Custom error pages (404, 500) in Flask - Step-by-Step Execution

Concept Flow - Custom error pages (404, 500)

User requests URL

↓

Flask checks route

Yes↓

Route found -> Return page

No↓

Error triggered (404 or 500)

↓

Custom error handler catches error

↓

Render custom error page

↓

Send error page to user

When a user visits a URL, Flask checks if the route exists. If not, or if an error occurs, Flask triggers an error. Custom error handlers catch these errors and show friendly error pages.

Execution Sample

Flask

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

@app.errorhandler(404)
def not_found(e):
    return render_template('404.html'), 404

@app.errorhandler(500)
def internal_error(e):
    return render_template('500.html'), 500

This code sets custom pages for 404 and 500 errors by telling Flask to use the 404.html and 500.html templates when a page is not found or an internal error occurs.

Execution Table

Step	User Request	Route Found?	Error Triggered	Handler Called	Page Rendered	Response Code
1	/home	Yes	No	No	Home page	200
2	/unknown	No	Yes (404)	Yes (404 handler)	Custom 404 page	404
3	/cause-error	Yes	Yes (500)	Yes (500 handler)	Custom 500 page	500
4	End	-	-	-	-	-

💡 Execution stops after sending the response page to the user.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
user_request	None	/home	/unknown	/cause-error	None
route_found	False	True	False	True	N/A
error_triggered	None	No	404	500	N/A
handler_called	None	No	404 handler	500 handler	N/A
page_rendered	None	Home page	Custom 404 page	Custom 500 page	N/A
response_code	None	200	404	500	N/A

Key Moments - 2 Insights

Why does Flask call the 404 error handler instead of returning a normal page?

Because the route was not found (see execution_table row 2), Flask triggers a 404 error and calls the custom 404 handler to show a friendly page.

How does Flask know to use the custom 500 error page?

When an internal error happens (row 3), Flask triggers a 500 error and calls the registered 500 error handler to render the custom page.

Visual Quiz - 3 Questions

Look at the execution table, what is the response code when the user requests '/unknown'?

A 200

B 500

C 404

D None

💡 Hint

Check the 'Response Code' column for the row where 'User Request' is '/unknown'.

At which step does Flask call the 500 error handler?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Look at the 'Handler Called' column to find when the 500 handler is used.

If the route '/home' did not exist, what would change in the execution table?

A Step 1 would show error triggered as 404 and handler called as 404 handler

B Step 1 would show error triggered as 500 and handler called as 500 handler

C Step 1 would remain the same with response code 200

D Step 1 would be removed

💡 Hint

Refer to how missing routes trigger 404 errors in the execution_table rows.

Concept Snapshot

Custom error pages in Flask:
- Use @app.errorhandler(code) decorator
- Define a function returning a template and status code
- Flask calls handler on matching error (404, 500)
- Shows friendly error pages instead of default messages
- Helps users understand errors clearly

Full Transcript

When a user visits a website built with Flask, the app checks if the requested page exists. If it does, Flask shows the page normally with a 200 status code. If the page is missing, Flask triggers a 404 error. If something goes wrong inside the app, it triggers a 500 error. Developers can write special functions called error handlers to catch these errors. These handlers return custom pages like 'Page Not Found' or 'Server Error' to help users understand what happened. This makes the website friendlier and easier to use. The code example shows how to create a custom 404 page by using the @app.errorhandler decorator and returning a template with the 404 status code.