Flash message categories in Flask - Step-by-Step Execution

Concept Flow - Flash message categories

Trigger flash message

↓

Store message with category

↓

Redirect or render template

↓

Retrieve flashed messages

↓

Display messages grouped by category

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User sees categorized messages

Flash messages are stored with categories, retrieved later, and displayed grouped by those categories.

Execution Sample

Flask

from flask import flash, get_flashed_messages

flash('Login successful', 'success')
flash('Invalid password', 'error')
messages = get_flashed_messages(with_categories=true)

This code flashes two messages with categories and retrieves them grouped by category.

Execution Table

Step	Action	Message Stored	Category	Messages Retrieved
1	flash('Login successful', 'success')	Login successful	success	[]
2	flash('Invalid password', 'error')	Invalid password	error	[]
3	get_flashed_messages(with_categories=true)	-	-	[('success', 'Login successful'), ('error', 'Invalid password')]
4	Display messages grouped by category	-	-	Show 'Login successful' as success, 'Invalid password' as error

💡 All flashed messages retrieved and ready for display grouped by their categories.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
messages	[]	[]	[]	[('success', 'Login successful'), ('error', 'Invalid password')]	[('success', 'Login successful'), ('error', 'Invalid password')]

Key Moments - 2 Insights

Why do we use categories with flash messages?

Categories help group messages by type (like success or error) so the display can style or organize them differently, as shown in step 3 and 4 of the execution_table.

What happens if we call get_flashed_messages without with_categories=true?

Only message texts are returned without categories, so you lose the ability to group or style messages by type. This is why step 3 uses with_categories=true.

Visual Quiz - 3 Questions

Look at the execution_table at step 3, what does get_flashed_messages(with_categories=true) return?

A []

B ['Login successful', 'Invalid password']

C [('success', 'Login successful'), ('error', 'Invalid password')]

D [('Login successful', 'success'), ('Invalid password', 'error')]

💡 Hint

Check the Messages Retrieved column at step 3 in the execution_table.

At which step are the flash messages actually stored with their categories?

A Step 1 and Step 2

B Step 3

C Step 4

D Step 0

💡 Hint

Look at the Message Stored and Category columns in steps 1 and 2.

If we call get_flashed_messages() without with_categories=true, how would the messages variable change after step 3?

A It would be empty

B It would contain only message texts without categories

C It would contain categories but no messages

D It would cause an error

💡 Hint

Refer to key_moments about get_flashed_messages without with_categories=true.

Concept Snapshot

Flash messages in Flask store short messages for the next request.
Use flash(message, category) to add a message with a category.
Retrieve with get_flashed_messages(with_categories=true) to get (category, message) pairs.
Categories help display messages differently (e.g., success, error).
Common categories: 'success', 'error', 'warning', 'info'.

Full Transcript

In Flask, flash messages let you send short messages to users that appear after a page reload or redirect. You add a message with flash(message, category), where category is a label like 'success' or 'error'. These categories help you show messages in different styles or places. When you want to show the messages, you call get_flashed_messages(with_categories=true) to get a list of pairs with category and message. Then you can loop over them in your template and display each message with its style. This flow helps keep user feedback clear and organized.