HTTP status codes for APIs in Flask - Step-by-Step Execution

Concept Flow - HTTP status codes for APIs

Client sends API request

↓

Server receives request

↓

Server processes request

↓

Server selects HTTP status code

↓

Server sends response with status code

↓

Client receives response

↓

Client acts based on status code

This flow shows how an API request is handled, with the server choosing an HTTP status code to tell the client what happened.

Execution Sample

Flask

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

@app.route('/item/<int:id>')
def get_item(id):
    if id == 1:
        return jsonify({'item': 'apple'}), 200
    else:
        return jsonify({'error': 'Not found'}), 404

This Flask API returns a 200 status with data if item id is 1, otherwise a 404 status with error message.

Execution Table

Step	Input id	Condition	Status Code	Response Body
1	1	id == 1 is True	200	{"item": "apple"}
2	2	id == 1 is False	404	{"error": "Not found"}
3	0	id == 1 is False	404	{"error": "Not found"}
4	exit	No more requests	-	-

💡 No more requests to process, server waits for next client call

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
id	-	1	2	0	-
status_code	-	200	404	404	-
response_body	-	{"item": "apple"}	{"error": "Not found"}	{"error": "Not found"}	-

Key Moments - 2 Insights

Why does the server return 404 instead of 200 when id is not 1?

Because the condition id == 1 is False, the code returns a 404 status to tell the client the item was not found, as shown in execution_table rows 2 and 3.

What does the 200 status code mean in this API?

The 200 status code means the request was successful and the server is sending back the requested data, as seen in execution_table row 1.

Visual Quiz - 3 Questions

Look at the execution table, what is the status code when id is 2?

A 200

B 404

C 500

D 301

💡 Hint

Check execution_table row 2 under Status Code column

At which step does the server send a successful response?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look for status code 200 in execution_table

If the API added a new item with id 3 returning 201, how would the status code change at step 3?

A It would stay 404

B It would change to 500

C It would change to 201

D It would change to 200

💡 Hint

201 means created, so a new item added would return 201 instead of 404

Concept Snapshot

HTTP status codes tell the client what happened with their API request.
200 means success and data returned.
404 means resource not found.
Use Flask return syntax: return jsonify(data), status_code.
Choose status codes based on request result.
Clients use codes to decide next steps.

Full Transcript

This lesson shows how HTTP status codes work in APIs using Flask. When a client sends a request, the server processes it and picks a status code to explain the result. For example, if the requested item exists, the server returns 200 with the data. If not, it returns 404 with an error message. The execution table traces requests with different ids and shows the status codes and responses sent. Key moments clarify why 404 is used for missing items and what 200 means. The quiz asks about status codes at different steps and how adding a new item changes the response. Remember, status codes help clients understand if their request succeeded or failed and what to do next.

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