Route with dynamic parameters in Flask - Step-by-Step Execution

Concept Flow - Route with dynamic parameters

Client sends URL request

↓

Flask checks route patterns

↓

Match route with dynamic parameter?

No→404 Not Found

Yes↓

Extract parameter value from URL

↓

Call view function with parameter

↓

Return response with parameter used

Flask receives a URL, matches it to a route pattern with a dynamic part, extracts that part, and passes it to the function to create a response.

Execution Sample

Flask

from flask import Flask
app = Flask(__name__)

@app.route('/user/<username>')
def show_user(username):
    return f"User: {username}"

Defines a route that captures the username from the URL and returns it in the response.

Execution Table

Step	URL Requested	Route Pattern Matched	Parameter Extracted	Function Called	Response
1	/user/alice	/user/<username>	alice	show_user('alice')	User: alice
2	/user/bob	/user/<username>	bob	show_user('bob')	User: bob
3	/user/	No match	-	-	404 Not Found
4	/profile/alice	No match	-	-	404 Not Found

💡 Execution stops when a matching route is found and response returned, or 404 if no match.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4
username	-	alice	bob	-	-
response	-	User: alice	User: bob	404 Not Found	404 Not Found

Key Moments - 2 Insights

Why does /user/ (with no username) cause a 404 error?

Because the route expects a dynamic parameter after /user/, as shown in execution_table step 3, no parameter means no match.

How does Flask know what part of the URL is the username?

Flask uses the <username> placeholder in the route pattern to capture that part of the URL, as seen in steps 1 and 2.

Visual Quiz - 3 Questions

Look at the execution table, what is the response when the URL '/user/bob' is requested?

A 404 Not Found

B User: alice

C User: bob

D Error

💡 Hint

Check row 2 under the Response column in the execution_table.

At which step does the URL not match any route pattern?

A Step 3

B Step 2

C Step 1

D Step 4

💡 Hint

Look for 'No match' in the Route Pattern Matched column in execution_table.

If the route was changed to '/profile/<username>', what would happen when requesting '/user/alice'?

A It matches and returns 'User: alice'

B It returns 404 Not Found

C It matches but returns an error

D It redirects to /profile/alice

💡 Hint

Refer to execution_table step 4 where '/profile/alice' does not match '/user/'.

Concept Snapshot

Flask routes can include dynamic parts using <parameter> syntax.
When a URL matches, Flask extracts the parameter value.
The value is passed as an argument to the view function.
If no route matches, Flask returns a 404 error.
Example: @app.route('/user/<username>') captures username from URL.

Full Transcript

In Flask, routes can have dynamic parameters marked by angle brackets like <username>. When a client requests a URL, Flask checks if it matches any route pattern. If it matches a route with a dynamic parameter, Flask extracts the value from the URL and passes it to the function. The function then uses this value to create a response. If no route matches, Flask returns a 404 error. For example, the route '/user/<username>' matches URLs like '/user/alice' and passes 'alice' to the function. URLs without the parameter or with a different path do not match and cause 404 errors.