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

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Server selects HTTP status code

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Server sends response with status code

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Client receives response

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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.