What is Flask - Visual Explanation

Concept Flow - What is Flask

Start Flask App

↓

Define Routes

↓

Receive HTTP Request

↓

Match Route

↓

Run View Function

↓

Return Response

↓

Send Response to Browser

This flow shows how Flask starts, listens for requests, matches routes, runs code, and sends back responses.

Execution Sample

Flask

from flask import Flask
app = Flask(__name__)

@app.route('/')
def home():
    return 'Hello, Flask!'

This code creates a simple Flask app that shows 'Hello, Flask!' when you visit the home page.

Execution Table

Step	Action	Details	Result
1	Create Flask app	app = Flask(__name__)	App instance ready to handle requests
2	Define route '/'	@app.route('/')	Route '/' linked to home() function
3	Receive HTTP GET request	Browser visits '/' URL	Request received by Flask
4	Match route	Request URL '/' matches route '/'	home() function selected
5	Run view function	Execute home()	Returns string 'Hello, Flask!'
6	Send response	Send 'Hello, Flask!' back to browser	Browser displays 'Hello, Flask!'
7	End	No more actions	Request cycle complete

💡 Request handled and response sent, Flask waits for next request

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 5	Final
app	None	Flask instance	Flask instance with route '/'	Flask instance with route '/'	Flask instance with route '/'
request	None	None	None	HTTP GET '/' processed	None
response	None	None	None	'Hello, Flask!'	'Hello, Flask!' sent

Key Moments - 2 Insights

Why do we use @app.route('/') before the function?

The @app.route('/') tells Flask which URL should run the function. See execution_table step 2 where the route is linked to the function.

What happens when the browser visits the URL '/'?

Flask receives the request, matches it to the route '/', runs the linked function, and sends back the response. See steps 3 to 6 in the execution_table.

Visual Quiz - 3 Questions

Look at the execution_table, what is the result after step 4?

A Response is sent to browser

B home() function is selected to run

C Flask app is created

D Route '/' is defined

💡 Hint

Check the 'Result' column in row for step 4 in execution_table

At which step does Flask send the response back to the browser?

A Step 3

B Step 5

C Step 6

D Step 2

💡 Hint

Look for the step where 'Send response' action happens in execution_table

If we add another route, how would variable 'app' change after step 2?

A It would have multiple routes linked

B It would lose the first route

C It would become None

D It would not change

💡 Hint

See variable_tracker for 'app' after step 2 and think about adding routes

Concept Snapshot

Flask is a small web framework in Python.
You create an app with Flask(__name__).
Use @app.route('path') to link URLs to functions.
Functions return responses sent to browsers.
Flask listens for requests and runs matching functions.
Simple and flexible for web apps.

Full Transcript

Flask is a lightweight web framework for Python. You start by creating a Flask app instance. Then you define routes using the @app.route decorator to connect URLs to Python functions. When a browser sends a request to a URL, Flask matches it to a route, runs the linked function, and sends back the function's return value as the response. This process repeats for each request, allowing you to build web applications easily.