Flash messages for user feedback in Flask - Step-by-Step Execution

Concept Flow - Flash messages for user feedback

User triggers action

↓

Server processes request

↓

Server sets flash message

↓

Server redirects or renders template

↓

Template reads flash messages

↓

Flash messages displayed to user

↓

User sees feedback on page

Flash messages are set on the server after an action, then shown once on the next page load to give user feedback.

Execution Sample

Flask

from flask import Flask, flash, redirect, render_template, request, url_for

app = Flask(__name__)
app.secret_key = 'your_secret_key'

@app.route('/submit', methods=['POST'])
def submit():
    flash('Form submitted successfully!')
    return redirect(url_for('index'))

This code flashes a success message after form submission and redirects to the index page where the message is shown.

Execution Table

Step	Action	Flash Message Set	Redirect or Render	Message Displayed
1	User submits form	No	No	No
2	Server receives POST /submit	No	No	No
3	flash('Form submitted successfully!') called	Yes: 'Form submitted successfully!'	No	No
4	return redirect(url_for('index'))	Yes	Redirect to /index	No
5	User browser requests /index	No	No	No
6	Server renders index template	No	Render index.html	Yes: 'Form submitted successfully!' shown
7	Flash message removed after display	No	No	No

💡 Flash message shown once on /index page, then cleared.

Variable Tracker

Variable	Start	After Step 3	After Step 6	Final
flash_messages	[]	['Form submitted successfully!']	['Form submitted successfully!']	[]

Key Moments - 3 Insights

Why does the flash message only appear once and then disappear?

Because flash messages are stored in the session and removed automatically after being displayed once, as shown in execution_table step 7.

What happens if you forget to display flash messages in the template?

The message is set in the server but never shown to the user, so the user gets no feedback. See execution_table step 6 where rendering includes message display.

Why do we redirect after flashing instead of rendering directly?

Redirecting avoids form resubmission on page refresh and ensures flash messages appear on the next page load, as shown in execution_table steps 4 and 5.

Visual Quiz - 3 Questions

Look at the execution_table, at which step is the flash message actually displayed to the user?

A Step 3

B Step 6

C Step 4

D Step 7

💡 Hint

Check the 'Message Displayed' column in execution_table.

According to variable_tracker, what happens to flash_messages after step 7?

A It becomes empty

B It keeps the message for future requests

C It duplicates the message

D It throws an error

💡 Hint

Look at the 'Final' column for flash_messages in variable_tracker.

If you remove the redirect and render the template directly after flashing, what changes in the execution flow?

A Flash message will not be set

B Flash message will appear twice

C Flash message will appear immediately without redirect

D Flash message will never appear

💡 Hint

Consider the purpose of redirect in execution_table steps 4 and 5.

Concept Snapshot

Flash messages in Flask:
- Use flash('message') to store feedback
- Redirect to another route to show message
- In template, use get_flashed_messages() to display
- Messages show once then clear
- Helps inform users about actions like form submission

Full Transcript

Flash messages in Flask help give users feedback after actions like submitting a form. When the user submits, the server calls flash() to save a message. Then it redirects to another page. On that page, the template reads and shows the message once. After showing, the message is removed automatically. This prevents repeated messages on refresh. The flow is: user action triggers flash, server redirects, template displays message, then clears it. This is a simple way to keep users informed about what happened.