Login form and verification in Flask - Step-by-Step Execution

Concept Flow - Login form and verification

User opens login page

↓

Display login form

↓

User enters username & password

↓

User submits form

↓

Server receives data

↓

Verify credentials

↓

Login success

↓

Redirect to

↓

dashboard

The user opens the login page, fills the form, submits it, then the server checks credentials and either logs in or shows an error.

Execution Sample

Flask

from flask import Flask, request, render_template_string
app = Flask(__name__)

@app.route('/login', methods=['GET', 'POST'])
def login():
    if request.method == 'POST':
        username = request.form['username']
        password = request.form['password']
        if username == 'user' and password == 'pass':
            return 'Login successful'
        else:
            return 'Login failed'
    return render_template_string('<form method="post">Username: <input name="username"><br>Password: <input name="password" type="password"><br><input type="submit"></form>')

This Flask app shows a login form and checks if username and password match fixed values.

Execution Table

Step	Action	Request Method	Form Data	Verification Result	Response
1	User opens /login page	GET	None	N/A	Show login form
2	User fills form and submits	POST	username='user', password='pass'	Check if username=='user' and password=='pass'	Login successful
3	User fills form and submits	POST	username='user', password='wrong'	Check fails	Login failed
4	User opens /login page again	GET	None	N/A	Show login form

💡 Execution stops after sending response to user for each request.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4
username	None	'user'	'user'	None
password	None	'pass'	'wrong'	None
request.method	GET	POST	POST	GET
verification_result	N/A	True	False	N/A

Key Moments - 2 Insights

Why does the form show again after a failed login?

Because when verification fails (see step 3 in execution_table), the server returns 'Login failed' but the user can try again by reloading the form (step 4). The form is shown on GET requests.

Why do we check request.method before accessing form data?

Because form data is only sent on POST requests. On GET requests, there is no form data, so we show the form instead (see steps 1 and 4).

Visual Quiz - 3 Questions

Look at the execution_table, what is the response when username='user' and password='pass'?

A Login successful

B Login failed

C Show login form

D Error page

💡 Hint

Check step 2 in execution_table where form data matches correct credentials.

At which step does the server receive a GET request?

A Step 2

B Step 3

C Step 1

D Step 4

💡 Hint

Look at the Request Method column in execution_table for GET requests.

If the password check was changed to 'password == "1234"', what would be the response at step 2?

A Login successful

B Login failed

C Show login form

D Server error

💡 Hint

Compare the password in form data at step 2 with the new check condition.

Concept Snapshot

Flask login form:
- Use @app.route with GET and POST
- On GET, show form HTML
- On POST, get username/password from request.form
- Check credentials with if condition
- Return success or failure message
- Form resubmits on failure for retry

Full Transcript

This example shows a simple login form in Flask. When the user visits the /login page with a GET request, the server sends back a form with username and password fields. When the user fills the form and submits it, the browser sends a POST request with the form data. The server then checks if the username and password match the expected values. If they do, it returns a success message. If not, it returns a failure message. The user can then try again by reloading the form. This flow repeats for each request.