available:
        return make_response(jsonify(data), 200)
    else:
        return make_response(jsonify({'error': 'Data not found'}), 404)

# Add app run command
# Your code here

Hint

Use the standard Flask app run block with debug=True for easy testing.

Practice

1. What HTTP status code should a Flask API return when a request is successful and data is sent back?

easy

A. 200 OK

B. 404 Not Found

C. 500 Internal Server Error

D. 201 Created

Solution

1. Step 1: Understand the meaning of status codes

   200 means the request was successful and the server is returning the requested data.

2. Step 2: Match the code to the scenario

   Since the request succeeded and data is returned, 200 OK is the correct code.

3. Final Answer:

   200 OK -> Option A

4. Quick Check:

   Success response = 200 OK [OK]

Hint: Success with data? Use 200 OK [OK]

Common Mistakes:

• Confusing 201 Created with simple success
• Using 404 when resource exists
• Using 500 for client errors

2. Which of the following is the correct way to return a JSON response with a 201 status code in Flask?

easy

A. return data, code=201

B. return data, status=201

C. return json.dumps(data), 201

D. return jsonify(data), 201

Solution

1. Step 1: Recall Flask's return syntax for JSON and status

   Flask uses jsonify() to convert data to JSON and returns a tuple (response, status_code).

2. Step 2: Identify the correct syntax

   return jsonify(data), 201 correctly uses jsonify(data), 201 to return JSON with status 201.

3. Final Answer:

   return jsonify(data), 201 -> Option D

4. Quick Check:

   Flask JSON + status = jsonify(data), 201 [OK]

Hint: Use jsonify() and return tuple (data, status) [OK]

Common Mistakes:

• Using json.dumps without jsonify
• Incorrect keyword arguments like status= or code=
• Returning raw data without JSON conversion

3. Consider this Flask route code:

from flask import Flask, jsonify
app = Flask(__name__)

@app.route('/item/')
def get_item(id):
    items = {1: 'apple', 2: 'banana'}
    if id in items:
        return jsonify({'item': items[id]}), 200
    else:
        return jsonify({'error': 'Item not found'}), 404

What HTTP status code will be returned when accessing /item/3?

medium

A. 200

B. 404

C. 500

D. 201

Solution

1. Step 1: Check if id 3 exists in items dictionary

   Items only have keys 1 and 2, so 3 is not found.

2. Step 2: Identify the returned status code for missing item

   The else block returns 404 Not Found with an error message.

3. Final Answer:

   404 -> Option B

4. Quick Check:

   Missing resource = 404 Not Found [OK]

Hint: Missing item? Return 404 Not Found [OK]

Common Mistakes:

• Assuming 200 even if item missing
• Confusing 404 with 500 server error
• Returning 201 for missing resource

4. This Flask API code is intended to return a 400 Bad Request if the 'name' field is missing in JSON input:

@app.route('/add', methods=['POST'])
def add():
    data = request.get_json()
    if 'name' not in data:
        return {'error': 'Name required'}, 400
    return {'message': 'Added'}, 201

What is the main problem with this code?

medium

A. It returns a dict instead of using jsonify

B. It returns 201 instead of 200 on success

C. It does not import request from flask

D. It uses wrong status code for missing name

Solution

1. Step 1: Check imports needed for request

   The code uses request.get_json() but does not show importing request from flask.

2. Step 2: Identify the error caused by missing import

   Without importing request, the code will raise a NameError at runtime.

3. Final Answer:

   It does not import request from flask -> Option C

4. Quick Check:

   Missing import request causes error [OK]

Hint: Always import request to use request.get_json() [OK]

Common Mistakes:

• Ignoring missing imports causing runtime errors
• Thinking returning dict is invalid (Flask accepts dict)
• Confusing 201 Created as wrong success code here

5. You want to create a Flask API endpoint that creates a new user. If the username already exists, return 400 Bad Request. If creation succeeds, return 201 Created with the new user's ID. Which code snippet correctly implements this behavior?

hard

A. def create_user():
data = request.get_json()
if data['username'] in users:
return {'error': 'User exists'}, 400
new_id = len(users) + 1
users[new_id] = data['username']
return {'id': new_id}, 201

B. def create_user():
data = request.get_json()
if data['username'] in users:
return {'error': 'User exists'}, 404
new_id = len(users) + 1
users[new_id] = data['username']
return {'id': new_id}, 200

C. def create_user():
data = request.get_json()
if data['username'] in users:
return {'error': 'User exists'}, 500
new_id = len(users) + 1
users[new_id] = data['username']
return {'id': new_id}, 201

D. def create_user():
data = request.get_json()
if data['username'] in users:
return {'error': 'User exists'}, 201
new_id = len(users) + 1
users[new_id] = data['username']
return {'id': new_id}, 400

Solution

1. Step 1: Check status code for existing user

   400 Bad Request is correct when username already exists, indicating client error.

2. Step 2: Check status code for successful creation

   201 Created is the correct code for successful resource creation.

3. Step 3: Verify code matches these codes

   def create_user(): data = request.get_json() if data['username'] in users: return {'error': 'User exists'}, 400 new_id = len(users) + 1 users[new_id] = data['username'] return {'id': new_id}, 201 returns 400 for existing user and 201 for success, matching requirements.

4. Final Answer:

   Option A code snippet -> Option A

5. Quick Check:

   Existing user = 400, success = 201 [OK]

Hint: Use 400 for bad input, 201 for successful creation [OK]

Common Mistakes:

• Using 404 or 500 for existing user error
• Returning 200 instead of 201 on creation
• Mixing up success and error codes