Custom status codes in Flask - Step-by-Step Execution

Concept Flow - Custom status codes

Client sends request

↓

Flask route receives request

↓

Process request logic

↓

Create response with custom status code

↓

Send response back to client

↓

Client receives response with custom status code

The client sends a request, Flask processes it, then returns a response with a custom status code back to the client.

Execution Sample

Flask

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

@app.route('/custom')
def custom_status():
    return jsonify(message='Hello'), 202

This Flask route returns a JSON message with a custom status code 202 (Accepted).

Execution Table

Step	Action	Status Code Set	Response Content	Client Receives
1	Client sends GET /custom	N/A	N/A	Request sent
2	Flask route '/custom' called	N/A	N/A	Waiting for response
3	Return jsonify with message and status 202	202	{"message": "Hello"}	Response received
4	Client processes response	202	{"message": "Hello"}	Message shown with status 202

💡 Response sent with custom status code 202, ending request cycle

Variable Tracker

Variable	Start	After Step 3	Final
status_code	None	202	202
response_content	None	{"message": "Hello"}	{"message": "Hello"}

Key Moments - 2 Insights

Why do we return a tuple with (response, status_code) in Flask?

Flask allows returning a tuple where the second item is the status code. This sets the HTTP status code explicitly, as shown in execution_table step 3.

Can we use any number as a custom status code?

You can use standard HTTP codes or custom ones, but it's best to use valid HTTP status codes. The example uses 202 which means 'Accepted'. See execution_table step 3.

Visual Quiz - 3 Questions

Look at the execution table, what status code is set at step 3?

A 404

B 202

C 200

D 500

💡 Hint

Check the 'Status Code Set' column at step 3 in the execution_table.

At which step does the client receive the response?

A Step 3

B Step 4

C Step 1

D Step 2

💡 Hint

Look at the 'Client Receives' column to find when the response is received.

If we changed the status code to 404, what would change in the execution_table?

A The client would send a different request

B The 'Response Content' would change to an error message

C The 'Status Code Set' at step 3 would be 404

D Nothing would change

💡 Hint

Focus on the 'Status Code Set' column at step 3 in the execution_table.

Concept Snapshot

Flask lets you return a response with a custom status code by returning a tuple: (response, status_code).
Use jsonify() to send JSON data.
Example: return jsonify(message='Hi'), 202
The status code tells the client how to interpret the response.
Common codes: 200 OK, 404 Not Found, 202 Accepted.
Custom codes should follow HTTP standards.

Full Transcript

In Flask, you can send a custom status code by returning a tuple from your route function. The first item is the response data, often created with jsonify for JSON responses. The second item is the status code number, like 202 for Accepted. When a client sends a request, Flask runs the route function, creates the response with the custom status code, and sends it back. The client then receives the response and can act based on the status code. This lets you communicate more precise information about the request result beyond the default 200 OK.

Practice

1. What is the correct way to return a custom status code 450 from a Flask route?

easy

A. return "Error", 450

B. return 450, "Error"

C. return Response(status=450)

D. return make_response(450, "Error")

Solution

1. Step 1: Understand Flask return tuple format

   Flask routes return a tuple with (response_body, status_code) to set custom status codes.

2. Step 2: Match the tuple with custom code 450

   Returning "Error", 450 matches the format (body, status_code) correctly.

3. Final Answer:

   return "Error", 450 -> Option A

4. Quick Check:

   Tuple (body, status) = return "Error", 450 [OK]

Hint: Return (body, code) tuple to set custom status [OK]

Common Mistakes:

• Swapping order of status and body in return tuple
• Using Response without import or wrong parameters
• Using make_response incorrectly

2. Which of the following is the correct syntax to return a JSON response with a custom status code 499 in Flask?

easy

A. return jsonify(message="Fail"), 499

B. return jsonify(message="Fail", status=499)

C. return jsonify(message="Fail", 499)

D. return jsonify(message="Fail").status_code(499)

Solution

1. Step 1: Use jsonify to create JSON response

   jsonify returns a Response object with JSON content.

