Flask route as API endpoint - Step-by-Step Execution

Concept Flow - Flask route as API endpoint

Start Flask app

↓

Receive HTTP request

↓

Match URL to route

Yes↓

Call route function

↓

Process data / logic

↓

Return response (JSON)

↓

Send response to client

↓

End

This flow shows how Flask listens for a web request, matches it to a route, runs the function, and sends back a JSON response.

Execution Sample

Flask

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

@app.route('/api/data')
def get_data():
    return jsonify({'message': 'Hello API'})

This code creates a Flask API endpoint at /api/data that returns a JSON message.

Execution Table

Step	Action	Input/Request	Function Called	Output/Response
1	Start Flask app	N/A	N/A	App running, waiting for requests
2	Receive HTTP GET request	GET /api/data	N/A	Request received
3	Match URL to route	/api/data	get_data()	Route matched
4	Call route function	N/A	get_data()	Function executes
5	Process data	N/A	get_data()	{'message': 'Hello API'}
6	Return JSON response	N/A	get_data()	JSON response sent
7	Send response to client	N/A	N/A	Client receives JSON
8	End	N/A	N/A	Request cycle complete

💡 Request cycle ends after sending JSON response to client

Variable Tracker

Variable	Start	After Request	After Function Call	Final
request.path	N/A	/api/data	/api/data	/api/data
response	N/A	N/A	{'message': 'Hello API'}	{'message': 'Hello API'}

Key Moments - 3 Insights

Why does Flask call the get_data() function only when the URL matches '/api/data'?

Flask uses the @app.route decorator to link URLs to functions. The execution_table row 3 shows the URL matching step before the function is called.

What format does the API endpoint return data in?

The function returns data using jsonify(), which converts Python dictionaries to JSON format. This is shown in execution_table row 6.

What happens if a request is made to a URL not defined in Flask routes?

Flask will not find a matching route and returns a 404 error. This is implied by the route matching step in execution_table row 3.

Visual Quiz - 3 Questions

Look at the execution table, what is the output at step 5?

A {'message': 'Hello API'}

B App running, waiting for requests

C JSON response sent

D Request received

💡 Hint

Check the 'Output/Response' column at step 5 in the execution_table

At which step does Flask match the incoming URL to the route function?

A Step 2

B Step 4

C Step 3

D Step 6

💡 Hint

Look at the 'Action' column for URL matching in the execution_table

If the route function returned a plain string instead of jsonify, how would the output at step 6 change?

A It would still be JSON formatted

B It would be a plain text response

C Flask would throw an error

D The client would receive no response

💡 Hint

Think about what jsonify() does compared to returning a string directly

Concept Snapshot

Flask API endpoint basics:
- Use @app.route('/path') to define URL
- Define a function to handle requests
- Return data with jsonify() for JSON response
- Flask matches URL, calls function, sends JSON back
- Unmatched URLs return 404 error

Full Transcript

This visual trace shows how a Flask app runs an API endpoint. First, the app starts and waits for requests. When a client sends a GET request to /api/data, Flask matches this URL to the get_data function. The function runs and returns a Python dictionary converted to JSON by jsonify. Flask sends this JSON response back to the client. If the URL does not match any route, Flask returns a 404 error. Variables like request.path hold the URL, and response holds the JSON data. This step-by-step flow helps beginners see how Flask routes work as API endpoints.