HTTP status codes for APIs in Flask

Introduction

HTTP status codes tell the client if the API request worked or if there was a problem. They help both the server and client understand what happened.

When sending a success response after creating a new resource

When the client sends bad data that the server cannot process

When a requested resource is not found on the server

When the client is not authorized to access a resource

When the server encounters an unexpected error

Syntax

Flask

from flask import Flask, jsonify

app = Flask(__name__)

@app.route('/example')
def example():
    data = {'message': 'Success!'}
    return jsonify(data), 200

The status code is returned as the second value in the return statement.

Common status codes include 200 (OK), 201 (Created), 400 (Bad Request), 404 (Not Found), 500 (Internal Server Error).

Examples

Returns a 201 status code to indicate a new resource was created successfully.

Flask

return jsonify({'message': 'Created'}), 201

Returns a 400 status code when the client sends invalid data.

Flask

return jsonify({'error': 'Bad request'}), 400

Returns a 404 status code when the requested resource does not exist.

Flask

return jsonify({'error': 'Not found'}), 404

Returns a 500 status code when the server has an unexpected error.

Flask

return jsonify({'error': 'Server error'}), 500

Sample Program

This Flask app has two routes: one to get an item by ID and one to create a new item.

It uses HTTP status codes to tell the client if the request was successful or if there was an error like not found or bad request.

Flask

from flask import Flask, jsonify, request

app = Flask(__name__)

items = {'1': 'apple', '2': 'banana'}

@app.route('/items/<item_id>', methods=['GET'])
def get_item(item_id):
    if item_id in items:
        return jsonify({'item': items[item_id]}), 200
    else:
        return jsonify({'error': 'Item not found'}), 404

@app.route('/items', methods=['POST'])
def create_item():
    data = request.get_json()
    if not data or 'id' not in data or 'name' not in data:
        return jsonify({'error': 'Bad request, id and name required'}), 400
    if data['id'] in items:
        return jsonify({'error': 'Item already exists'}), 400
    items[data['id']] = data['name']
    return jsonify({'message': 'Item created'}), 201

if __name__ == '__main__':
    app.run(debug=True)

Important Notes

Always return the correct status code to help clients handle responses properly.

Use 2xx codes for success, 4xx for client errors, and 5xx for server errors.

Flask lets you return status codes easily by returning a tuple with the response and code.

Summary

HTTP status codes communicate the result of an API request.

Use Flask's return syntax to send both data and status codes.

Common codes: 200 (OK), 201 (Created), 400 (Bad Request), 404 (Not Found), 500 (Internal Server Error).

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