2. Step 2: Return tuple with jsonify and status code

   To set custom status code, return (jsonify(...), status_code) tuple.

3. Final Answer:

   return jsonify(message="Fail"), 499 -> Option A

4. Quick Check:

   jsonify + tuple (response, code) = return jsonify(message="Fail"), 499 [OK]

Hint: Return (jsonify(...), code) tuple for JSON + custom code [OK]

Common Mistakes:

• Passing status code inside jsonify arguments
• Trying to call status_code as method
• Incorrect tuple order or syntax

3. What will be the HTTP status code returned by this Flask route?

from flask import Flask
app = Flask(__name__)

@app.route('/test')
def test():
    return "Custom", 499

medium

A. 404

B. 200

C. 499

D. 500

Solution

1. Step 1: Check the return statement in route

   The route returns a tuple ("Custom", 499), where 499 is the status code.

2. Step 2: Flask uses the second tuple element as status code

   Flask sets HTTP status code to 499 as specified in the return tuple.

3. Final Answer:

   499 -> Option C

4. Quick Check:

   Return tuple second value = status code 499 [OK]

Hint: Second tuple value in return sets status code [OK]

Common Mistakes:

• Assuming default 200 status code
• Confusing return order
• Thinking 499 is invalid and defaults to 500

4. Identify the error in this Flask route that tries to return a custom status code 499:

from flask import Flask
app = Flask(__name__)

@app.route('/error')
def error():
    return "Oops", status=499

medium

A. Route function must return a Response object

B. Custom status code 499 is not allowed in Flask

C. Missing jsonify for JSON response

D. Using 'status' keyword argument in return is invalid

Solution

1. Step 1: Check return statement syntax

   Flask does not accept 'status' keyword in return statement directly.

2. Step 2: Correct way is to return tuple (body, status_code)

   Use return "Oops", 499 instead of return "Oops", status=499.

3. Final Answer:

   Using 'status' keyword argument in return is invalid -> Option D

4. Quick Check:

   Return tuple (body, code) not keyword args [OK]

Hint: Return tuple (body, code), not keyword args [OK]

Common Mistakes:

• Using status=code in return instead of tuple
• Assuming Flask auto-detects status from keywords
• Confusing with Response object parameters

5. You want to create a Flask route that returns a JSON message with a custom status code 460 only if a query parameter 'valid' is 'no'. Otherwise, return status 200 with a success message. Which code correctly implements this?

hard

A. from flask import Flask, request, jsonify
app = Flask(__name__)

@app.route('/check')
def check():
if request.args.get('valid') == 'no':
return jsonify(error="Invalid", status=460)
return jsonify(message="OK")

B. from flask import Flask, request, jsonify
app = Flask(__name__)

@app.route('/check')
def check():
if request.args.get('valid') == 'no':
return jsonify(error="Invalid"), 460
return jsonify(message="OK"), 200

C. from flask import Flask, request, jsonify
app = Flask(__name__)

@app.route('/check')
def check():
if request.args.get('valid') == 'no':
return jsonify(error="Invalid").status_code = 460
return jsonify(message="OK")

D. from flask import Flask, request, jsonify
app = Flask(__name__)

@app.route('/check')
def check():
if request.args.get('valid') == 'no':
return jsonify(error="Invalid")
return jsonify(message="OK"), 200

Solution

1. Step 1: Check conditional logic for query parameter

   Use request.args.get('valid') to check if it equals 'no'.

2. Step 2: Return JSON with custom status code 460 if invalid

   Return (jsonify(...), 460) tuple to set custom status code correctly.

3. Step 3: Return success JSON with status 200 otherwise

   Return (jsonify(...), 200) tuple for success response.

4. Final Answer:

   Correct conditional returns with tuples for custom and normal codes -> Option B

5. Quick Check:

   Return (jsonify, code) tuples for all cases [OK]

Hint: Return (jsonify(...), code) tuples for all responses [OK]

Common Mistakes:

• Passing status inside jsonify arguments
• Trying to set status_code attribute directly
• Missing status code tuple on error response